Lay Summaries Archive

Read Lay Summaries from previous volumes of Functional Ecology here:

Early View Lay Summaries

 

Experimental effects of early-life corticosterone on the HPA axis and pre-migratory behaviour in a wild songbird

Jesse J. Pakkala, D. Ryan Norris, James S. Sedinger and Amy E.M. Newman A banded Savannah sparrow perches upon a gooseberry shrub on its breeding territory, Kent Island (New Brunswick, Canada).  Photo credit: Stéphanie Doucet.

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The early-life environment has profound and lasting effects on behaviour and physiology, which is especially obvious when the early-life environment involves exposure to severe stressors. For example, evidence from children born in the year of the Dutch Hunger Winter of 1945 is a potent example of the lasting effects of early-life stress. Although stress may be considered a so-called “fact of life”, how early-life stress influences the fitness of animals in the wild is not well understood. To understand such ecophysiological links, one must be able to follow individuals in their natural environment from birth to later-life stages, and have knowledge of the physiological effects of stress. The vertebrate stress axis, also called the hypothalamic-pituitary-adrenal axis (or HPA axis) is highly conserved among vertebrates and is an important mechanism that connects organisms to their environment. Stressors in the environment activate the HPA axis, causing an increase in circulating stress hormones, which have many downstream effects.

Contrary to popular myth, Savannah sparrows are far more than mere ‘little brown jobs’. An experiment on a long-term, marked population breeding on a remote island in the Bay of Fundy has revealed how increased levels of stress hormones during the early days of life influence both the physiology of nestlings and their subsequent behaviour as independent juveniles. In an intensive field experiment, we examined hypotheses to explain stress exposure. Nestling sparrows treated with the stress hormone, corticosterone, on days 2-6 post hatch, were more sensitive (had higher circulating stress hormones) to inclement weather when they were 7 days old. And, importantly, after birds had fledged the nest, mark-recapture analysis revealed that early-life corticosterone exposure increased temporary emigration from the study site, highlighting an important carry-over effect of early-life stress on behaviour later in life. Tracking individuals across multiple stages of the life cycle will advance our understanding of how early-life stress and ecology interact to influence physiology, behaviour and fitness.

Image caption: A banded Savannah sparrow perches upon a gooseberry shrub on its breeding territory, Kent Island (New Brunswick, Canada). Photo credit: Stéphanie Doucet.
You can read the article in full here.

 

The importance of biotic interactions for the prediction of macroinvertebrate communities under multiple stressors

Nele Schuwirth, Anne Dietzel and Peter ReichertMayfly with eggs, taken by Nele Schuwirth.

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In this study we tested a computer model to predict the occurrence of macroinvertebrates in streams. Macroinvertebrates are aquatic organisms that live on the river bed or in the sediment, like insect larvae, snails, worms, or crustaceans. They are very diverse and have different environmental requirements, e.g. regarding temperature, flow velocity, substrate types, water quality, and they are specialized on different food sources. Because their community composition depends on the environmental conditions, they can be used to assess the ecological state of rivers and the influence of human impacts, like water pollution, channelization, or modifications of the flow regime. For decision support in river management it is important to predict the influence of different management activities on the ecological state of the rivers. To this end, predictive models can be useful tools.

Based on existing knowledge about the requirements of different macroinvertebrate taxa and some basic principles, like mass conservation or the scaling of vital rates with body-size and temperature, we constructed the process based model called "Streambugs". The model aims to predict which species, genera or families are expected to occur at which sites. We used existing monitoring data from 36 sites in the Glatt catchment at the Swiss Plateau to compare model predictions with observations. Even without fitting the model parameters (factors that influence growth, respiration and death processes) and just relying on prior knowledge about these parameters, for 79% of the taxa at the 36 sites the difference between the observed and predicted frequency of occurrence is less than 50%. This is considerably better (+18%) than a random model that assigns a 50% chance to observe or not observe each taxon at each site and each sampling event. By adjusting factors that increase or decrease the growth rates of the different organisms, we can increase model compliance with data. Furthermore, we tested which properties of the organisms contributed most to the predictive power of the model. In this catchment, sensitivity to organic toxic substances as well as feeding-types were most important for making good predictions. These results highlight that biotic interactions between the organisms (competition for food and predator-prey interactions) should be taken into account when trying to predict the occurrence of aquatic organisms in streams.

Image caption: Mayfly with eggs, taken by Nele Schuwirth.
You can read the article in full here.

 

Diet determines movement rates and size of area used for herbivores

Zulima Tablado, Eloy Revilla, Dominique Dubray, Sonia Said, Daniel Maillard and Anne LoisonPhoto provided by authors.

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Mammalian herbivores have specialized on a variety of diets, ranging from species eating almost exclusively grasses (grazers) to species feeding mostly on browse, other herbaceous plants, and fruits (browsers). Since these vegetation types are not equally distributed in the space, species feeding on them should not move equally either. Moreover, grasses and therefore grazers are usually associated with open spaces (grasslands) in which animals are more easily detected by predators and tend to group in herds in order to maintain high levels of vigilance and decrease per capita predation risk. By contrast, browsers are linked to areas with protective cover (bushes and forests), rely more on hiding as a strategy to avoid predation, and are usually solitary. In-between these two extremes, we find the species called mixed feeders. In an area of the French Alps where three species of large herbivores coexist (roe deer, chamois, and mouflon), we marked individuals with GPS collars. We investigated how movement patterns and home ranges at different temporal scales differed for these three species, expecting shorter movements and smaller home ranges at all scales for browsers than grazers and intermediate species. Interestingly, no differences in movement occurred at fine temporal scale: all species move as much when looking for food (20 minutes time scale), whatever their diet. But differences emerged at larger scales (hours, day, season scales). As expected, mouflons, which are grazers and form large herds, performed larger displacements and depended on larger areas, probably as a results of competition within groups. Further, their movements and range areas were affected the most by environmental factors, such as weather and human disturbance, which occurred mostly in open areas. At the opposite extreme, roe deer, which are solitary browsers, performed smaller displacements, moving back and forth within smaller range areas. Their movements seem also to be less affected by the variability of external factors. Finally, chamois, which are mixed feeders, showed patterns that were intermediate between the other two species. Food distribution, feeding type, and local competition with related animals should be considered jointly to understand large herbivore movements, home range and response to external factors.

Image caption: Photo provided by authors.
You can read the article in full here.

 

Impacts of toxic nectar on three pollinators

Erin Jo Tiedeken, Paul A. Egan, Philip C. Stevenson, Geraldine A. Wright, Mark J. F. Brown, Eileen F. Power, Iain Farrell, Sharon M. Matthews, and Jane C. Stout Invasive Rhododendron ponticum.

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Pollinator decline is a serious concern worldwide; pollinators are important for global food production, and they are also essential in maintaining the diversity of flowering plants on our planet. One cause of pollinator declines is thought to be the spread of highly abundant and ecologically damaging species beyond their native ranges (invasive alien species). Nevertheless, researchers still do not understand the direct impacts that invasive plants have on pollinators.

In this study, we show that an invasive plant in the United Kingdom and Ireland, Rhododendron ponticum, has very different effects on three economically and ecologically important pollinating bee species. Rhododendron nectar was toxic to native Irish honeybees, killing individuals within hours of consumption. The nectar also had negative impacts on the foraging behaviour of a native Irish solitary bee. The solitary bees became paralyzed after feeding on Rhododendron nectar, and ate less food than bees fed a control nectar. In contrast, the native buff-tailed bumblebee was able to consume the nectar with no negative responses, even when the bumblebee was weakened by lack of food or parasite infection. Invasive Rhododendron may therefore provide an important novel nectar resource for this bumblebee species, especially in the early spring when bumblebee queens are establishing their colonies.

Our study shows that the toxicity of Rhododendron nectar is due to the presence of a neurotoxin that humans are also susceptible to, called grayanotoxin I. Many plant species contain such “nectar toxins,” chemicals that are usually associated with defence against foliar herbivores. Our paper demonstrates that invasive plants may differentially impact pollinators, providing new food resources only to those that can tolerate their nectar toxins. In addition, this work demonstrates the importance of comparing the susceptibility of multiple species to natural and synthetic chemicals before making conclusions about the impact of substances on pollinators as a whole.

Image caption: Invasive Rhododendron ponticum.
You can read the article in full here here.

 

Energy expenditure at rest affects reproduction and survival in house sparrows

Bernt Rønning, Juli Broggi, Claus Bech, Børge Moe, Thor Harald Ringsby, Henrik Pärn, Ingerid J. Hagen, Bernt-Erik Sæther and Henrik JensenAfter the experimental period, birds were released at the same location as they were captured. Photo: H. Jensen.

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The body must use a minimum amount of energy to keep all its systems functioning correctly. This rate of energy use, termed basal metabolic rate (BMR), has been measured in birds all over the world and found to vary between birds living in different environments. The fact that this trait is adapted to different environments implies that it has been shaped through natural selection and therefore probably is related to fitness, i.e. survival and reproduction. Despite this, we know surprisingly little about how BMR is related to fitness. One could expect that an individual which uses little energy to sustain its basic body processes i.e. has low BMR, has more energy available for producing offspring and staying alive, thereby enhancing fitness compared to an individual with high BMR. On the contrary, it may be that high BMR reflects a high working capacity. In other words, an individual with high BMR can eat more food and process it at a higher rate, thereby having more energy to spend on activities that increase fitness. To test the relationship between BMR and fitness in a songbird we measured BMR in two populations of house sparrows (Passer domesticus) in northern Norway. Thereafter, we collected data on how many offspring each bird produced and their survival from one year to the next.

We found that, although there was an overall tendency for birds with low BMR to produce more offspring, this negative relationship was significant only for females in one of the populations. Furthermore, the relationship between BMR and survival also differed between sexes and populations. An effect of BMR on survival was found only in one population. In that population females with an average BMR were more likely to survive, whereas males with an average BMR were less likely to survive.

The fact that the relationship between BMR and fitness diffed both between sexes and populations implies that this relationship may not be always either positive or negative as often predicted, but instead be context dependent, varying in time and space in relation to e.g. food availability and weather conditions.

Image caption: After the experimental period, birds were released at the same location as they were captured. Photo: H. Jensen.
You can read the article in full here.

 

Short-term rainfall, not temperature, controls lizard microhabitat use in a piñon-juniper woodland

Mason J Ryan, Ian M Latella, J. Tomasz Giermakowski, Howard Snell, Steven Poe, Robert E Pangle, Nathan Gehres, William T Pockman & Nate G McDowellImage provided by authors.

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As climate change increases temperature globally and alters regional rainfall, the activity and foraging behavior of cold-blooded animals (ectotherms) like lizards must also change if individuals are to grow and reproduce. Lizards respond to higher temperatures by decreasing activity and reducing foraging time, particularly in sunlit locations, to limit heat gain. While these behavioral changes at higher temperature are linked to population declines, little is known about how lizards will respond to decreases in rainfall in arid environments predicted to accompany warming. Moreover, tests of these responses are difficult in natural environments because experimental manipulations of temperature and rainfall are uncommon.

To examine how wild lizards respond to changing temperature and rainfall, we measured lizard responses in a piñon-juniper woodland in New Mexico where rainfall and temperature were manipulated on 40 x 40 m plots assigned to one of four treatments: Drought imposed by plastic troughs diverting 45% of natural rainfall, Warming plots with inverted plastic troughs that raised temperatures without diverting rainfall, Irrigation plots with sprinklers that simulated rainfall, and unmanipulated Control plots. In the context of these treatments, we observed lizards and recorded their activity and use of shade or sun microhabitats. This design allowed us to determine how rainfall and/or temperature influenced lizard activity and decisions about foraging in sun vs. the shade of trees.

Contrary to recent studies, we found that rainfall, not temperature, strongly influenced lizard behavior. During dry periods, lizards foraged almost exclusively in shade under trees, whereas lizards foraged in sunny areas across the landscape following natural or simulated rainfall events. Temperature did not influence lizard behavior, even in the covered plots with the warmest temperatures, a surprising result considering the wealth of research showing lizard temperature sensitivity. We conclude that our study lizard is more sensitive to moisture and relies on tree shade as a refuge during dry periods. The long-term drying trends that are increasing tree mortality in piñon-juniper woodlands may decrease the availability of this critical lizard microhabitat.

Image caption: Image provided by authors.
You can read the article in full here.

 

Friend or foe? Lessons from cushion alpine plants and their neighbours

Giuliano Bonanomi , Adriano Stinca, Giovanni Battista Chirico, Giampiero Ciaschetti, Antonio Saracino and Guido Incerti Cushion of Silene acaulis hosting a suite of beneficiary species in the Apennine mountains (Italy).

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Historically, the struggle for resources and predation by herbivores were thought to be the main processes controlling the number and abundance of different species in plant communities. But in recent years evidence that some species can help other plants under harsh environmental conditions has become prominent. Such facilitative effects have been observed worldwide in deserts, wetlands, grasslands and scrublands, where nurse species facilitate beneficiary plants by buffering extreme temperatures, reducing exposure to solar radiation by shading, protecting palatable species from grazing, and increasing soil moisture and nutrient availability.

In alpine environments, where plants face extreme weather conditions, cushion-shaped plants, thanks to their low stature and compact structure, are among the organisms best adapted to the critical combination of low temperatures, strong winds and severe desiccation. Several studies have reported that alpine cushion plants act as nurses, hosting a suite of stress-tolerant species within their canopy, where warmer, more protected and stable conditions occur. Among alpine cushion plants, Silene acaulis is well known as a nurse plant, and here we explore whether it always has a positive influence on coexisting species by analysing plant responses along an altitudinal severity gradient ranging from ~2,000 to ~2,800 m in the Italian Apennine mountains. We found that the ability of Silene to facilitate other plants considerably shifts along the gradient, being greatest at the intermediate elevation. On the other hand, cushion morphology dramatically changes with altitude, being lax, soft, and flat-shaped at low elevation and tightly knit and dome-shaped at high elevation. Silene compactness progressively increases with elevation because the plant stems are progressively more closely paired, likely to avoid heat dissipation. We discovered that cushions effectively act as heat traps only at medium and high elevations, while at lower altitude the soft flat cushions avoid excessive heat accumulation that may damage plant functions. However, at the upper end of the gradient, cushion compactness is so high that space available for hosting other species is greatly reduced, thus outweighing the positive heating effect for other plants.

Image caption: Cushion of Silene acaulis hosting a suite of beneficiary species in the Apennine mountains (Italy).
You can read the article in full here.

 

Agricultural stressors affect stream ecosystems in unexpected ways

Andreas Bruder, Romana K. Salis, Nicola J. McHugh and Christoph D. MatthaeiThe experimental set-up on the banks of a New Zealand stream. The insert shows a close-up of a channel simulating a stream ecosystem after colonisation by river organisms. Photo credit: P. Schenker.

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Streams draining agricultural land are affected by many stressors caused by human land use, including water diversion for irrigation and elevated levels of nutrients from fertilizers, agrochemicals and deposited fine sediment. These stressors can interact, which makes their combined effect hard to predict based on the individual stressor effects. Such situations complicate management and restoration of agricultural streams. Our study addresses this challenge in an experiment conducted in New Zealand where agriculture has been rapidly intensifying. We used model stream ecosystems created in 128 small circular channels, which were subjected to the four stressors mentioned above. Channels were supplied continuously with water from a nearby river, enabling their colonisation by river invertebrates, algae and microorganisms. We studied stressor effects on decomposition of leaf litter from two tree species common alongside stream banks in New Zealand, evergreen mahoe and deciduous birch. Leaf litter is an important source of energy and nutrients for stream food webs, and one group of organisms that specialises in utilizing leaf litter as a resource are microscopic fungi.

Reduced flow velocity (a consequence of streamwater diversion for irrigation) had the strongest negative effects of the stressors tested. It reduced the growth of fungi on both leaf species and slowed down leaf decomposition. Slower flow velocity curtails the supply of dissolved oxygen and nutrients from the streamwater to the fungi, which depend on these chemicals for growth. Deposited fine sediment also had strong effects, slowing down birch decomposition but accelerating mahoe decomposition. For birch, the effects of sediment addition and flow reduction caused stronger combined effects than one would have expected based on the individual effects of each stressor. Effects of elevated levels of nutrients and agrochemicals were relatively rare and subtle.

These findings are both novel and important. The two stressors with the strongest impacts, flow reduction and deposited fine sediment, are considered less often in river management and restoration projects than nutrients or agrochemicals, and their interactions are rarely assessed. Finally, the contrasting stressor effects on mahoe and birch leaves show that stream ecosystem responses to agricultural stressors depend on the composition of the stream-bank vegetation.

Image caption: The experimental set-up on the banks of a New Zealand stream. The insert shows a close-up of a channel simulating a stream ecosystem after colonisation by river organisms. Photo credit: P. Schenker.
You can read the article in full here.

 

Biotic and abiotic interactions, but not plant chemistry alone, contribute to chemical identity of invaded soils

Vidya Suseela, Peter Alpert Cindy H. Nakatsu, Arthur Armstrong and Nishanth TharayilJapanese knotweed encroaching into an old-field in Massachusetts.

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Spread of introduced, invasive plants can alter soil chemistry and microbial communities, and thus impact native ecosystem processes including nutrient cycling and soil carbon storage. Influence of some of these exotic species on native ecosystems could linger even after the removal of the invasive, contributing to a legacy effect. However the attributes of invasive plants that facilitate the creation and persistence of the new ecosystem properties is poorly understood. Discovering the mechanisms for this effect could help show why some introduced plants are more invasive than others and suggest how to restore invaded communities. Phenolic compounds, due to their abundance, persistence and biological activity, could be one of the more important classes of compounds that contribute to the lasting impact of plant identity on the chemistry of soil carbon in invaded soils. Despite this likely influence of plant inputs on invaded ecosystems, our knowledge of the significance of plant inputs in changing plant-soil feedbacks remains fragmentary.

To address this knowledge-gap we investigated the changes in composition of phenolic compounds and microbial functional activities in soils following the invasion of Japanese knotweed (Polygonum cuspidatum; Fallopia japonia) in four sites across eastern US. Soils under knotweed stands were twice as rich in phenolic compounds compared to soils under native vegetation. Also, the invaded soil across the wider geographic region were more similar in soil phenolic profile and microbial communities compared to soils under and adjacent to knotweed stands at the same site. However, the composition of phenolic compounds in invaded soil had low resemblance to phenolics found in knotweed tissues, indicating a potential role for microbial metabolism and abiotic transformations in shaping the chemical identity of invaded soils. Knotweed influenced the chemistry not only of extractable phenolic compounds, but also of the non-extractable bulk soil carbon with a turnover rate of decades or more, suggesting a long-lasting influence of this invader in these ecosystems.

Our results suggest that interactions between plant inputs, abiotic reactions, and biotic transformations could create and maintain new states in the soil that are chemically and biologically less diverse, and could outlive the invader themselves. Restoration could thus require not just removal of the species, but also post-removal interventions such as soil amendments.

Image caption: Japanese knotweed encroaching into an old-field in Massachusetts.
You can read the article in full here.

 

Predation, food, and male-male competition drive natural variation in lizard tail autotomy

Chi-Yun Kuo and Duncan J. IrschickSide-blotched lizard couple sharing a burrow. Photo credit: Chi-Yun Kuo.

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Autotomy, or the voluntary shedding of body parts, is a behavior many animals (> 100 vertebrate and invertebrate families) use to survive predation. In some animals the detached body parts would continue moving to divert the attention of the predators away from the escaping animal. Autotomy is normally triggered by a force exerted on the discardable body part. In most cases the force comes from a predator but can sometimes be the result of male-male combats. Despite its benefit in enhancing the chance of survival in the field, autotomy imposes numerous short- and long-term costs, such as risk of infection, lower locomotor performance, lower mating success, and ultimately long-term survival. High costs of autotomy imply that selection should not take the use of this behavior lightly, and how easily autotomy occurs should reflect the underlying cost-benefit dynamics. Indeed, how easy it is to induce autotomy (i.e. the propensity for autotomy) exhibits remarkable variation among populations. In this study, we combined model simulations and field data to explicitly test the hypothesis that the ecological environment drives the variation in autotomy among populations. We first built a simple model to examine the roles of three major ecological factors: predation, food abundance, and male-male competition. Our predictions for the three factors are as follows: high predation and an abundance of food should favor high propensities for autotomy (i.e. easier to induce), as the former situation would increase the benefit of autotomy while the latter makes autotomy less expensive. On the contrary, more male-male fighting should favor lower propensities for autotomy because it is undesirable to lose body parts under non-life threatening situations. Simulation results confirmed all our predictions above. We then collected data on predation, food abundance, the intensity of male-male competition, and the propensity for tail autotomy from five side-blotched lizard populations in western U.S. With the ecological information, our model was able to predict how those five populations should vary in the propensity for tail autotomy. Again, our model successfully predicted the pattern of variation observed in the field, thereby providing further evidence that predation, food abundance, and male-male competition jointly drive natural variation in autotomy among populations.

Image caption: Side-blotched lizard couple sharing a burrow. Photo credit: Chi-Yun Kuo.
You can read the article in full here.

 

Do non-native plants have the same traits at home? A comparison in France and New York

J. Mason Heberling ,Thomas Kichey, Guillaume Decocq, and Jason D. FridleyCommon buckthorn on the forest edge in the native France (left) and invasive New York, USA (right). Photo by J.M. Heberling.

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Non-native species become invasive when they actively spread in their introduced range. Trait comparisons between invasive and native species that occur in the same habitat have provided important explanations for how certain non-native species are so successful in their new range. However, it is unclear if these invasive species function similarly in their native range. In other words, do “invasive traits” arise after introduction or do some invasive species also display these traits in their native range?

We compared leaf traits associated with plant function of two important forest invaders in their native (home) and introduced (invaded) ranges: black cherry, a tree native to the Eastern North America but invasive in Europe, and common buckthorn, a shrub native to Europe but invasive in Eastern North America. This transatlantic comparison was unique in that where one species is native the other is invasive, and vice-versa. Because the two species occur in the same regions, we could better control for the potential effect of regional conditions like climate to understand the effect of whether the species is invasive to the region.

We found striking differences in leaf traits associated with plant strategy between native and invaded ranges for both species. Black cherry had similar rates of photosynthesis in the forests of native New York and invasive France, but leaves in France contained more carbon. Trait differences between home and invaded regions were more pronounced for common buckthorn, with 50-60% higher rates of photosynthesis in the invaded region (New York) compared to native France. Buckthorn leaves in New York also tended to have greater leaf nitrogen costs (an important nutrient for photosynthesis) and resorbed proportionately less of this nitrogen in autumn prior to dropping leaves. Autumn leaf nitrogen concentrations were 69% lower in France than New York. This trait might explain how buckthorn is able to drop its leaves later than other species in the invaded range.

Overall, our study shows that traits, which together form a plant’s resource-use strategy, can be different between native and non-native ranges.

Image caption: Common buckthorn on the forest edge in the native France (left) and invasive New York, USA (right). Photo by J.M. Heberling.
You can read the article in full here.

 

Comparing the decomposition of plant litter in aquatic and terrestrial ecosystems

Pablo García-Palacios, Brendan G. McKie, I. Tanya Handa, André Frainer andStephan HättenschwilerLeaf litter layer in an Amazonian rainforest with fungi as key decomposer organisms. Photo credit: Stephan Hättenschwiler.

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The decomposition of dead plant material, for example leaf litter, takes places in both terrestrial (for example, forest floors) and aquatic (for example, forest streams) ecosystems. Litter decomposition is one of the main components of the global carbon cycle and plays a fundamental role in soil fertility. Despite being one of the most studied ecological processes, litter decomposition has traditionally been approached independently by terrestrial and stream ecologists. Thus, although we have a pretty good idea about the main drivers of litter decomposition, we lack an understanding of the commonalities that may exist between aquatic and terrestrial ecosystems, and whether those drivers operate similarly under different climates such as tropical or Mediterranean.

We analyzed data from an existing global litter decomposition experiment conducted in two ecosystems (forest floors and streams) across five climates (tropical, Mediterranean, temperate, subarctic and boreal). Litter from 15 different tree species combinations, originating from each climate, was incubated in the field inside custom-made mesh microcosms. The litter-filled microcosms were placed in direct contact with the soil surface (terrestrial) or submerged (aquatic) to simulate natural conditions. Three different mesh-sizes were used to limit the access of different components of the decomposer community to the litter inside the microcosms: fine (microbes), intermediate (microbes + mesofauna) and coarse (microbes + mesofauna + macrofauna). By doing so, we could test the contribution of different decomposer communities to the decomposition process.

The most important result we found is that many commonalities exist between forest floors and streams at a global scale. For example, the initial litter concentration of little-studied micronutrients such as magnesium and calcium played a major role in litter decomposition in both ecosystems. In addition, the relative importance of decomposer organisms for litter decomposition was larger in streams than in forest floors.

Our results can be used to improve global models developed to predict the effects of climate change on the carbon cycle. A useful approach would be to incorporate the global similarities found between aquatic and terrestrial ecosystems.

Image caption: Leaf litter layer in an Amazonian rainforest with fungi as key decomposer organisms. Photo credit: Stephan Hättenschwiler.
You can read the article in full here.

 

Cold hardiness does not limit range shifts of Mediterranean pines to Central and Western Europe

Christoph Bachofen, Thomas Wohlgemuth, Jaboury Ghazoul and Barbara MoserPinus nigra seedlings growing in the common garden in the Central Alps. Photograph by Christoph Bachofen.

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Mediterranean tree species are adapted to long dry periods during summer, thus they might be well adapted to drought spells, which are projected to increase in Central Europe if climatic conditions become warmer and dryer. Frost, on the other hand, is an important factor limiting the distribution ranges of many tree species. It causes damage to buds and needles during winter and spring. Species distribution models predicting latitudinal range shifts might underestimate the risk from freezing damage, because they rely on average climate data and knowledge of the loss of cold hardiness from winter to spring, called dehardening, is sparse for many tree species.

We estimated the vulnerability of Mediterranean pines to winter and spring frost, and thus their potential to migrate northwards, by comparing the cold hardiness of Mediterranean populations of Pinus sylvestris (Scots pine), Pinus nigra (black pine) and Pinus halepensis (Aleppo pine) to Central Alpine populations of P. sylvestris. Seedlings were grown in a common garden in a cold Central Alpine valley. We artificially froze needles of every species and population at several temperatures on seven dates between February and July 2013. Cold hardiness was determined by estimating the temperature at which the majority of the needles were injured. Needle injury was measured in terms of the amount of electrolytes leaking from the ruptured needle cells.

We found no difference between the cold hardiness of Mediterranean P. sylvestris and P. nigra and Central Alpine P. sylvestris. The cold hardiness was not related to the temperature at the seed origin of the populations. All populations of P. sylvestris and P. nigra maintained extremely wide safety margins against frost, which exceed that of deciduous trees considerably. P. halepensis, on the other hand, exhibited a very narrow safety margin until late spring and its cold hardiness was in the range of temperatures that regularly recur in Central Europe in winter.

Our study shows that Mediterranean P. sylvestris and P. nigra populations are equally well adapted to the late winters and spring frosts of Central Europe as P. sylvestris from the Central Alps. P. halepensis might be resistant to winter temperatures occurring in the Atlantic region of Europe but not to current frost events in Central Europe.

Image caption: Pinus nigra seedlings growing in the common garden in the Central Alps. Photograph by Christoph Bachofen.
You can read the article in full here.

 

Additive genetic variance and effects of inbreeding, sex and age on heterophil to lymphocyte ratio in song sparrows

Sylvain Losdat, Peter Arcese, Laura Sampson, Nacho Villar, Jane M. ReidSong Sparrow. Image provided by authors.

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An individual’s ability to survive and reproduce, defined as individual fitness, can be predicted and influenced by physiological traits such as the amount of stress the individual experiences. As a consequence, individual stress has been hypothesized to influence the evolution of life-history traits and strategies. However, the hypothesis that variation in physiological traits could influence life-history evolution requires that such traits can be genetically inherited. Yet, the genetic and environmental determinants of individual physiological traits linked to stress remain relatively unknown, particularly in vertebrates under natural environmental conditions.

We used long-term data from a study on wild song sparrows in Western Canada to quantify genetic variation in the ratio between heterophils and lymphocytes, which are blood cells with an immune function. The heterophil to lymphocyte (H:L) ratio is a physiological metric that reflects individual stress. We collected and analysed blood samples from 350 fully developed individuals for which we have a complete genetic pedigree. We subsequently estimated the effect of individual coefficient of inbreeding on H:L ratio to test whether being inbred was influencing individual stress. We also tested the effect of sampling period, age and sex. Last, we estimated the magnitude of genetic inheritance of H:L ratio using the genetic pedigree.

Overall, H:L ratio increased with individual coefficient of inbreeding, indicating that more inbred individuals have higher stress levels. This implies that individuals that mate with closer relatives could produce offspring with relatively high individual stress, which might in turn impact their offspring’s survival and reproductive success. Additionally, H:L ratio was higher in older individuals, but did not differ between males and females.

The estimated genetic inheritance of H:L ratio was small (h2=0.04) and did not differ significantly from zero, suggesting that individual stress is primarily influenced by environmental factors rather that being genetically inherited from parents. This low genetic inheritance suggests that individual stress has low potential to evolve because any advantage will not be transmitted to the new generation.

Image caption: Song Sparrow. Image provided by authors.
You can read the article in full here.

 

Fewer new species colonize at low frequency N addition in a temperate grassland

Yunhai Zhang, Carly J. Stevens, Xiaotao Lü, Nianpeng He, Jianhui Huang, Xingguo HanYunhai Zhang worked in the field for simulation wet nitrogen deposition.

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Increasing availability of biologically-active nitrogen (N), from either human fertilization or atmospheric deposition, has caused biodiversity loss in diverse ecosystems . Based on a field study that independently manipulated the rates and frequencies of N inputs in a temperate grassland in Inner Mongolia, China (photographed by Wei Liu), we were able to test the responses of ecological processes to the frequency and rate of N addition. Because of the different frequencies of N added to ecosystems between fertilizer application and atmospheric deposition, traditional N addition experiments may have overestimated the effects of atmospheric N deposition on biodiversity loss, as it has been found that low frequency N additions, as used in traditional studies, lead to more rapid biodiversity loss. It remains unclear, however, whether new species colonization (gain) or old species extinction (loss) is the cause of such differences.

In this study we explore both processes and demonstrate that there are changes in composition masked by considering species richness alone. We found that the gain of new species was higher at a high frequency of N addition than at a low addition frequency, whilst loss of existing species was similar at the two frequencies of N addition. The number of species gained decreased and species lost increased with increasing rate of N addition at both annual and five-year intervals. Cumulative gain of new species was negatively correlated with soil acidification, ammonium concentration and community biomass accumulation, whereas cumulative loss of old species was positively correlated with these variables.

Our findings indicate lower new species gain rather than higher old species loss accounted for the lower species richness at low frequency of N addition. Given the lower new species colonization at low frequency of N addition, previous N fertilization experiments may have overestimated the effects of N deposition on biodiversity loss via understatement of the capacity for new species colonization. Moreover, our study suggests that practices that prevent soil acidification and ammonium accumulation, or remove biomass (such as grazing or mowing), may decrease the rate of biodiversity loss under N enrichment via fertilization or atmospheric deposition.

Image caption: Yunhai Zhang worked in the field for simulation wet nitrogen deposition.
You can read the article in full here.

 

Environmental cue to germinate in gaps is closely associated with seed sizes

Qingqing Xia, Mariko Ando and Kenji SeiwaSeed germination experiment in a plant growth chamber in which we provided a R: FR ratio by stick-type light-emitting diode lamps.

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It is well known that seeds of pioneer species use either temperature fluctuations or spectral composition of light as gap detection signals. The relative importance of these two signals, however, remains unknown due to the lack of germination experiments testing the interactive effects of the different signals. We hypothesized that the relative importance of these two factors differs among species with different seed sizes, because the environmental signals may change with soil depth in different manner.

We monitored changes in these environmental signals at different soil and litter depths in forest gaps and under adjacent forest understorey. We also conducted a seed germination experiment in darkness and under red to far-red (R: FR) light ratios of 0.1, 0.4, and 1.0 under fluctuating (30/10 °C day/night) and constant (20 °C) temperatures using 10 pioneer tree species with different seed sizes.

In forest gaps, light is a germination cue that is detectable only at the soil surface or immediately below it when litter is absent, whereas temperature signals can be detected in deeper soil. The proportion of seeds germinating in small-seeded species (seed mass ≤1.05 mg) was strongly promoted by higher R: FR ratios, thereby ensuring germination of seeds buried close to the soil or litter surface. In contrast, percent germination of large-seeded species (seed mass ≥200 mg) was strongly facilitated by temperature fluctuations, ensuring germination of seeds even if buried in deeper soil and litter. In three medium-seeded species (2.84–9.50 mg), interactive effects of the two signals were observed.

Our results suggest that in temperate pioneer trees, differences in seed size across species were associated with differences in the relative importance of light vs. temperature regimes as germination cues for gap detection.

Image caption: Seed germination experiment in a plant growth chamber in which we provided a R: FR ratio by stick-type light-emitting diode lamps.
You can read the article in full here.

 

Dung beetles reduce drought stress in plants without increasing plant susceptibility to an aboveground herbivore

Scott N. Johnson, Goran Lopaticki, Kirk Barnett, Sarah L. Facey, Jeff R. Powell and Susan E. Hartley Rain exclusion shelters at the DRI-GRASS research platform.

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It’s reckoned that insects provide well in excess of USD $500 billion of ecosystem services to mankind every year in the form of pollination and pest control. Some of the most important providers, however, have the less glamorous role of consuming animal faeces – the dung beetles. There are over 6,000 species of dung beetle living on all continents except Antarctica. These beetles provide ecological functions and services by virtue of their physical manipulation of the ecosystem, making them miniature ecosystem engineers. In addition to consuming animal dung, these engineers disperse and incorporate it into the soil via burrowing activity. This drives a number of ecological processes including nutrient cycling, soil aeration, seed burial and parasite suppression. This is estimated to contribute an amazing USD $380 million in ecosystem services to the US cattle industry alone. We were interested in whether these ecosystem services might help mitigate the negative impacts of climate change, particularly altered rainfall patterns, on plant growth and productivity.

Using rain exclusion shelters in SE Australia, we tested how dung beetles (Bubas bison) might alleviate the negative impacts of drought on plants (Brassica oleracea) by enhancing soil water retention. Provisioning plants with resources, however, might make them more nutritious for herbivores and potentially cancel out any benefits that dung beetles conferred, so we also tested whether plant suitability to a herbivorous pest (Plutella xylostella) changed when dung beetles were present. We found that dung beetles increased soil water retention by 10% and increased plant growth by almost three fold under drought conditions. Dung beetles also allowed plants to capture more nutrients such as nitrogen and carbon. Contrary to our predictions, however, dung beetles did not make the plants more susceptible to the herbivore, potentially because they increased nutrient content but not the concentration in leaves. These results point to a potential beneficial role for insect ecosystem engineers in climate change adaptation and crop protection.

Image caption: Rain exclusion shelters at the DRI-GRASS research platform.
You can read the article in full here.

 

Seasonal changes in temperature and bush lupine availability drive bordered plant bug abundance

Christopher A. Johnson, Renato M. Coutinho, Erin Berlin, Kimberly E. Dolphin, Johanna Heyer, Britney Kim, Alice Leung, Jamie Lou Sabellon, and Priyanga Amarasekare Left photo: Our field site in Santa Barbara County, CA, with bush lupine. Right photo: adult bordered plant bugs on bush lupine in the laboratory; note the yellow egg cluster on the bottom-right and adults preparing to mate on the top-left. Photo credits: Christopher A. Johnson

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Studying species’ population dynamics (how population abundances change over time) is important for understanding how species persist in variable environments as well as predicting which species may be vulnerable to extinction due to climate change. Both biological factors, such as food availability, and non-biological factors, such as temperature, may be important drivers of population dynamics.

Mathematical models are useful tools for ecologists to study population dynamics because models can simultaneously consider the effects of multiple factors and generate predictions that can be tested using experiments or empirical data. Models are powerful tools because they can investigate which processes are likely to be important and make predictions that may be otherwise impossible. It is critical to develop models using real-world “study systems”, which guide and validate models.

We noticed a striking pattern in the abundance of the bordered plant bug in coastal Santa Barbara Co., California (left photo): juveniles and adults are completely absent during certain times of the year and extremely abundant during others. Because plant bugs are ectotherms (species that depend on external heat sources) and primarily eat bush lupine, which drops its leaves in winter, we hypothesized that plant bug population dynamics are driven by seasonal changes in temperature and food availability.

To investigate how plant bug populations respond to changes in temperature and food availability, we raised plant bugs in the laboratory (right photo) at four temperatures and measured their birth, development, and death rates. We then developed a model using the laboratory data, and then varied temperature and food availability as observed in nature. The model predicts similar population dynamics to our field data only when it includes both temperature and food availability; thus, plant bug population dynamics are likely driven by both seasonal changes in temperature and bush lupine availability.

More broadly, this study is important because it provides a mathematical framework, validated using the bordered plant bug, that can investigate ectotherm population dynamics and predict which species may be vulnerable to extinction due to atypical changes in temperature or food sources under climate change.

Image caption: Left photo: Our field site in Santa Barbara County, CA, with bush lupine. Right photo: adult bordered plant bugs on bush lupine in the laboratory; note the yellow egg cluster on the bottom-right and adults preparing to mate on the top-left. Photo credits: Christopher A. Johnson.
You can read the article in full here.

 

Liverworts to the rescue: an investigation of their efficacy as mycorrhizal inoculum for vascular plants

Jill Kowal, Silvia Pressel, Jeffrey G. Duckett, and Martin I. Bidartondo Septate hyphae emanating from a microscopic leafy liverwort rhizoid following resynthesis with the ericoid mycorrhizal fungus isolate.

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Healthy plant life aboveground is largely dependent on the function of belowground fungal communities. Mycorrhizal fungi are ubiquitous in the soil, forming mutualistic plant-fungal interactions in all terrestrial ecosystems. Mycorrhizal fungi are not transmitted directly from parent to offspring plants, so mycorrhizal mutualisms must form de novo when plants establish from seed.

A widespread mycorrhiza-forming fungus (Pezoloma ericae (D.J. Read) Baral) associates with plants in the order Ericales and also forms mutualistic associations with several families of liverworts, many of which are also widespread throughout the world. The ecological significance of this ‘shared’ mycorrhizal partnership remains largely unknown. The Ericales number approximately 8,000 species and include familiar plants such as heathers, blueberries and cranberries.

We investigated the functional associations among ericaceous plants, liverworts and mycorrhizal fungi in wet lowland heathland dominated by heathers, a top priority habitat for conservation. The research aimed to determine whether pioneering liverwort plants harbouring P. ericae can act as inoculum for the re-establishment of heather plants and can therefore be proposed as a practical application in heathland restoration ecology. The process of becoming mycorrhizal can be limiting to plant establishment in ecosystems poor in nutrients, such as heathlands.

Our molecular analyses confirmed that the liverworts and heather (Erica tetralix and Calluna vulgaris) share the same mycorrhizal fungus, P. ericae. We cultured liverworts from spores, and introduced the P. ericae isolate. Within weeks, the liverworts were colonized by the fungus, as found in nature. These liverworts were then planted together with heathers (both seedlings and cuttings) in field-simulating conditions, to assess the functional role of the colonized liverworts on the heathland plants. Germination, rooting success, plant survival, and growth were measured after several weeks and months.

Our research showed, for the first time through molecular identification, that British species of leafy liverworts harbour a fungus that forms mycorrhizas with vascular plants. We unequivocally demonstrated that these fungal symbionts emanate from leafy liverwort rhizoids and repeatedly colonize roots forming typical "ericoid mycorrhizas" in seedlings and cuttings. Plant establishment is significantly more successful when plants are co-planted with liverworts containing these fungi. Growth is significantly increased as well, but can be initially suppressed – there may be an initial cost to partnering with fungi.

The liverworts tested are adaptable to considerable swings in temperature and moisture levels in nature, and are relatively easy to cultivate and apply for inoculation treatments. We therefore conclude these liverworts are a viable inoculum delivery mechanism for heather establishment, and preferable to a slurry of the fungus isolate or other Ericaceae plants as they are more cost effective and practical on a large scale. We plan to test this inoculum in nature now that we have established its efficacy in our experiments.

Image caption: Septate hyphae emanating from a microscopic leafy liverwort rhizoid following resynthesis with the ericoid mycorrhizal fungus isolate.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

Old birds become worn and rusty too

Katherine A. Herborn, Francis Daunt, Britt J. Heidinger, Hanna M. V. Granroth-Wilding, Sarah J. Burthe, Mark A. Newell and Pat MonaghanFamily of Isle of May shags, Phalacrocorx aristotelis. ©Lucie Bernardova.

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While we know that our own bodies deteriorate as we get older, relatively little is known about these processes in wild animal populations. Much of what we know about aging in other species comes from research on species with relatively short lifespans, such as insects and rodents, which can be studied from birth to death in laboratories. To determine whether the same patterns of aging occur in long lived organisms requires long term programmes and long scientific careers. Moreover, understanding aging under natural conditions, instead of in the relatively homogenous and safe laboratory environment, presents the additional challenge of finding the same animal at different stages of its life in the wild. One reason why individuals deteriorate with age is thought to be akin to rusting – that is accumulated oxidative damage to tissues. The term ‘oxidative stress’ describes a state where oxidising molecules, which are mostly a natural by-product of metabolism, exceed the body’s level of antioxidant defences, and thus are free to react with and damage body tissue. We examined age-related changes in oxidative stress exposure in the Isle of May population of a long-lived seabird, the European Shag (Phalacrocorax aristotelis). Chicks were given unique coloured and lettered leg rings, allowing them to be recognised throughout their lives. Over three consecutive breeding seasons, we collected tiny blood samples from individuals aged from 2 to 22 years at the start of the study. Individuals of all ages showed an increase in oxidative stress exposure from one year to the next, and those with the highest levels were more likely to die. This is consistent with the expectation that older individuals will invest less energy and resources into self-maintenance in favour of more investment into reproduction, as a last ditch attempt at passing on their genes. Our study, therefore, provides a rare insight into the process of aging in a long-lived species; they become rusty much like ourselves.

Image caption: Family of Isle of May shags, Phalacrocorx aristotelis. ©Lucie Bernardova.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

Functional replacement across species pools of vertebrate scavengers separated at a continental scale maintains an ecosystem function

Chantal M. Huijbers, Thomas A. Schlacher, Rosemary R. McVeigh, David S. Schoeman, Andrew D. Olds, Marion B. Brown, Kasun B. Ekanayake, Michael A. Weston and Rod M. Connolly Removal of fish carcasses from ocean beaches, a key ecological function, is maintained across a continental scale via functional species replacement: scavenger assemblages on tropical beaches were dominated by raptors, such as white-bellied sea eagles (left image), and substantially different from temperate scavenger assemblages, which were dominated by red foxes (right image). Photographs taken while scavengers were actively removing beach carrion during the experiments. Credits: C. Huijbers.

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The composition of the total variety of species available to colonise a particular site (the ‘species pool’), and those that actually occur together (the local assemblage), often show large geographical variation, reflected in distinct ‘faunas’ and ‘bio-regions’ across the globe. Whilst such structural differences are often prominent, much less is known about whether these geographic patterns in species identity change the functioning of ecosystems.

Scavenging, the consumption of animal carcasses, is an important ecological function. We tested how differences in the composition of local scavenger assemblages affect the removal rates of carrion (dead fish) from sandy beaches. We deployed fish carcasses on beaches in tropical north Australia and temperate south Australia and used motion-triggered cameras to monitor which animals scavenge on the carcasses.

Tropical assemblages of vertebrate beach scavengers were dominated by raptors such as brahminy kites and white-bellied sea eagles, while carrion on temperate beaches was mostly removed by invasive red foxes. Despite the distinct differences in terms of which species make up the scavenger assemblages, the rates at which carrion was removed from beaches was similar between regions separated by thousands of kilometres. This indicates that two very distinct species assemblages can maintain a pivotal ecological process via functional replacement of species.

Image caption: Removal of fish carcasses from ocean beaches, a key ecological function, is maintained across a continental scale via functional species replacement: scavenger assemblages on tropical beaches were dominated by raptors, such as white-bellied sea eagles (left image), and substantially different from temperate scavenger assemblages, which were dominated by red foxes (right image). Photographs taken while scavengers were actively removing beach carrion during the experiments. Credits: C. Huijbers.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

Re-growing a tropical dry forest: functional plant trait composition and community assembly during succession

Vanessa Buzzard, Catherine M. Hulshof, Trevor Birt, Cyrille Violle and Brian J. EnquistRecently disturbed TDF in Sector Santa Rosa of Area de Conservación Guanacaste (ACG) in northwestern Costa Rica. Photo credited to Catherine Hulshof.

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Most of the world’s remaining tropical forests have experienced some degree of disturbance. Tropical dry forests (TDFs), which are characterized by seasonal drought, currently occupy 42% of all forested tropical regions throughout the world but today less than 2% of TDFs throughout Central America remain undisturbed. The primary conversion of this forest type is a result of a long history of human land use. In order to restore these tropical ecosystems, ecological studies are needed to help understand how the structure and function of these forests change over time.

Ecological theory suggests that the environmental conditions following disturbance are stressful. Therefore, directly following a disturbance, communities are likely to be characterized by drier and more variable soil moisture, higher light levels, increased temperatures, and lower relative humidity acting as a filter where only a few species can successfully establish. Over time, as environmental conditions improve, more species establish and begin to compete with one another for resources resulting in greater differences between species. This shift in resource availability and turnover in species composition through time has been the primary focus of successional ecology for many years, resulting in little understanding of the change in function over time. Therefore, in this study we take a more mechanistic approach to assess community function over time by measuring physiological and chemical traits of leaves that are related to the success of an individual (functional traits) for woody plants within 14 plots that have varying times since disturbance.

When we compare species composition and community function, we find that older TDF communities differed significantly from younger forests in species composition, above ground biomass and functional traits. The observed trait shifts appear to be primarily determined by a concomitant shift in resource availability, e.g., water availability (reflected in increased soil moisture) and light availability. Further, early stages of succession were uniformly characterized by traits related to strategies important for avoiding water loss, whereas in later successional stages traits reflected increased water and nutrient availability. Interestingly, in contrast to expectations, functional trait variation did not generally change through succession. Our results suggest that regenerating TDFs are resilient and can be restored within a human lifetime.

Image caption: Recently disturbed TDF in Sector Santa Rosa of Area de Conservación Guanacaste (ACG) in northwestern Costa Rica. Photo credited to Catherine Hulshof.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

Visual ecology of Eulemur suggests a cathemeral origin for the primate cone opsin polymorphism

Kim Valenta, Melissa Edwards, Radoniaina R. Rafaliarison, Steig E. Johnson, Sheila M. Holmes, Kevin A. Brown, Nathaniel J. Dominy, Shawn M. Lehman, Esteban J. Parra, Amanda D. Melin A brown lemur (Eulemur fulvus) searches for fruits under daylight conditions in Ankarafantsika National Park, Madagascar. Photo credit: Travis Steffens.

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Unlike most primates, which are predominantly active during either the day or night, lemurs in the genus Eulemur are active across the 24-hour daily cycle. Although debated, recent evidence suggests that this activity pattern has a long evolutionary history for these lemurs, and was possessed by their ancestors. The impact of nighttime vs. daytime activity on the evolution of color vision in primates is actively debated and we explore the visual ecology of lemurs that are active at both times to shed new light on this debate. We studied three groups of wild brown lemurs (Eulemur fulvus) in Ankarafantsika National Park, Madagascar and collected stool samples for DNA analysis to study the opsin genes underlying color vision. We measured the color and brightness of dietary fruits and modeled the conspicuousness of food objects to different color vision types under daylight, twilight, and moonlight conditions. We found the color vision of E. fulvus to be routinely dichromatic – all males and females had the same color vision, consisting of two color channels. Our models suggest that dichromacy is well-suited to the foraging ecology of this species. The color vision of E. fulvus differs from its close relative E. flavifrons, for which polymorphic trichromacy – the addition of a third (red-green) color channel in some females - has been reported. We suggest that ecological differences between species of Eulemur could reveal thresholds for the origins of polymorphic trichromacy, which preceded the evolution of routine trichromatic vision in humans and other primates.

Image caption: A brown lemur (Eulemur fulvus) searches for fruits under daylight conditions in Ankarafantsika National Park, Madagascar. Photo credit: Travis Steffens.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

Food as fuel: How food protein-carbohydrate content affects resting metabolic rates

Rebecca M. Clark, Anthony J. Zera, and Spencer T. BehmerFemale cricket housed in a flow-through respirometry chamber used to measure carbon dioxide production and oxygen consumption.  Photo by S.T. Behmer.

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Metabolic rate is a fundamental physiological characteristic of animals that summarizes overall energy usage by an organism. Because of its central role in organismal physiology, metabolic rate is widely thought to be influenced by external and internal factors, especially food nutrient content and physiological state (e.g. reproductive status). However, despite decades of study in the context of nutritional ecology and life-history evolution, the effects of food nutrient content and animal reproductive status on resting metabolic rates are poorly understood.

We tested how the balance and concentration of protein and carbohydrate in food affected resting metabolic rate in females of the sand cricket, Gryllus firmus. Females of this species occur in two physiologically distinct forms (morphs) that specialize in either flight ability or egg production. Because these forms differ dramatically in many aspects of fat and amino acid metabolism related to production of fat flight fuel and eggs, we also expected them to differ in metabolic rate. Surprisingly, we found that the dramatic differences in organismal biochemistry did not lead to differences in resting metabolic rates between the morphs. Likewise, resting metabolism was similar across diets, even though the diets had different protein-carbohydrate profiles. The maintenance of constant resting metabolic rate on different diets occurred because crickets shifted the nutrients that they used as metabolic “fuel,” to compensate for changes in the amount and ratio of protein and carbohydrate in their diet.

Therefore, counter to expectations, individuals can experience dramatic variation in nutrient input and have vastly different physiological characteristics, without showing any signs of these major distinctions at the level of whole-body resting metabolic rate. Conversely, the absence of differences among individuals in resting metabolic rate does not necessarily mean that those individuals do not differ dramatically in underlying aspects of biochemical function. Physiological studies need to be undertaken both at the whole-organism and biochemical levels to identify adaptive biochemical differences among individuals and to determine the extent to which these differences give rise to differences in overall energy usage.

Image caption: Female cricket housed in a flow-through respirometry chamber used to measure carbon dioxide production and oxygen consumption. Photo by S.T. Behmer.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

Does morphology matter in sexual deception?

Marinus L. de Jager and Rod PeakallThe wasp pollinator, Neozeleboria cryptoides, exhibiting the Forward orientation during attempted copulation with the orchid Chiloglottis trapeziformis, by Rod Peakall.

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Orchids exhibit some of the most ingenious modes of pollination, and often resort to deceptive ploys. About a third of all orchids produce no floral rewards and deceive their pollinators by mimicking the signals of other rewarding flowers. The orchids’ most remarkable floral deception, however, must be the sexual mimicry of their own pollinator’s females. Male insects searching for mates are attracted to, and even attempt to copulate with, these intriguing faux female flowers. Previous studies have revealed that this interaction is largely driven by the flower’s scent mimicry of female insects’ sex pheromones, which males of the same species are highly sensitive and responsive to.

But what about floral morphology? Sexually deceptive orchids typically have unusual and complex flowers that look rather insect-like. If only accurate scent mimicry of female sex pheromones is required for pollination, these morphologically complex flowers pose an evolutionary mystery. One hypothesis is that flowers are also acting as morphological mimics of the pollinator’s females, each species resembling the female of their specific pollinator. Another hypothesis is that flower morphology is under selection via mechanical fit to the pollinating male’s morphology, to ensure that pollen is successfully transferred during copulation attempts.

Our study focussed on two morphologically divergent Chiloglottis orchids that employ the exact same scent compound for attracting their respective pollinators. This allowed us to explore the importance of floral morphology in the absence of scent differences. The pollinators of these Australian orchids belong to the wasp genus Neozeleboria, which exhibit strong sexual dimorphism in size between male and female wasps. This allowed us to make distinct predictions for floral morphological adaptation to a) the small wingless females via female mimicry, or b) the large winged males via mechanical fit, and thus distinguish between these two hypotheses. We reveal that floral morphology is shaped by both morphological mimicry of the pollinators’ females, and mechanical fit to the pollinating males. We conclude that floral morphology is indeed important in the pollination of sexually deceptive orchids, and that selection by pollinators on this largely overlooked component of sexual deception may be more common than previously thought.

Image caption: The wasp pollinator, Neozeleboria cryptoides, exhibiting the Forward orientation during attempted copulation with the orchid Chiloglottis trapeziformis, by Rod Peakall.
You read the article in full here here.

 

Trade-offs in juvenile growth potential vs. shade tolerance among subtropical rainforest trees on soils of contrasting fertility

Kerrie M. Sendall, Christopher H. Lusk and Peter B. ReichPhoto provided by authors.

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Coexistence of tree species is often associated with a trade-off between juvenile shade tolerance and growth rate in gaps. On the other hand, species sorting along/across soil fertility gradients is thought to involve trade-offs between the nutrient and carbon economies of plants. Little is known, however, about how these two types of trade-off interact. We asked if the growth versus shade tolerance trade-off in Australian subtropical rainforest is influenced by soil phosphorus (P) availability, and if similar trade-offs occur on P-rich basalt and low-P rhyolite.

We measured growth of juveniles (400-1200 mm tall) of 15 common tree species across a wide range of light intensity in an Australian subtropical rainforest. Nine common species were measured on the basaltic soil and seven on the rhyolitic soil; one species occurred on both soils. We used hemispherical photographs to quantify light environments above each plant. We then used relationships of growth with light to compare growth of each species at 3, 6, 12, and 20% light availability, and to calculate two measures of species’ light requirements: (a) the light compensation point (the light level at which stem growth averages zero) and (b) the minimum light requirement (the bottom 5% of the range of light environments occupied by each species).

Growth in 3 and 6% light was negatively related to minimum light requirements, but positively related at 20% light. Similar growth versus shade tolerance trade-offs were found among species adapted to the two different substrates. However, the species growing on P-rich basalt encompassed a wider range of light compensation points and growth rates than species growing on low-P rhyolite.

Our results suggest that differential adaptation to soil P availability has little effect on the relationship between shade tolerance and maximum growth rates. However, they do suggest an effect on functional diversity: both the fastest-growing species and the most shade-tolerant species are limited to P-rich soils.

Image caption: Photo provided by authors.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

Colour pattern mimicry in flowers- the functional importance of complex floral colour pattern in a food-deceptive orchid

Xiaokai Ma, Jun Shi, Hans Banziger, Yangna Sun, Yanyan Guo, Zhongjian Liu, Steven D. Johnson and Yibo Luo Mimic (silver slipper orchid Paphiopedilum micranthum, centre) and its co-occurring food flowers on a background of their habitat, framed in bee hexagonal vision.  Image courtesy of Xiaokai Ma.

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Mimicry, where one species evolves to resemble another, unrelated species, is a renowned textbook example of natural selection and adaptation. Mimicry is often based on imitation of special visual patterns, such as the gorgeous colour patterns on wings of butterflies and body surface of snakes. Flower mimics, which are widespread among plants, often dupe animal pollinators by providing no reward (i.e. nectar), but imitating the colour of nectar-providing flowers that share the same habitat. Moreover, both the mimics and the food flowers often display complex colour patterns, including contrast between outer and central flower parts, that could be discriminated by pollinators.

To detect whether the colour pattern is the key to flower mimicry, we tested the effects of floral colour patterns in the silver slipper orchid, Paphiopedilum micranthum, on pollinator bumblebee choices and pollination success using behavioural tests in a community context. Using a bee vision model and evolutionary analysis, we also compared the colour patterns of the orchid with those of its relatives, and the food flowers that it mimics.

Our results reveal that orchid pollination success might be enhanced by colour patterns that mimic those of food flowers in the local community. Such mimicry enhances attraction of the orchid to bumblebees. Experimental disruption of the colour pattern clearly hampers decision-making by bumblebees, and this decreases pollination success of the orchid. Evolutionary analysis indicate that the colour pattern of orchid mimics might have been shaped by multiple evolutionary histories, including evolutionary innovation of the outer floral colour and a pre-adaptation of the ancestral centre floral colour.

Our study is novel in its inclusion of colour pattern in the study of floral mimicry in the context of the associated plant community, and provides the first detailed evidence for the functional importance of colour pattern mimicry for a deceptive flower. This finding advances our understanding of the evolutionary processes governing floral deception via the functional significance of colour pattern traits that influence signalling efficacy. This highlights the importance of complex signals in facilitating species interactions.

Image caption: Mimic (silver slipper orchid Paphiopedilum micranthum, centre) and its co-occurring food flowers on a background of their habitat, framed in bee hexagonal vision. Image courtesy of Xiaokai Ma.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

Special Feature: The functional role of silicon in plant biology

Molecular evolution of aquaporins and silicon influx in plants

Rupesh Deshmukh and Richard R. BélangerHorsetail plant (Equisetum arvense) in the field (left). Scanning electron micrograph of horsetail leaf (top right) and X-ray microanalysis mapping of silicon presence (bottom right).

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Silicon (Si) is one of the most abundant elements in the earth crust, but whether it is essential for plant growth remains a matter of debate. Plants take up Si through the roots in the form of silicic acid, and can accumulate concentrations as high as 10% on a dry weight basis. Nevertheless, most plants (with the notable exception of horsetail) can complete their life cycle without Si. For this reason, Si is not considered an essential element, in spite of the multiple studies that have shown its beneficial role for plants, especially under conditions of biotic and abiotic stress.

The benefits plants derive from Si are well correlated with their ability to take up Si from the soil, and this ability varies greatly among plant species. In the context of better defining the ecological role of Si in plants, it thus becomes very important to understand which and how plant species can take up silicon.

Si uptake in plants depends on two specific proteins, an influx transporter and an efflux transporter, both with unique characteristics. Recent studies suggest that the presence of an influx transporter is the indispensable key for a plant to be able to absorb Si. Based on DNA sequence analyses and comparisons, influx transporters appear to bear conserved features that allow us to classify plant species as Si-competent or not. While it is unclear how and why plants have acquired or lost this trait, genomic data now offer a reliable molecular tool to predict with accuracy which plant species are predisposed to benefit from Si. This work presents a detailed review of the molecular features inherent to Si influx in plants, a property that has a profound impact on Si biogeochemical cycling and the role of Si in many fundamental aspects of ecology.

Image caption: Horsetail plant (Equisetum arvense) in the field (left). Scanning electron micrograph of horsetail leaf (top right) and X-ray microanalysis mapping of silicon presence (bottom right).
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

How do precipitation gradient and evolutionary history shape the variations in leaf and root traits in the Inner Mongolia grassland?

Junhui Cheng, Pengfei Chu, Dima Chen and Yongfei BaiRoot systems of six grasses widely distributed in the Inner Mongolia grassland.

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The attributes of leaves and roots are fundamental for understanding different strategies that plants use to acquire resources, like water, soil nutrients, and light, in different environmental conditions. This is important because these resource acquisition strategies are essential for species coexistence, community assembly and functioning of terrestrial ecosystems. Species with less dry matter investment per leaf area and root length have high growth rate but short life span, indicating a resource acquisitive strategy. In contrast, species with high dry matter investment per leaf area and root length have low growth rate but long life span, suggesting a resource conservative strategy. It has been assumed that variation in environmental factors (e.g. precipitation and temperature) and evolutionary history (species divergence time on the evolutionary tree) shape both leaf and root traits. The branching architecture of fine roots also affects the functional role of living roots, such as acquisition and transport of water and nutrients. However, few studies have examined the effects of evolutionary history and environmental gradients on species leaf and root traits, or considered how plants are able to access increasingly scarce resources like water and nutrients as climate shifts from relative moist to very arid?

In this study, we examine how leaf and root traits vary across 55 species and 21 plant communities along a 2,000-km transect in the Inner Mongolia grassland, part of the largest contiguous grassland in the world, the Eurasia steppe. Our results suggest that acquisition of soil resources, such as water and nutrients, is a fundamental determinant of plant dry mass investment per root length in arid and semiarid grasslands. Across the transect, species that evolved relatively late tended to have high dry matter investment per leaf area and root length for the first root branching order, and were distributed in dry and infertile habitats, while earlier-evolving species exhibited low dry matter investment per leaf area and root length and were found in wet and fertile habitats. These findings provide new insights for predicting the response of species and ecosystems to changes in temperature and precipitation in arid and semiarid grasslands.

Image caption: Root systems of six grasses widely distributed in the Inner Mongolia grassland.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

Cold adaptation in insects involves adaptive modifications of the cell membrane phospholipid composition

Stine Slotsbo, Jesper G. Sørensen, Martin Holmstrup, Vladimir Kostal, Vanessa Kellermann and Johannes OvergaardThis photo shows different fruit flies along a cold tolerance gradient, and how the phospholipid fatty acids in the membrane are expected to become more unsaturated with increasing cold tolerance. Photo provided by Heath A. MacMillan.

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Maintaining function and structural integrity of cell membranes is central for the performance and survival of animals. Because temperature influences the physical properties and fluidity of animals’ biological membranes it is hypothesized that temperature stress may compromise membrane function. In accordance with this hypothesis it is often found that acquisition of thermal tolerance following acclimation involves modifications of the cell membrane lipid composition. Each membrane phospholipid consists of a head group attached to two fatty acids, and modification can be in type of head group, types of fatty acids, or a combination of these factors. Membrane modifications are associated with changes in the membranes' physical properties such that appropriate function is secured at the acclimation temperature. Parallel differences in membrane lipid composition have been found between species and between populations of insects that are adapted to different thermal environments. However, there are few large multi-species studies that investigate directly the interspecific association between membrane lipid composition and thermal tolerance in insects.

Here we present the most comprehensive interspecific investigation of phospholipid fatty acids composition within insects (and cold-blooded animals in general). Using 57 species of fruit flies originating from both temperate, subtropical and tropical environments we found clear differences between species in phospholipid fatty acid composition. When we related these differences to species’ cold tolerance, which constitute a powerful proxy for their cold distribution limits, we found that phospholipid fatty acids could explain more than 20 % of the variation in cold tolerance found between species. Considering that a number of other important membrane parameters (phospholipid head group composition and concentration of sterols and other lipid components) were not measured, our finding highlights that phospholipid fatty acids and membrane adaptations in general play a highly significant role in thermal tolerance in insects.

Image caption: This photo shows different fruit flies along a cold tolerance gradient, and how the phospholipid fatty acids in the membrane are expected to become more unsaturated with increasing cold tolerance. Photo provided by Heath A. MacMillan.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

Macronutrients, weapons and genital traits in male broad horned beetles

Clarissa M. House, Kim Jensen, James Rapkin, Sarah Lane, Kensuke Okada, David J. Hosken and John Hunt Male Gnatocerus cornutus fight using an enlarged mandible which is a secondary sexual trait. Photo credit: Matthew Silk.

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The calorific content and the nutritional composition of food that is acquired from the environment is likely to be vitally important for individual survival, growth and morphology (i.e. the physical characteristics of an animal). This could be especially important if the nutritional requirements of traits differ, such that the optimization of one trait occurs at the expense of another, and therefore trade-offs occur due to nutrient imbalance in the diet that is consumed. In this study we used 24 well-defined, artificial diets to test the sensitivity of a flour beetle, Gnatocerus cornutus, to variation in the calorific content and ratio of two major nutrients, protein (P) and carbohydrates (C) consumed during larval development to adulthood. We found that extreme calorific restriction was fatal or slowed the rate of development. However, beyond a minimum amount of food, the ratio of consumed macronutrients was critically important for the size of the morphological traits that we measured. Male weapons grew most in response to the consumption of P and in particular C whereas the genitalia had the weakest response. Even though different traits grew at different rates the P:C ratio that maximized survival, development rate and the size of all morphological traits was 1:2. This shows that the balance of P:C in the diet that is optimal for survival also optimizes development rate and the size of all morphological traits. Therefore as the nutritional needs of the different traits are similar, the balance of P:C in the diet of this beetle is unlikely to lead to trade-offs. Nonetheless, nutrition will have lasting effects on male fitness as larvae that develop on optimal diets developed fastest and were larger adults with larger weapons and small genitalia, which confers an advantage in male-male competition, access to mates and insemination success.

Image caption: Male Gnatocerus cornutus fight using an enlarged mandible which is a secondary sexual trait. Photo credit: Matthew Silk.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

Why baby birds differ in the speed at which they grow?

Riccardo Ton and Thomas E. MartinSometimes bird nests are not easily reachable, and creative thinking is required to access their contents. Photo provided by authors.

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Baby songbirds grow at very different speeds. In particular, tropical species tend to grow more slowly than similar species in temperate regions. Understanding why is important because speed of growth can have important consequences for the future and survival of the offspring. Growing slowly can be good and give baby birds time to better develop important organs like the brain, muscles and feathers. However slow growth also increases the probability that a predator like a snake or a hawk will find and eat the babies. Conversely, growing quickly helps a baby bird leave the nest early and avoid predators but with organs of lower quality. Despite these implications for survival, it is still unclear why baby birds differ in the speed at which they grow.

Generally babies of bigger species grow more slowly than those of smaller species. However, songbirds tend not to follow this natural rule. A possible explanation is that metabolism, the set of biochemical reactions that occur in each cell of a living organism, may be the engine propelling growth, but available data on this topic are contradictory. The oxygen that organisms breathe fuels this engine; therefore, if metabolism is the mechanism promoting growth, species that consume more oxygen should grow faster.

To test the hypothesis that metabolic rate may regulate differences in growth, we measured the oxygen consumed by baby birds sleeping on a nest inside a dark chamber for 59 species in three different continents. We sampled fast growing species in Arizona, USA, slow growing species in tropical Malaysia, and species that grow at intermediate speed in South Africa.

We found that, for a given body size, growing baby birds from Arizona consumed substantially more oxygen (had higher metabolism) than species from Malaysia or South Africa. We also found that babies of species consuming more oxygen grew faster compared to those species with lower metabolism. Finally, we observed that when differences in metabolism among species were accounted for, baby songbirds do follow the natural pattern that bigger species grow more slowly. However the reasons why metabolism differs among species and latitudes remain unclear and should be the focus of further studies.

Image caption: Sometimes bird nests are not easily reachable, and creative thinking is required to access their contents. Photo provided by authors.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

Effects of neonatal size on maturity and escape performance in the Trinidadian guppy

Terry Dial, David Reznick and Elizabeth BrainerdImage provided by authors.

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Newborn offspring are always smaller than the parent, but not all offspring are born the same size or same level of maturity, so what factors ultimately control the evolution of offspring size? Parents balance the tradeoff between offspring size and offspring number, and the optimal balance depends largely on the environment.

In response to high predation pressure by large predatory fish, Trinidadian guppies produce lots of small babies, flooding their environment with numbers. Above waterfall barriers, in environments with low or no predation, guppies produce fewer, larger babies.

To avoid a predator, fish will initiate an escape start by first bending their body into a C-shape and then undulating out of it, away from the threat. In other fish studied, larger individuals are known to produce higher performance escape starts. Thus, it is somewhat paradoxical that an adaptation to living with predators is to produce babies that are born with an inherent size disadvantage, but are they?

Here we measure escape performance of newborn guppies from five populations adapted either to low predation, high predation, or extreme high predation environments. We also measured detailed morphology, both external and internal, to assess whether these offspring possess different body proportions or levels of maturity. We find that those fish responding maximally to our stimulus perform at the upper limit for their size, and that both size and maturity influence this escape performance limit.

The smallest guppies are extremely slow at performing the escape start, even for their size, and these smallest offspring are also less developmentally mature compared with their larger counterparts. Within the Trinidadian guppy, it appears that both size and maturity tradeoff with number of offspring, and that offspring abundance outweighs performance in environments with the highest predation pressure.

Image caption: Image provided by authors.
You can read the article in full here.

 

Wood decomposition inside living trees shows that hollows develop slowly but cause a considerable loss of forest biomass

Zheng Zheng, Shubin Zhang, Carol Baskin, Jerry Baskin, Doug Schaefer and Xiaodong Yang Zhang Shubin is sampling decaying wood from a living tree hollow for measurement of its decay rate by CO2 release, in the Ailao Mountain Forest of China. Picture by Zheng Zheng.

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Living trees with decayed hollows provide nesting places for animals, especially in old-growth forests throughout the world. These hollows are formed by microorganisms that enter wounds in trunks or large branches and then decompose heartwood. Formation of tree hollows appears to be a slow process because tree hollows usually are found in large trees. However, large hollows formed in some trees indicates that wood decomposition rates there can be high compared to tree growth rates, and that hollow trees store less carbon. While abundance and use by animals of tree hollows have been well studied, the rates at which they form have not been measured.

In an old-growth subtropical montane forest preserve in southwest China, we measured decomposition rate of wood within tree hollows. From this we estimated the time required to form them. Wood inside tree hollows decomposed about half as fast as decomposing wood on the forest floor. Decomposition was slower because this wood was drier, and had higher density. Tree hollows enlarged by 2 mm per year on each side horizontally and about 8 times faster than that vertically. At that rate it would take 100 years for hollows to become large enough for nests of a local bee species. In addition, decomposition inside hollow trees means stems of living trees in this forest have lost 8·7% of their biomass.

Image caption: Zhang Shubin is sampling decaying wood from a living tree hollow for measurement of its decay rate by CO2 release, in the Ailao Mountain Forest of China. Picture by Zheng Zheng.
You can read the article in full here.

 

Black and yellow plumage signals resistance to oxidative stress in a bird

Amberleigh E. Henschen, Linda A. Whittingham, and Peter O. Dunn Photo by Matt Tillett. Flickr: via Wikimedia Commons. CC BY 2.0.

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Choosing a healthy mate is a vital part of female reproductive success as healthier mates may provide benefits to females, such as a better territory or ‘good genes’ that will be inherited by offspring. Male ornaments can help females choose wisely as these often elaborate traits may honestly advertise male health. In particular, it has been suggested that healthier males will have more effective immune systems and efficient metabolism, which will lead to the production of fewer free radicals (e.g., Oxygen ions and hydrogen peroxide). An excess of free radicals leads to an imbalance in cells called oxidative stress, which can damage cells and lead to poorer quality ornaments. Therefore, it is hypothesized that elaborate male ornaments honestly signal the ability of a male to produce immune responses that do not result in oxidative stress. We tested this hypothesis in the common yellowthroat, a warbler in which males have two ornaments, a black face mask and a yellow bib. These ornaments have been studied in both Wisconsin (WI) and New York (NY) and where females choose mates based on the size of the black mask and the size and color of the yellow bib, respectively. As the yellow bib is an honest indicator of oxidative stress in NY, our aim was to determine if the black mask signals similar information in WI. We assessed oxidative stress in males by measuring how resistant red blood cells from each male were to free radicals as well as oxidative stress in the plasma, or non-cellular portion of the blood. As predicted, we found that males with larger masks had red blood cells with greater resistance to oxidative stress. Furthermore, males with larger and more colorful bibs had lower levels of free radicals in the plasma. These results are consistent with the hypothesis that male ornaments indicate the ability of males to manage oxidative stress, but perhaps not in all components of the blood. Photo caption: Male common yellowthroats have two plumage ornaments, a black mask and yellow bib, which are used by females to choose mates. In this study, the authors found that each ornament was related to different measures of the ability to manage oxidative stress.

Image caption: Photo by Matt Tillett. Flickr: via Wikimedia Commons. CC BY 2.0.
You can read the article in full here.

 

Every plant needs good neighbours

Henry E. Creissen, Tove H. Jorgensen and James K.M. BrownFour different Arabidopsis genotypes competing whilst under disease pressure.

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The relationship between plant diversity, ecological stability and ecosystem productivity has been studied extensively in recent decades. Plant herbivores, diseases and insect pests alter such relationships by affecting plant fitness, reducing the growth and competitive ability of diseased plants. This can impact heavily upon plant population and community structure. Yet, despite their importance, experimental tests of mechanisms of pathogen-induced changes to plant competitive ability, productivity and diversity are rare.

We conducted competition experiments involving different genotypes of a small plant (Arabidopsis thaliana) to examine the relationships between population diversity, composition, productivity and stability in diseased and non-diseased plant populations. Plant genotypes varied greatly in both competitive ability and response to two pathogens, a virus and an oomycete. In certain populations we observed compensatory competitive interactions in which some genotypes increased productivity and compensated for the loss of yield by others. These interactions ultimately resulted in increased stability and productivity of the population as a whole. This study shows the importance of pathogen-mediated competition in maintaining plant genotypic diversity and productivity. A key finding is that the genotypic composition of the plant population, specifically the presence and maintenance of resistant genotypes within that population, is responsible for the capacity of the population to maintain productivity, stability and diversity.

As well as increasing our understanding of natural processes occurring in genotypically and phenotypically diverse plant populations, this study has potential applications to agriculture by demonstrating methods that can inform decisions about suitable plant cultivars for cultivation as variety mixtures.

Image caption: Four different Arabidopsis genotypes competing whilst under disease pressure.
You can read the article in full here.

 

Feed or fight: Testing the impact of food availability and intraspecific aggression on the functional ecology of an island lizard

Colin M. Donihue, Kinsey M. Brock, Johannes Foufopoulos, and Anthony HerrelPodarcis erhardii from the Greek Islands with tell-tale bite scar on its belly.

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Islands are often considered ideal biological laboratories as they are isolated and vary tremendously in size, structure, and habitats. These factors impose different selective pressures that can drive adaptations of organisms on islands. Our study capitalizes on an island-size gradient in the Greek Archipelago to investigate inter-island divergence in the body size, head shape, and bite force of a lizard, Podarcis erhardii.

We hypothesized that strong bites would be an advantage for lizards on small islands. For example, a strong bite might enable that lizard to eat relatively hard or large insects, so if food resources were sparse on a small island the lizard might not starve. A harder bite might also enable the lizard to win fights with the other individuals competing for the same valuable resources like food, mates, or nesting sites on a small island where lizard densities are often high.

We caught lizards on 11 islands in the Greek Cyclades ranging in size from 0.004 to 450 km2. We measured the lizards’ body and head size, and measured bite force. We then flushed their stomachs to see what they were eating and compared the stomach contents to insects we caught in traps. Finally, we counted the bite scars and lost toes, which typically result from battles with other lizards (see inset picture for an example bite scar).

We found, first, that bite force did increase on small islands. Second, we found that the competition indicators, i.e., bite scars and lost toes, best predicted the pattern in bite force. Hardness of prey in the diet did not vary significantly between lizards on these different islands. These results shed new light on the drivers of body size and performance differences among island populations.

Image caption: Podarcis erhardii from the Greek Islands with tell-tale bite scar on its belly.
You can read the article in full here.

 

Individual-level trait diversity indices

Simone Fontana, Owen L. Petchey and Francesco Pomati© Studio Asparagus.

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As humans, we are increasingly concerned about the potential effects of our activities on the structure and functioning of ecosystems. The challenge is to understand if ecosystems will be able to maintain processes and essential functions in a future of lasting human impact (e.g. climate change, pollution). The effects of human-induced environmental change on ecosystem processes might be direct or mediated by changes in the living organisms.

For this reason, researchers are increasingly considering traits (measurable features of individual organisms, e.g. morphological and physiological characteristics) as a powerful way to explain processes in ecological systems. However, traits are generally assumed not to vary at the species level, thus disregarding the importance of differences between individuals belonging to the same species.

Our approach considers both differences among individuals and multiple traits at the same time: each individual is placed within a space defined by every measured trait (axes). Thus a species occupies a ‘cloud’ in trait space, and there are basically three different ways to quantify this cloud; trait richness (= trait space occupied by individuals in a community), trait evenness (= regularity in the distribution of individuals within the trait space) and trait divergence (= how spread-out are individuals within the trait space). These characteristics can in turn be measured by various indices, and we can define how we expect each index to change (increase/decrease) if individual organisms are deleted or moved within the trait space. We tested different indices using simulated and real phytoplankton data, to find those with the most appropriate behaviour (that is, the ones changing as expected as a result of our manipulations). The resulting validated set of indices includes FDis (trait divergence) and two newly proposed metrics: TOP (= Trait Onion Peeling; trait richness) and TED (= Trait Even Distribution, trait evenness).

These three components of trait diversity, used in a complementary approach, may shed light on the importance of differences among individuals in natural ecosystem processes and improve our understanding of the pathways by which environmental changes affect ecosystem functioning through biodiversity change.

Image caption: © Studio Asparagus.
You can read the article in full here.

 

Water use by Swedish boreal forests in a changing climate

Thomas B. Hasper, Göran Wallin, Shubhangi Lamba, Marianne Hall, Fernando Jaramillo, Hjalmar Laudon, Sune Linder, Jane L. Medhurst, Mats Räntfors, Bjarni D. Sigurdsson, Johan UddlingSwedish boreal landscape (left; © Thomas B. Hasper) and Flakaliden Whole-Tree Chamber Experiment (right; © Bengt-Olof Vigren).

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The ongoing increases in atmospheric carbon dioxide concentration ([CO2]) and temperature have the potential to alter the flux of water vapor through plant leaf ‘stomata’, tiny and actively regulated pores in the leaf surface. This flux, transpiration, in turn regulates the terrestrial water and energy balance and, thus, influences local and regional hydrology and climate.

Combined stomatal-photosynthesis models employed in ecosystem and climate models predict decreases in stomatal conductance and land evapotranspiration (ET) at elevated [CO2]. This stomatal water-saving response to increased [CO2] has been used to explain the observed increase in global river runoff during the past century. Plant water use is, however, also affected by changes in air temperature, precipitation and land-use, and there is yet no consensus about the contribution of different drivers to temporal trends of ET and river runoff.

In this study, we examined the water-use responses of Swedish boreal forests to climate change by using long-term monitoring as well as experimental data. We used climate and runoff data of large-scale boreal landscapes from the past 50 years to explore historical trends and patterns in ET. In addition, we examined explicit tree water-use responses to elevated [CO2] and/or air temperature in a whole-tree chamber experiment using mature Norway spruce trees.

The results demonstrated that ET increased by 18% over the past half century while river runoff did not significantly change. The increase in ET was related to increasing precipitation and a steady increase in forest standing biomass over this period. The whole-tree experiment showed that Norway spruce trees did not use less water under elevated [CO2] and that elevated air temperature did not increase plant transpiration as decreased stomatal conductance neutralized the effect of higher evaporative demand in warmed air.

Our findings have important implications for projections of the future hydrology of European boreal coniferous forests, indicating that changes in precipitation and standing biomass are more important than effects of elevated [CO2] or temperature on tree transpiration rates.

Image caption: Swedish boreal landscape (left; © Thomas B. Hasper) and Flakaliden Whole-Tree Chamber Experiment (right; © Bengt-Olof Vigren).
You can read the article in full here.

 

Nitrogen deposition affects Scots pine stoichiometry

Jordi Sardans, Rocio Alonso, Ivan Janssens, Jofre Carnicer, Stavros Vereseglou, Mathias Rillig, Marcos Fernández-Martínez, Tanja Sanders and Josep Peñuelas Pinus sylvestris forests at the Southern limit of its distribution area, such as in this photo in a Pyrenees valley, are subject to several stresses, from drought to excess light, that threaten their survival. The impacts of these stresses on plant-soil nutrient cycles and nutrient ratios can be crucial for the future of this species in several parts of Europe. Credit Dr. Oriol Grau.

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The spatial and temporal patterns of nitrogen (N) and phosphorus (P) concentrations and of N:P ratios in terrestrial plant communities have been shown to be related to several ecosystem processes. Little is known, however, about the flexibility of N and P concentrations or of N:P ratios in terrestrial plant species along their natural ranges of distribution in response to natural and anthropogenic gradients, or of their ecological significance. Pinus sylvestris (Scots pine) is one of the most important forest species in Eurasia, with a broad distribution extending from Spain to Siberia. This broad distribution across Europe offers a unique opportunity to study the relationships between soil and plant nutrient concentrations, and how these vary with climate, atmospheric N deposition and plant growth. In this study, we investigated needle N and P concentrations, N:P ratios and soil elemental composition and their shifts in response to climatic gradients and atmospheric N deposition in P. sylvestris forests. We also investigated the relationship of these variables with P. sylvestris growth by analyzing a compiled dataset of 2245 stands of P. sylvestris throughout Europe. The results showed that N deposition was positively correlated with needle N concentration and N:P ratio and negatively with needle P concentrations. This was especially pronounced at sites where high levels of N deposition coincided with higher mean annual temperature and higher mean annual precipitation, such as in central Europe. Atmospheric N deposition was also negatively correlated with soil plant-available P and K concentrations. Higher soil-solution nitrate:P ratios coincided with higher needle N:P ratios at higher levels of N deposition. The relationships between needle and soil-solution nutrient concentrations indicated that other elements, such as potassium, could also be involved in soil nutritional disequilibrium. Despite these nutrient imbalances, N deposition was positively related to Pinus sylvestris absolute basal diameter growth.

These results thus indicate a tendency of European P. sylvestris forests to store N in trees and soil in response to N deposition, and strongly suggest a trend toward increased nutrient losses in runoff related to higher soil-solution N concentrations.

Image caption: Pinus sylvestris forests at the Southern limit of its distribution area, such as in this photo in a Pyrenees valley, are subject to several stresses, from drought to excess light, that threaten their survival. The impacts of these stresses on plant-soil nutrient cycles and nutrient ratios can be crucial for the future of this species in several parts of Europe. Credit Dr. Oriol Grau.
You can read the article in full here.

Special Feature: The functional role of silicon in plant biology

The Importance of Agriculture in Global Biogenic Silicon Production

Joanna C. Carey and Robinson W. FulweilerCorn farm in central Pennsylvania. Photo: fishhawk via Flickr (CC BY).

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Our human footprint on the Earth is so large that many scientists assert we have ushered in a new geological epoch – the Anthropocene. Human impacts on the Earth are well-documented. We have cut down forests, dammed rivers, overfished the seas, and added enough extra carbon dioxide to our atmosphere to increase global temperatures. We have also fundamentally changed how we grow our food. Industrialized agriculture has altered a range of ecosystem processes, perhaps the most fundamental of which is nutrient cycling. While the impacts of agriculture on the nitrogen and phosphorus cycles are well-described, we know much less about how agriculture has changed the global silicon (Si) cycle.

We care about Si for many reasons. Weathering of silicate rocks plays a key role in regulating atmospheric carbon dioxide concentrations over long time periods. Additionally, Si is an essential nutrient for diatoms, small photosynthetic plankton (think ‘grasses of the sea’) that consume carbon dioxide. Diatoms also support economically, nutritionally, and culturally important marine food webs. Si also turns out to be a ‘quasi-essential’ nutrient for land plants, as it protects them from stressors such as drought, herbivory, and heavy metal toxicity.

Land plants take up dissolved silica and it becomes deposited within their tissues as biogenic Si. Agricultural crops account for approximately 35% of the biogenic silica fixed globally by land plants, not only because of their large biomass, but also because they tend to have high Si concentrations in their tissues. In the last fifty years (1961-2012) biogenic silica production in the ten most productive agricultural crops has more than tripled, and we predict that by 2050 biogenic silica production may increase by another 50%.

Compared to mineral silicates, biogenic silica is considered ‘bio-available’ and is rapidly regenerated and available for subsequent uptake by terrestrial or aquatic organisms. In turn, the substantial increase in biogenic silica production is augmenting the reservoir of biologically available Si on Earth. As a result, the fate of the biogenic silica removed from agricultural areas via plant harvest is important, with implications for global carbon cycling and marine food webs.

Image caption: Corn farm in central Pennsylvania. Photo: fishhawk via Flickr (CC BY).
You can read the article in full here.

 

Unseen consequences of losing large wildlife: increases in rodent immune function following large mammal defaunation

Hillary S. Young, Rodolfo Dirzo, Kristofer M. Helgen, Douglas J. McCauley, Charles L Nunn, Paul Snyder, Kari E. Veblen, Serena Zhao and Vanessa O. EzenwaZebras. Photo provided by authors.

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Ecosystems around the world are losing large wildlife due to overexploitation, development of agriculture, and increased human densities. While research has examined the consequences of losing large-bodied animals for the abundance and composition of the smaller animals that remain, little research has considered how these changes affect animal physiology. This is a critical oversight because changes in animal physiology can have enormous consequences for ecosystem functioning and processes, including spread of disease.

We examined the effect of large wildlife removal on immune function in rodents using a large-scale exclosure experiment in Kenya, where high voltage electrified fences have effectively remove all large wildlife from plots for the past 20 years. Examining a suite of immune parameters in the most common mouse species in these plots, we found evidence of significant increases in immune function of rodents when large wildlife are absent. This effect may result from a higher number of parasites in defaunated ecosystems, which drives an increase in need for immune protection. Alternatively, observed changes may result from increased food availability in the landscape when large herbivores are removed, which allows for more energy to be devoted to immune functions. Either way, our research indicates that loss of large wildlife could have major consequences for rodent-borne disease dynamics via changes in immune function. For example, such changes may reduce the likelihood that increased parasite density in landscapes without large wildlife may translate to realized increases in disease prevalence in rodents.

Image caption: Zebras. Photo provided by authors.
You can read the article in full here.

Special Feature: Mechanisms and consequences of facilitation in plant communities

Facilitation among plants as an insurance policy for diversity in Alpine communities

Lohengrin A. Cavieres, Carolina Hernández-Fuentes, Angela Sierra-Almeida and Zaal Kikvidze  Laretia acaulis cushion harbouring native grasses in the high alpine of central Chile Andes.

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Positive interactions among plant species occur when the presence of one plant species enhances the chances that another species co-occurs in the same place, indicating that positive interactions may determine biological diversity. Although early research focused on (negative) interactions such as competition and predation as the main factors structuring plant communities and regulating biological diversity, positive interactions have started to gain a place in the ecological literature. However their role in determining diversity at different spatial scales has been poorly explored.

Nurse plants are those that provide shelter to other plant species from environmental severity (e.g. strong winds, extreme low temperature, etc.) or herbivory, and are one of the most commonly recognized examples of positive interactions among plants. It has been suggested that the presence and importance of nurse plants are higher in environmentally severe environments such as alpine or desert habitats, where the mitigation of extreme conditions by the nurse species can be beneficial to many other species.

The majority of the studies addressing the consequences of nurse plants for diversity have compared the diversity of plant assemblages growing within nurses vs. those growing outside them, reporting contrasting results among them. Nonetheless, nurses and their alternative microhabitats (open areas between nurses) are part of the same community. Thus, if nurses allow for the persistence of species that otherwise would be excluded, a net increase in species diversity of the whole community will be generated even if areas under nurse plants contain fewer species than the open areas outside them.

In this study we conducted a bibliographic search using the ISI-Web of Knowledge database and reviewed the literature on alpine plant communities where assessments of the diversity of plants growing within and outside nurse species were available. We found that in most cases, nurse species substantially increased species richness, despite the fact that in some cases they contained lower species numbers than surrounding open areas. Strikingly, nurse species enhanced species richness more in sites with more severe environmental conditions, suggesting that facilitative interactions in alpine habitats act as an insurance policy that sustains diversity under very harsh environmental conditions.

Image caption: Laretia acaulis cushion harbouring native grasses in the high alpine of central Chile Andes.
You can read the article in full here.

 

Perturbations in growth trajectory due to early diet affect age-related deterioration in performance

Who-Seung Lee, Pat Monaghan and Neil B. MetcalfeMale (left) and female (right) three-spined sticklebacks.

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Dietary conditions affect the growth rates of organisms, which in turn influences future survival and reproduction. Animals can often compensate for a period of slow growth early in life by accelerating their growth when conditions improve, resulting in normal adult size, but this rapid growth can prove costly in later life. In this study we investigated whether this compensatory growth has long-term consequences for patterns of senescence. Using a small species of fish, the three-spined stickleback (Gasterosteus aculeatus), we showed that a brief period of reduced food availability in early life affected skeletal growth rate not only during the manipulation itself, but also during a subsequent compensatory phase when fish caught up in size with controls. However, fish that had gone through this period of growth acceleration had a faster decline in their swimming performance and a shorter period of breeding over the following two breeding seasons, suggesting that they were ageing faster. This was confirmed by their having a shorter lifespan. The effects were strongest if the acceleration of growth happened just before the breeding season. These results, complementary to those found when growth trajectories were altered by temperature rather than dietary manipulations, show that the costs of accelerated growth can last well beyond the time over which growth rates differ.

Image caption: Male (left) and female (right) three-spined sticklebacks.
You can read the article in full here.

 

Flowers avoiding bees? The case of Costus arabicus colour variation sheds light on bee sensorial exclusion hypothesis for hummingbird red-flowers

Pedro Joaquim Bergamo, André Rodrigo Rech, Vinícius L. G. Brito and Marlies SazimaPink flowers of Costus arabicus, presumably avoiding bee-pollination.  Photo credit: Camila Silva Oliveira.

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Floral colour plays a role in plant-pollinator communication by signalling floral resources. Hummingbirds are often associated with red coloured flowers, and there are two tentative hypotheses to explain this association: 1. hummingbirds are attracted to red due its easier detection and, 2. bees are sensorialy excluded from red flowers. The second hypothesis is based on bees' red colour blindness, which leads them to be less frequent and less important than hummingbirds as pollinators of red-reflecting flowers. In this sense, red-reflecting flowers could adopt a "bee avoidance" strategy. Here we empirically tested the above hypotheses. We chose Costus arabicus, which has white- and pink-flowered individuals and is pollinated by both bees and hummingbirds. Specifically, we tested whether differences in red reflectance attracts hummingbirds (expecting more hummingbird visitation of pink flowers) or excludes bees (and then, expecting bee avoidance of pink flowers) and the consequent implications for the plant’s reproduction. Flower colour morphs of C. arabicus differed only in petal red-reflectance. Using vision models, the white flowers were revealed to be easily detected by bees and the pink flowers by hummingbirds. Bees preferentially visited the white flowers, whereas hummingbirds visited both colours at the same rate - both patterns corroborating the bee avoidance hypothesis. Pollen loads deposited on flowers stigmas did not differ between flower colour morphs, indicating that bees and hummingbirds play a similar role in the quantity aspect of pollination. However, bees are more likely to self-pollinate C. arabicus flowers than hummingbirds. We also found that self-pollination limits C. arabicus reproduction, and red-reflecting flowers may enhance the quality aspect of pollination by discouraging bee visitation. Sensory exclusion of bees seems to be the pressure for evolution of red-reflecting flowers, driving specialisation in hummingbird-pollinated flowers due to the costs of bee pollination on plant reproduction.

Image caption: Pink flowers of Costus arabicus, presumably avoiding bee-pollination. Photo credit: Camila Silva Oliveira.
You can read the article in full here.

 

Ambient temperature, body condition and sibling rivalry explain feather corticosterone levels in developing black kites

Lidia López-Jiménez, Julio Blas, Alessandro Tanferna, Sonia Cabezas, Tracy Marchant, Fernando Hiraldo and Fabrizio SergioBlack kite feeding its young. Photo credit: Fabrizio Sergio.

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When an animal is psychologically or physically challenged (e.g. upon exposure to inclement weather, disease, food shortage or predators) a number of internal mechanisms that promote survival become activated. In birds, one such mechanism results in the secretion of the hormone corticosterone into the bloodstream. Rises in circulating corticosterone levels elicit a variety of changes in both physiology and behaviour which allow dealing with perturbations as quickly as possible. Recently, the development of a new technique to quantify the amount of corticosterone in feathers has been received with enthusiasm in the field of ecophysiology on account of its potential advantages over more traditional methods such as blood sampling. Feathers develop in specialised areas in the skin called follicles, where they receive a large supply of blood throughout growth (but not afterwards). During this period, numerous materials which circulate at the time, including corticosterone, passively diffuse out of the blood plasma and become sequestered into the forming feather keratin structure. In this sense, feathers have the potential to serve as permanent records of levels of past exposure to perturbations during a comparatively long period (days to weeks). In this study, we aimed to contribute further to the validation of this technique by measuring corticosterone levels in the feathers of the nestling of a raptor species (the black kite, Milvus migrans). Specifically, do these reflect variation between individuals in their responses to a number of potentially challenging environmental, social and physiological variables? As predicted, we found higher feather corticosterone in nestlings that: 1) had been exposed to lower ambient temperatures; 2) had poorer body condition; or 3) had been born in multiple-chick broods, but especially in those hatched lower down the brood hierarchy, which are more prone to suffer aggression from their older siblings during the establishment of dominant-subordinate relationships. Overall, our results underscore the potential of using feather corticosterone to infer levels of exposure to challenging situations in wild birds and their potential power as indicator tools in conservation biology.

Image caption: Black kite feeding its young. Photo credit: Fabrizio Sergio.
You can read the article in full here.

 

Multiple environmental drivers structure plant traits at the community level in a pyrogenic ecosystem

Gregory M. Ames, Steven M. Anderson and Justin P. WrightA longleaf pine savanna experiencing a prescribed burn, Fort Bragg, NC. Photo by Gregory Ames, 2012.

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Biodiversity is declining at unprecedented rates and this decline may negatively impact ecosystem functions that we depend on. Preserving functional diversity requires an understanding of how it responds to changes in environmental conditions. Measureable properties of plants that affect performance, i.e. functional traits, provide a measure of functional diversity that is independent of species identity and can be compared across systems containing different species. However, little is known about the relationship between functional traits and environmental forcing, or how local trait variability influences our understanding of this relationship.

We measured traits that are strongly associated with growth and survival strategies of plants in the understory communities of longleaf pine forests that experience periodic fire, to investigate how this strong disturbance interacts with environmental conditions to drive changes in functional traits. We also investigated which environmental factors are most associated with changes in community composition without considering functional traits.

We found that a small set of environmental factors explained a large amount of the variation in key traits, but that the factors explaining community composition and functional traits were different. In particular, we found that functional traits were primarily explained by interactions between environmental factors such as fire, precipitation, and soil texture. This shows that strong environmental gradients cannot be considered independently of one another.

Furthermore, our interpretations of which environmental factors were most important depended on whether or not we used functional trait values from across the landscape or just the local plot-level measurements. This shows that it is essential to account for local variability in species traits when trying to infer relationships with the environment, especially in landscapes with strong disturbance gradients like those imposed by fire.

Our results are important because they show that understanding the response of functional diversity to the environment requires consideration of multiple environmental factors as well as their interactions, as well as the use of locally measured trait values. Incorporating these elements into future trait-based studies will greatly improve our understanding of how functional diversity responds to environmental changes.

Image caption: A longleaf pine savanna experiencing a prescribed burn, Fort Bragg, NC. Photo by Gregory Ames, 2012.
You can read the article in full here.

 

Frog habitat preferences do not maximize jumping performance

Allegra Mitchell and Philip J. Bergmann Photo credit: Philip J Bergmann.

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Predicting species responses to a changing climate is difficult because of the dynamic relationship between animals and their environment, but it is necessary to reduce species losses. Amphibians in particular are suffering population declines globally, and understanding how the environment influences their habitat preferences and physical performance is crucial to understanding how these animals will be influenced by climate change. The biology of amphibians makes them especially sensitive to changes in environmental temperature and moisture because they are cold blooded and have highly water-permeable skin. Therefore, changes in environmental temperature and moisture directly affect amphibian body temperature and hydration level. Amphibians should select habitats with the optimal combination of temperature and moisture to perform tasks necessary for survival, like escaping predators or capturing prey. However, interactions between environmental temperature and moisture can influence habitat selection and task performance in different and often unpredictable ways.

In our study, we tested how environmental temperature and moisture influenced temperature and moisture preferences, and jumping performance in Green Frogs (Lithobates clamitans) in the laboratory and the field. We found that frogs in the laboratory selected environmental conditions that minimized water loss through the skin, rather than maximized body temperature to allow them to jump better. Frogs in the field also preferred to be well hydrated, and allowed their body temperatures to be much more variable. Jumping performance was highest at higher temperatures, but higher temperatures also increase dehydration. Therefore, conditions that minimized water loss frequently did not maximize jumping performance, creating a discrepancy between remaining hydrated and moving effectively.

The ecology of Green Frogs may explain this discrepancy because the frogs remain near bodies of water that serve as refuges from both dehydration and many predators. However, when it is warmer and drier, Green Frogs may be more likely to select microhabitats that minimize the risk of dehydration at the expense of their ability to forage and escape from predators. As the climate changes, all amphibians may be at greater risk of dehydration and predation, particularly those that are already declining.

Image caption: Photo credit: Philip J Bergmann.
You can read the article in full here.

 

Mercury could reduce parental care behaviour by disrupting parenting hormones in Arctic seabirds

Sabrina Tartu, Paco Bustamante, Frédéric Angelier, Ádám Z. Lendvai, Børge Moe, Pierre Blévin, Claus Bech, Geir W. Gabrielsen, Jan Ove Bustnes, Olivier ChastelUnattended black-legged kittiwake hatchling. Credit: Sabrina Tartu.

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Despite its remote location, the Arctic is a fallout region for long-range transport pollutants, among them mercury, a naturally occurring element that has been enriched in the environment by human activities such as coal combustion and mining. When converted to methylmercury by microbial processes in the ocean, it can accumulate in organisms and magnify along the food web. Consequently, the levels of methylmercury reached in top predators like Arctic seabirds are very elevated. Methylmercury is a toxic element which can have serious health effects on Arctic wildlife. Less known are its effects on free-living animals as an endocrine disruptor, that is, its ability to disrupt the normal functioning of natural hormones. Hormones are chemical substances that act like messenger molecules in the body, and control and regulate the activity of certain cells or organs. To this end, an international team collaborated to assess the effects of mercury contamination on the hormone prolactin and breeding performance in Black-legged kittiwakes from Svalbard, in the high Arctic. Prolactin is a hormone that stimulates lactation in mammals and incubation or chick feeding in birds. Kittiwakes were captured on their nest and a blood sample was taken to measure mercury and prolactin concentrations. It appeared that high mercury levels in male kittiwakes were associated with decreased prolactin secretion and with reduced breeding success. An experimental study was also conducted to see if environmental stress could exaggerate the negative effect of mercury on prolactin secretion. To do so, male kittiwakes were implanted with corticosterone, a stress hormone which naturally rises during unfavorable conditions (e.g. food shortage, predation). Because stress is known to decrease prolactin concentrations, we predicted that mercury and corticosterone would act synergistically and would steepen the mercury- prolactin relationship. Hatching success was significantly lower in corticosterone-implanted males, but contrary to our prediction, mercury and stress did not act synergistically. Results from this study strongly suggest that mercury can impair seabird reproductive performance through a disruption of prolactin secretion and highlights the need to further explore the complex interactions between environmental stressors and anthropogenic contaminants.

Image caption: Unattended black-legged kittiwake hatchling. Credit: Sabrina Tartu.
You can read the article in full here.

Review

Functional characterizations of Ellenberg Indicator Values – a review on ecophysiological determinants

Maik Bartelheimer & Peter PoschlodControlled screening experiment to relate Ellenberg Indicator Values for soil moisture to species plasticity in rooting depth. Photo by Jessica Rossow.

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Ellenberg Indicator Values (EIV) provide a numerical system for Central European plant species to describe their peak occurrence along environmental gradients. EIV are available for the soil factors moisture (M-numbers), nutrients (N-numbers), and soil pH/reaction (R-numbers) as well as for the climatic factors light (L-), temperature (T-), and distance to the Atlantic Ocean with its climate-modifying effects (continentality, C-numbers). By finding correlations of EIVs to morphological or ecophysiological plant properties we hope to identify those properties that determine species distributions along environmental gradients. To identify such determinants this review surveys existing literature containing species comparisons from controlled experiments and combines them with EIV. The picture emerging is that multiple determinants can be identified for nutrient-, reaction- and also moisture-numbers, while only few can be found for light- and especially for continentality- and temperature-numbers.

As an example, for nutrient-numbers 16 different determinants could be identified. These include many that might be expected like ‘specific leaf area’ (a measure of the amount of leaf material deployed per unit area of leaf), ‘seed weight’, and ‘relative growth rate’, but also some that are less obvious, like ‘regrowth after defoliation’, ‘germination response to ammonium nitrate’, and ‘germination temperature requirement’. At the same time, just a single determinant for continentality-numbers could be identified from existing data sets (timing of leaf unfolding and related frost resistance).

Functional characterizations of the different EIV can thus be deduced, which helps us to understand the mechanisms and processes driving the ecological niche of a plant. The approach described is a powerful tool to analyse the ecological significance of different plant properties. In the future, species screenings specifically designed to allow for correlations with EIV have great potential for high explanatory power.

Image caption: Controlled screening experiment to relate Ellenberg Indicator Values for soil moisture to species plasticity in rooting depth. Photo by Jessica Rossow.
You can read the article in full here.

 

Tracking woodland water use efficiency under future atmospheric conditions

Teresa E. Gimeno, Kristine Y. Crous, Julia Cooke, Anthony P. O’Grady, Anna Ósvaldsson, Belinda E. Medlyn and David S. Ellsworth A view of the EucFACE study site from the top of a crane inside one of the study rings taken while making a gas-exchange measurement campaigns by Teresa E. Gimeno.

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The most certain prediction for our atmosphere is that carbon dioxide (CO2) concentration will continue to rise, due to fossil fuel burning. Rising CO2 affects the biosphere indirectly because it is the main driver of climate change, but it also has a large direct impact on vegetation because it is the main fuel for plant growth. However, when plants take up CO2, it is accompanied by an inevitable loss of water. This process of transpiration is regulated by the opening and closing of small pores (called stomata) found on the leaf surface of all higher plants. In an atmosphere with more CO2, plants can close stomatal pores to economize on water, while maintaining or even increasing CO2 uptake. This means that plants can increase how efficiently they use water with rising CO2. This increase is particularly relevant for ecosystems where plant growth is limited by water availability, including the vast majority of Australian forests and woodlands.

Previous experiments showed that rising CO2 could increase or decrease water use efficiency depending on plant types and climates. Fortunately, models help to predict optimal water use per unit of CO2 uptake, based on the sensitivity of stomatal pores to the environment. Here we tested the ability of one of these models to predict water use efficiency during carbon uptake in future atmospheric conditions, under variable climatic conditions. To test for optimal stomatal behaviour of the trees, we measured carbon uptake and water use at the tops of mature eucalypt trees under current and future CO2 atmospheric concentrations, over the course of the seasons. Our study was conducted at the EucFACE site, the only existing elevated CO2 experiment in a native Eucalyptus forest. We show that water use efficiency increases proportionally with the rise in CO2 concentration. So in a future global change scenario, mature trees will be more efficient in their water use, but this may only partially mitigate some of the adverse effects of the concomitant rise in temperature.

Image caption: A view of the EucFACE study site from the top of a crane inside one of the study rings taken while making a gas-exchange measurement campaigns by Teresa E. Gimeno.
You can read the article in full here.

 

Sperm production characteristics vary with level of sperm competition in Cataglyphis desert ants

Serge Aron, Pascale Lybaert, Claire Baudoux, Morgane Vandervelden and Denis Fournier Cataglyphis hispanica worker ant collecting pollen of Spergularia purpurea (Caryophyllaceae).

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In animal species where females mate with more than one male, ejaculates from different males coexist in the female tract and compete to fertilize ova. Competition among sperm from multiple males is recognized as an important force in the evolution of sperm traits that enhance fertilization success. In contrast to species with weak or no sperm competition, males of species with intense sperm competition usually produce more sperm to out-compete rivals and/or longer sperm cells that swim faster than shorter ones. We studied relationships between levels of sperm competition, sperm production and sperm size in a comparative study of 15 species of Cataglyphis desert ants. The mating system of ants imposes unique selective pressures on male ejaculates that are rarely, if ever, found in other animals. Males disperse with a finite amount of sperm, they typically mate with a single or, rarely, a few females and die shortly afterwards. However, males live posthumously as spermatozoa stored in a little reservoir of the queen (the spermatheca), for several years or more. Thus, the reproductive success of males is limited by the amount of sperm successfully stored in the spermathecae of the queens. A remarkable feature of Cataglyphis ants is that multiple queen-mating (queens can mate with 2 to 14 males) is the ancestral state, and reduction in mating frequency evolved secondarily in some lineages. This provides a unique opportunity to examine how reduction from multiple to single mating influences sperm traits. We counted sperm in the testis of males and showed that they produce more spermatozoa in species experiencing greater levels of sperm competition. In contrast, neither sperm length nor male size is significantly associated with the number of matings of queens. Our comparative analysis provides the first direct evidence that sperm production covaries with the level of sperm competition in a eusocial insect and, more generally, that reduction in sperm competition influences sperm traits in an organism.

Image caption: Cataglyphis hispanica worker ant collecting pollen of Spergularia purpurea (Caryophyllaceae).
You can read the article in full here.

 

How are leaf mechanical properties and water use traits coordinated by vein traits?—A case study in Fagaceae

Kiyosoda Kawai and Naoki Okada Hierarchal vein system in Quercus gilva (Fagaceae). Photo credited to authors..

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If you take a stroll into a forest, you will immediately find a variety of leaf shapes there. Broad-leaved (dicotyledonous) flowering plants (Angiosperms) are the most diversified in morphology and this is also true for leaf venation. But, venation in all Angiosperms shares one key property; a hierarchal vein system, i.e. many types (orders) of veins whose diameter and length are dramatically different (see attached image).

This system has been known for a long time, but its functional significance is still discussed. One fascinating explanation has been “division of labor”: thick but short primary and secondary veins transport sugars and water over long distances. On the other hand, thin and net-like minor veins distribute water and collect sugars produced by photosynthesis. But, earlier researches have not paid much attention to another important vein function: mechanical support to keep the light-intercepting leaf stable against gravity.

In this paper, we examined the contribution of leaf veins to leaf mechanical properties and water use and attempted to confirm the “division of labor” hypothesis in Angiosperm venation. For this purpose, we employed 8 Fagaceae species (oak/beech family) with a diversity of leaf shape, leaf habit and venation. We quantified mechanical properties (toughness) of leaves by tearing the leaf lamina and water relations by measuring conductance of leaf and water use efficiency (photo- synthetically gained carbon / loss of water). We measured vein and leaf traits to explain the variation of these functions. For vein traits, we measured vein density (vein length per unit area) for different order veins.

We found that different order veins were associated with different leaf functions. Primary and secondary vein density influenced leaf carbon cost and mechanical properties. In contrast, minor vein density is associated with leaf hydraulics. These results showed that the division of labor occurs in multiple leaf functions among contrasting vein orders. Further our study suggests the possibility that combinations of vein density of different orders could have led to the diversification of multiple leaf functions in the Angiosperms.

Image caption: Hierarchal vein system in Quercus gilva (Fagaceae). Photo credited to authors.
You can read the article in full here.

 

Female guppies reduce energetic costs of being harassed by males by becoming more efficient swimmers

Shaun S. Killen, Darren P. Croft, Karine Salin and Safi K. DardenImage provided by authors.

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Females and males of the same species are often in conflict over the frequency and timing of matings. Typically male reproductive success is limited by access to females and males of many species will try to overcome this using a number of behaviours, such as chasing and even attacking females in an attempt to gain a mating. These types of behaviours are considered sexually harassing as males are attempting to coerce females into mating with them. Females can spend a lot of energy avoiding males in these situations and can even be injured. To reduce these costs, one possibility is that females may be able to change their own behaviour or physiology in ways that reduce the negative energetic consequences of harassment, or allows them to more easily escape male coercion.

We studied this likelihood in a laboratory setting by housing female Trinidadian guppies (Poecilia reticulata) for several months with varying levels of male harassment that they would normally encounter in the wild. In the wild male guppies spend a large portion of their time chasing and harassing females in an attempt to mate with them. Females can attempt to avoid this harassment by rapidly swimming away from the male during pursuits.

After five months, females exposed to higher levels of harassment were able to swim much more efficiently, using less energy to swim at a given speed compared to those exposed to lower levels of harassment. It seems that prolonged increases in high-intensity swimming in females, caused by male harassment, leads to changes in the physiology or swimming mechanics of individual fish, which reduce costs of swimming. Indeed, females that experienced lower levels of harassment spent more time swimming with their pectoral fins extended, an indicator of an inefficient swimming technique.

These results show that female guppies can reduce the energetic costs of male sexual harassment through changes in their swimming physiology or technique. Increased swimming efficiency or performance could also allow female guppies to escape male coercion more easily, giving them more control over matings. An exciting opportunity now exists to examine the extent to which this phenomenon occurs in the wild.

Image caption: Image provided by authors.
You can read the article in full here.

 

Soil acidification exerts a greater control on soil respiration than soil nitrogen availability in grasslands subjected to long-term nitrogen enrichment

Dima Chen, Jianjun Li, Zhichun Lan, Shuijin Hu, and Yongfei BaiSemi-arid grassland in Inner Mongolia of China (Image provided by Qingmin Pan).

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In many areas of the globe and especially in Asia, deposition of atmospheric nitrogen compounds (N) is expected to continue to increase. The dramatic increases in N inputs have wide-ranging ecological impacts on the biotic community, biogeochemical cycles, and greenhouse gas emissions. As one of the largest carbon effluxes between the atmosphere and terrestrial ecosystems, soil respiration plays a vital role in regulating the atmospheric CO2 concentration and climate on Earth. N inputs can affect soil respiration through two mechanisms or pathways: direct N impacts through enhancing soil N availability, and thus increasing plant productivity, and indirect impacts through modifying soil acidity. Yet, few studies have experimentally assessed the relative effects of increased N availability vs. soil acidification on below-ground carbon cycling in natural ecosystems. To address this issue, we conducted a 12-yr N enrichment experiment and a 4-yr complementary acid addition experiment in a semi-arid Inner Mongolian grassland. We found that soil acidification exerts greater control than soil N availability on soil respiration in grasslands experiencing long-term N enrichment, suggesting the need to include soil acidification in predicting terrestrial ecosystem carbon balance under future N deposition scenarios. We also found that N enrichment enhanced soil depletion of base cations such as calcium and magnesium in this semi-arid ecosystem, indicating that researchers and policy makers should also consider the long-term effects of N enrichment on element availability for plants, wildlife, and domestic livestock.

Image caption: Semi-arid grassland in Inner Mongolia of China (Image provided by Qingmin Pan).
You can read the article in full here.

 

Mistletoe influences community seedfall patterns

Ana Mellado and Regino ZamoraIn this Mediterranean pineland, parasitized trees, constituting the only (or most abundant) nutritive resource offered on the canopy layer, are particularly noticeable for frugivorous birds. Frugivores respond to mistletoe patchiness by visiting parasitized trees preferentially to unparasitized ones, driving a differential deposition of mistletoe and co-fruiting species seeds towards parasitized trees. Photo by Ugo Mellone.

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Mistletoes are fleshy-fruited parasitic plants that dwell in forest canopies and show a strongly aggregated spatial distribution. Parasitized trees potentially concentrate fruit-eating bird activity in their canopies, where birds find food, places to perch, and protection against predators. Thus, seed-deposition patterns are expected to reflect the heterogeneity associated with the parasite. This becomes especially important in generalist dispersal systems; however, so far, we do not know the implications of mistletoe spatial heterogeneity for the seed-dispersal pattern of other plants that fruit at the same time.

In a Mediterranean pineland, we analyze the impact of Viscum album subsp. austriacum on the seed-deposition pattern of a bird-dispersed plant community, taking into consideration the spatial and temporal variability of environmental factors influencing the birds’ habitat use, such as fruit availability and forest tree density. For four consecutive years, we studied 55 pairs of trees parasitized and unparasitized by mistletoe, analyzing fruit availability, bird visits, and the bird-dispersed seed rain in selected trees.

As expected, fruit-eating birds responded to mistletoe heterogeneity by visiting parasitized trees preferentially to unparasitized ones, generating a differential deposition of mistletoe seeds on tree branches while dispersing seeds of co-fruiting species under the host canopy. Availability of understory fruits remained similar in patches of parasitized and unparasitized trees, but showed strong temporal fluctuations reflected in the seed rain. On the other hand, mistletoe was not only more copious in patches of parasitized trees, but their fruit crops varied little between years, making mistletoes reliable food resources likely to lead to consistency in fruit-deposition patterns.

In conclusion, mistletoes, by patchily growing in the canopy layer and concentrating bird-dispersed seeds underneath, can shape the spatial seed-deposition pattern of fleshy-fruited plants in the forest. Moreover, as seeds constantly reach the same deposition sites over long periods, the soil beneath the host canopy could become hotspots for community regeneration. In degraded areas, such mistletoe effects might be critical, possibly promoting recolonization and vegetation recovery through bird activity.

Image caption: In this Mediterranean pineland, parasitized trees, constituting the only (or most abundant) nutritive resource offered on the canopy layer, are particularly noticeable for frugivorous birds. Frugivores respond to mistletoe patchiness by visiting parasitized trees preferentially to unparasitized ones, driving a differential deposition of mistletoe and co-fruiting species seeds towards parasitized trees. Photo by Ugo Mellone.
You can read the article in full here.

 

Interactions under novel global change scenarios: How does ozone affect the triple interaction grass-endophyte-herbivore?

Andrea C. Ueno, Pedro E. Gundel, Marina Omacini, Claudio M. Ghersa, Lowell P. Bush andMaría Alejandra Martínez-GhersaLolium multiflorum and Rhopalosiphum padi.

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As human impact on the environment increases, biological systems, from individuals to ecosystems, are consequently exposed to novel situations. It is especially relevant to understand how symbiosis of plants with beneficial microorganisms will respond to rising atmospheric pollutants.

Tropospheric ozone is a very dynamic pollutant gas associated with human activities that causes oxidative stress on living organisms. At the same time, it has been proposed that episodic exposure to ozone might elicit plant defense mechanisms, increasing tolerance to biotic factors such as herbivory and pathogens. However, we propose that if the plant defense is conferred by a symbiotic microorganism, the opposite effect might be expected. The symbiosis between cool season grasses and fungal endophytes, which live inside the grass leaves, is a defensive mutualism since host plants obtain resistance against herbivory mediated by several fungal alkaloids. Here we show that endophyte-symbiotic and endophyte-free plants of annual ryegrass exposed to ozone for just a few hours were differently affected. We evaluated the effectiveness of the defensive mutualism by exposing the plants to herbivory by aphids. We found that ozone impaired the resistance to aphids in endophyte-symbiotic plants but it slightly increased that of endophyte-free plants. In conclusion, our work suggests that the growing incidence of ozone as a novel stress factor under some global change scenarios could change the rules of certain symbiotic interactions in nature.

Image caption: Lolium multiflorum and Rhopalosiphum padi.
You can read the article in full here.

 

Open wide! How and why gape reduces bite force in bats

Sharlene E. SantanaA Stripe-headed Round-eared Bat (Tonatia saurophila). Photo credit: Sharlene Santana.

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As the only flying mammals, bats completely rely on their skull and jaw muscles as major tools for grasping and processing food items. The outstanding diversity of bat diets, spanning fruits and insects of various sizes, to vertebrates and even blood, has resulted in the evolution of a plethora of cranial specializations that allow bats to bite with the appropriate amount of force and at the gapes necessary to consume their preferred foods. Theory predicts, however, that mammals experience trade-offs between their ability to produce a strong bite and to bite at wide gapes. First, opening the jaw at a wide gape stretches the fibers within jaw muscles, which makes their contraction weaker. Second, skull and muscle anatomies that allow for strong bites (e.g., short jaws, short muscle fibers) hinder the production of bite force at wide gapes, whereas anatomies that are more suited for biting at wide gapes (e.g., long jaws, long muscle fibers) are not optimal for producing strong bites.

Very few studies have confirmed these predictions in wild mammals, or harnessed the power of computer models to more deeply investigate this topic. This study documented the variation in bite force at low and wide gapes across free-ranging, tropical bat species, and built 3D computer models of their skulls to identify anatomical traits underlying the relationship between bite force and gape. Using these approaches, we corroborated that bites get weaker as bats have to open their mouth wider, but the drop in bite force varies substantially within and among species. In particular, bats that specialize in eating hard fruits experienced the steepest reduction in bite force at wide gapes. Computer models revealed that larger reductions in bite force at wide gapes are the product of a combination of anatomical features, including a shorter face, a larger temporalis (jaw closing) muscle, and a higher propensity for stretching this muscle during jaw opening. Altogether, these results suggest that gape-mediated changes in bite force can be explained both by behavior and cranial anatomy, and illustrate that these links are relevant for functional analyses of mammal diets.

Image caption: A Stripe-headed Round-eared Bat (Tonatia saurophila). Photo credit: Sharlene Santana.
You can read the article in full here.

 

Swimming in UV

Ensiyeh Ghanizadeh Kazerouni, Craig, E. Franklin and Frank SeebacherManly Dam, image provided by authors.

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As pleasant as it is to swim in the sunshine, as humans we know very well how damaging UV radiation from the sun can be. It turns out that we are not the only ones affected by UV radiation: small fish (mosquitofish, Gambusia holbrooki) are too. Here we show that rather than causing sunburn, UV radiation affects the metabolism and capacity to move in fish. For example, exposure to UV-B radiation at relatively low levels reduces swimming endurance of fish by up to 30-40%. This can obviously have serious consequences for foraging behaviour, dispersal, and other ecological functions in these and probably other fish that are exposed to sunshine at the water surface. Luckily, evolution has found a solution to this problem. Fish that are born in summer when the water is warm and UV-B is high are much more resilient to the damage caused by radiation than fish that are born in winter. In other words, the early developmental conditions experienced by fish prepare them for the environment they encounter later in life. There is more to the story, though, because the characteristics of fish as well as many other animals are not fixed even within adults. The process of acclimation means that physiological functions can change reversibly within individuals and thereby lessen potentially negative impacts of environmental conditions. However, our data show that the capacity for acclimation is also dependent on developmental conditions. For example, fish born in summer can cope very well with UV-B but only when chronically exposed to their favourite temperature of 28oC; winter-born fish can barely acclimate at all. Our study reveals how complex the biological effects of environmental change are, with at least two of the major global climate drivers, temperature and UV, interacting in their impact on animals. Similarly, animal responses are a complex mix of genetics and development, which affects average characteristics of animals as well as their mutability later in life. These insights are important because they reveal how animals manage to do well in variable environments, and they will help us predict what the impacts of future climate change will be.

Image caption: Manly Dam, image provided by authors.
You can read the article in full here.

 

Population growth and sequestration of plant toxins along a gradient of specialization in four aphid species on a common milkweed

Tobias Züst and Anurag A. AgrawalThe milkweed aphids Aphis nerii and Aphis asclepiadis sharing their host plant, the common milkweed Asclepias syriaca. T. Züst.

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Herbivorous insect species strongly differ in the number of plants they feed on. Generalist herbivores are often very efficient at consuming plants and reproducing rapidly, but may be susceptible to toxins which the plant produces as defensive compounds. In contrast, specialized herbivores are generally more tolerant of plant toxins, and even may gain the ability to accumulate (sequester) plant toxins in their bodies as a defense against their own enemies. Understanding the effects of plant variation on herbivore performance is thus important for predicting population dynamics of herbivorous insects.

We compared the performance of four aphid species on a set of naturally variable common milkweed plants. Aphids reproduce clonally and the four species vary greatly in their diet breadths, while milkweed plants produce variable amounts of toxic cardenolides. Populations of the two more generalist species grew the fastest overall, and both species grew best on plants growing at a high rate, thus these species were likely limited by low resources on slower-growing plants. Food consumption of aphids increased with decreasing population growth, indicating decreasing resource-use efficiency with increasing specialization.

All four aphid species contained cardenolides in their bodies, but the amount of sequestered toxin increased with degree of diet specialization. Contrary to our predictions, the only species that was negatively affected by increasing levels of cardenolides in the host plant was the most specialized aphid, perhaps due to the very high levels of cardenolides accumulating in its body. The type of cardenolides accumulated in aphid bodies indicates a mostly passive mode of sequestration, with body cardenolide content being directly related to the rate of food consumption.

Generalist aphids performed best overall and were limited only by low plant growth but not by plant toxins. Specialist aphids were not affected by variation in plant growth but appeared to use resources less efficiently. Their increased food consumption resulted in higher cardenolide sequestration to the point of causing negative effects on population growth at the very highest levels. Variation in both growth rate and cardenolide levels of milkweed plants is thus likely to determine the relative distribution of generalist and specialist herbivores in the field.

Image caption: The milkweed aphids Aphis nerii and Aphis asclepiadis sharing their host plant, the common milkweed Asclepias syriaca. T. Züst.
You can read the article in full here.

 

What features do plants use to survive drought?

Alexandria L. Pivovaroff, Sarah C. Pasquini, Mark E. De Guzman, Karrin P. Alstad, Jenessa S. Stemke, and Louis S. Santiago Photo by Louis Santiago.

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Tree mortality during drought has been documented in forests worldwide. It is thought that with global change, the frequency and intensity of extreme climatic events, such as drought, will increase. Although we have some information about the relative drought resistance of plant species, in many ecosystems, we still do not have a good physiological understanding of which species are most likely to die first. Physiologists are also currently trying to determine which mechanisms cause plants to die during drought. When water is limiting, plants can either close their stomata to conserve water at the risk of restricting photosynthetic carbon uptake, or they can open stomata to continue photosynthetic carbon uptake at the risk of losing too much water. Plants could therefore die of carbon starvation or hydraulic failure during drought, but which mechanism is more prevalent under what kind of conditions? And, what are the other traits that are important for maintaining internal carbon and water supplies during drought?

In this study, we measured seven drought survival traits to characterize how these traits combine in species and whether there were obvious drought survival strategies. We found that hydraulic architecture, the relative amount of water transport tissue and evaporative leaf surface area, was related to resistance of the stem tissue to water deficit. Yet, we also found that many traits occurred with no apparent pattern with regard to other traits, leading to a vegetation community with a large diversity of strategies for dealing with drought. This is likely because the study was conducted along a transition between two major vegetation types: California chaparral shrublands and the Mojave Desert. At the continental meeting of these two vegetation types, many species are on the edge of their range, so it is thought that areas such as the study site of this paper have a high chance of mortality during climate change. California is currently in a major drought, so if mortality begins to strengthen, the drought resistance characteristics studied in this paper should help us understand why certain species are dying and allow predictions for other sites.

Image caption: Photo by Louis Santiago.
You can read the article in full here.

 

Butterfly males can smell the mating status of females and use this information to design their ejaculate

Helena Larsdotter-Mellström, Kerstin Eriksson, Niklas Janz, Sören Nylin and Mikael A. CarlssonImage of a female Pieris napi. Photo by Mikael A Carlsson

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One of the most fascinating aspects of sexual selection is sperm competition. In many species, females mate with several partners, causing competition among sperm from the different males. One option for the males to maximise the number of fertilizations is to increase the size of the ejaculate. A large ejaculate would however be a more important investment when mating with an already mated female than with a virgin. Therefore, it would be beneficial for males to be able to assess the female mating history and accordingly adjust the size of the ejaculate.

So how can the male assess the mating status of a female? There is evidence that many animals can use odour cues, pheromones, for this purpose. A previous study showed that males of the Green Veined White butterfly, Pieris napi, transfer an anti-aphrodisiac signal, methylsalicylate, to females together with sperm at ejaculation. This compound signals to other males that the female is already mated, making her less attractive as a partner. Here, we asked if males may also use this signal to tailor the ejaculate accordingly, i.e. transfer more sperm to an already mated female than to a virgin.

We could show that males have a sense of smell that can distinguish between different concentrations of methylsalicylate. Females also have this ability but are less sensitive to it.

Next we found that males transferred a larger ejaculate to mated females than to virgins. But most interestingly, males also transferred a larger ejaculate to virgin females with artificially added anti-aphrodisiac.

Thus, we have shown a mechanism for how males can assess the mating status of females and we have furthermore shown that they can use this signal to tailor their ejaculates.

Image caption: Image of a female Pieris napi. Photo by Mikael A Carlsson
You can read the article in full here.

 

Breath of death: how a parasite favours its transmission through hijacking its host’s hypoxia-acclimation processes

Marie-Jeanne Perrot-Minnot, Matthieu Maddaleno, Frank Cezilly The thorny-headed worm Polymorphus minutus : larvae dissected from the crustacean host, and infective to the definitive host, a waterbird.

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Parasites with complex life cycles, involving more than one host, often alter the behaviour of their intermediate hosts in ways that increase their chances of getting transmitted to final hosts. For instance, the thorny-headed worm Polymorphus minutus is known to induce ‘reversed geotaxis’ in its amphipod host Gammarus roeseli, such that infected hosts swim closer to the water surface and presumably become more vulnerable to predation by aquatic birds (in which the parasite will complete its life cycle).

So far, the mechanisms underlying host manipulation by parasites remain poorly known. As hypoxia (shortage of oxygen) in aquatic invertebrates is known to induce complex physiological responses and reversed geotaxis, we conjectured that the reversal of geotaxis induced by P. minutus in amphipod hosts could result from a hypoxia-like state through two potential mechanisms. First a decrease in the metabolic rate of amphipods could be directly caused by infection with P. minutus. Second, the parasite could mimic a state of hypoxia in its host, as P. minutus has been previously shown to excrete both lactate and succinate (two end-products of its own anaerobic metabolism).

Under hypoxia, uninfected G. roeseli showed negative geotaxis and lower metabolic rate, two traits altered by infection with P. minutus, albeit with different intensities. The injection of a mixture of lactate and succinate in uninfected amphipods also mimicked the parasite-induced reversion of geotaxis, without affecting metabolic rate. In addition, both P. minutus-infected gammarids and uninfected ones conditioned to hypoxia for two days showed elevated levels of lactate in the brain, but not in the haemolymph (the fluid analogous to blood in vertebrates). Overall, our results strongly suggest that the pathways involved in anaerobic metabolism and hypoxia-signalling might be responsible for the changes in geotaxis and metabolic rate induced by P. minutus infection.

This study emphasizes the need to consider the tight and complex connections between physiological processes and behavioural adjustments, in particular at the brain level, in the understanding of parasitic manipulation, and more broadly of behavioural changes in infected hosts.

Image caption: The thorny-headed worm Polymorphus minutus : larvae dissected from the crustacean host, and infective to the definitive host, a waterbird.
You can read the article in full here.

 

Are elephant seals optimal divers?

Joffrey Jouma’a, Yves Le Bras, Gaëtan Richard, Jade Vacquié-Garcia, Baptiste Picard, Nory El Ksabi and Christophe GuinetFemale elephant seal and her pup on Kerguelen Island. Photograph by Joffrey Jouma’a.

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Optimal foraging theory offers a conceptual framework to describe and understand behavioural strategies adopted by an animal to maximise its fitness. Based on the optimisation principle, this theory aims to predict the best strategy for maximising efficient foraging under energetic and temporal constraints, e.g. where is the best place to feed on, what is the best type of prey to catch, and when to switch from one patch to another? Because marine predators must find prey in three dimensions and air-breathing divers are forced to come up to the surface to renew their oxygen store, several adaptations need to be considered to study foragers in a marine context. Things are twice as difficult, because only a few studies deal with the optimal theory applied to diving predators, and even fewer have tested in situ the associated assumptions. Here, we focus on the Southern Elephant Seal, a nearly continuous diver that can however be easily equipped with Time-Depth-Recorders (TDR) due to its annual presence on land for reproduction. Using TDR associated with accelerometers provides information on diving behaviour, such as the time spent in the foraging zone, the swimming effort or even the number of prey catch attempts. Accelerometers are a powerful tool that can also tell us about the animal’s buoyancy. We showed that elephant seals adjust precisely their time spent in the foraging zone with the depth targeted, their body condition, but also with prey encounter rate. For instance, an animal with a high body density will usually tend to stay less time in the foraging zone due to the energy required to come up to the surface. This study also highlights that using only the time spent in the foraging zone to estimate foraging success can be seriously misleading in the Southern Elephant Seal. Indeed, a long time spent in the foraging zone suggests a strong foraging success for shallow depths (>300 m), whereas at greater depths foraging success is lower.

Image caption: Female elephant seal and her pup on Kerguelen Island. Photograph by Joffrey Jouma’a.
You can read the article in full here.

 

Nonlinear responses of ecosystem carbon fluxes and water use efficiency to nitrogen addition

Dashuan Tian, Shuli Niu, Qingmin Pan, Tingting Ren, Shiping Chen, Yongfei Bai and Xingguo HanView of typical steppe, Inner Mongolia, China (left) and experimental site of nitrogen addition (right). Photo by Qingmin Pan.

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Human activities (e.g. fertilization and fossil fuel combustion) have greatly altered the global nitrogen cycle, leading to widespread reactive nitrogen inputs (nitrogen deposition), which may have profound impacts on ecosystem processes (e.g. biomass production, carbon and water cycles) and services. Therefore, it is an urgent task for scientists to explore how such elevated nitrogen inputs affect ecosystem processes. However, most previous simulation experiments dealing with the influences of nitrogen deposition on ecosystem carbon and water cycles used only two discrete levels of nitrogen treatment (i.e. with and without addition of nitrogen), which cannot reveal the likely nonlinear response of these processes to continuous nitrogen deposition in terrestrial ecosystems.

The Eurasian steppe is the largest remaining natural grassland in the world, and is subjected to increasing levels of nitrogen deposition. To evaluate the impacts of nitrogen enrichment on ecosystem functioning in this grassland ecosystem, we have started a long-term nitrogen addition experiment with multiple nitrogen input levels in a typical steppe ecosystem in Inner Mongolia of China, a representative area of Eurasian steppe, since 2000. In the current study, we focused on the responses of carbon and water processes to a gradient of nitrogen addition. We showed that both ecosystem carbon uptake and ecosystem respiration followed nonlinear patterns with increasing levels of nitrogen addition, and specifically that the magnitude of nitrogen-induced increases in these processes declined at high nitrogen levels relative to those at low nitrogen levels. In contrast, ecosystem-level water flux was not greatly affected by nitrogen addition. Therefore, the response patterns of ecosystem water use efficiency, the ratio of carbon uptake to water loss, were mainly determined by carbon rather than water processes. These findings have important implications for predicting the changes in ecosystem carbon and water balance under future nitrogen-enriched scenarios and for the management of the world’s largest natural grassland.

Image caption: View of typical steppe, Inner Mongolia, China (left) and experimental site of nitrogen addition (right). Photo by Qingmin Pan.
You can read the article in full here.

 

Small but active – pool size does not matter for carbon incorporation in belowground food webs

Johanna Pausch, Susanne Kramer, Anika Scharroba, Nicole Scheunemann, Olaf Butenschoen, Ellen Kandeler, Sven Marhan, Michael Riederer, Stefan Scheu, Yakov Kuzyakov and Liliane RuessConnectedness food web visualizing qualitative feeding relationships in the investigated arable soil (Photo credit: B. Lang).

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Soils store approximately 80% of global terrestrial carbon (C) and small changes of fluxes into and out of this pool may influence the atmospheric CO2 concentration and interact with climate change. Soil food webs are important drivers of major ecosystem functions such as C and nutrient cycling, and hence they play a central role in soil fertility as well as in C stocks. Despite this importance, the cryptic soil habitat and the complex interplay between multitudes of organisms hamper the understanding of C and energy fluxes through the soil food web.

Carbon resources enter the soil as shoot- and root-litter after plant death and as organic compounds released by living roots (rhizodeposits). In particular in agroecosystems, where aboveground plant biomass is harvested, C dynamics are largely controlled by the input of rhizodeposits. Rhizodeposits contain various substances that can easily be utilized as C and energy source by bacteria and fungi and are passed on to higher trophic levels.

To follow the flux of rhizodeposits into the food web of an arable system, maize plants were exposed to 13C-enriched CO2. 13C is a natural isotope of carbon, usually occurring in low amounts in the environment. After 13CO2 assimilation by plants, we followed the release of C from living roots into the soil and into various food web members. These were microorganisms (bacteria and fungi), microfauna (nematodes – small roundworms), mesofauna (mites and springtails) and macrofauna (spiders, beetles).

In contrast to general views we showed that the flux of root-derived C in soils is not necessarily driven by pool size, i.e., by food web members with large C stocks. Instead, members with small pool size but with high turnover rates (growth and death) dominate the C flux through the food web. This becomes evident at our arable site where easily available rhizodeposits are predominantly utilized by fungi and channelled through the fungal energy pathway to higher trophic levels. Our findings disprove the widely held idea of the dominance of the bacterial-energy channel and showed the importance of the fungal-energy channel for belowground utilization of root-derived C in arable soil.

Image caption: Connectedness food web visualizing qualitative feeding relationships in the investigated arable soil (Photo credit: B. Lang).
You can read the article in full here.

 

Nutrient foraging behaviour of four co-occuring perennial grassland plant species alone does not predict behaviour with neighbours

Gordon G. McNickle, Michael K. Deyholos, and James F. Cahill Jr.A pot with one individual of each of the four species competing in soil.

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To the untrained eye plants might appear to be more like inanimate objects than organisms that exhibit complex behavioural responses or engage in competitive games with neighbour plants. However, like any living organism, plants must solve problems in a world full of complex and ever changing stimuli. A plant that does respond adaptively to these changing stimuli should outperform a plant that does not. Indeed, if you know where to look, you will find that plants are remarkably good at assessing and responding to a variety of stimuli in ways that are often best described using behavioural models.

Here, we explored what has become known as the nutrient foraging behaviour of plants either growing alone or with neighbours. When plants are alone, it is well established that their foraging behaviour is to place more roots into nutrient rich patches than they do in nutrient poor patches. However, it is less well understood how plants forage in the presence of other plants that are competing for those nutrients in soil. We showed that the behaviour of plants grown alone does not predict the behaviour of plants grown with neighbours. Instead, plants grown with competitors engage in a sort of arms race for nutrients under competition: they produce more total roots in the presence of competitors than they do when grown alone. Broadly speaking this is a result that is predicted from game theory. Plants are playing games with each other!

Image caption: A pot with one individual of each of the four species competing in soil.
You can read the article in full here.

 

Visual and odour cues: how plants change after herbivore damage and pollination

Dani Lucas-Barbosa, Pulu Sun, Anouk Hakman, Teris A. van Beek, Joop J.A. van Loon and Marcel DickeA Hoverfly visiting a Black Mustard flower. Photograph credits: Dani Lucas-Barbosa.

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Plants in nature interact with many antagonistic organisms, such as insects that feed on them, and beneficial organisms, such as carnivorous and pollinating insects. To defend themselves against plant-feeding insects, plants can, for instance, produce odours that can be used by carnivores to find their herbivorous prey. However, odours that plants produce upon herbivore attack may repel pollinators that help the plant to reproduce, by carrying pollen from one flower to another. Thus, the attraction of pollinators may conflict with attraction of carnivorous insects. In this study, we investigated the role of plant chemistry in such a conflict. We assessed how plant chemistry changes after pollination or to damage by herbivores, and how this affects the behaviour of flower visitors. In terms of plant chemistry, we investigated how the production of odours and visual cues changes after the plant has been pollinated, and exposed to herbivore damage. Both herbivores and pollinators elicit important changes in the chemistry of flowers. Black mustard plants indeed change their odour production as well as the production of compounds that can confer colour to flowers, after having been pollinated or exposed to insect damage, or even to both. Our results show that butterflies use different cues when searching for a plant to deposit their eggs, or for a flower to feed from. Changes in chemistry, following pollination, influenced the behaviour of butterflies that feed on nectar, but not that of hoverflies that collect pollen from the flowers. We discuss the results in the context of the trade-off between plant defence and pollinator attraction, and suggest that changes after herbivory can interfere with changes after pollination. Therefore, these responses must be addressed in an integrated way because, in nature, plants are exposed to herbivores and pollinators at the same time.

Image caption: A Hoverfly visiting a Black Mustard flower. Photograph credits: Dani Lucas-Barbosa.
You can read the article in full here.

Review

Macrophysiology – a decade of novel insights

Steven L. Chown and Kevin J. GastonTiliqua rugosa (sleepy lizard) in a South Australian agricultural landscape. Macrophysiology provides a means to understand life’s responses to such impacts at broad scales.

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The investigation of variation in physiological characteristics over large geographic and temporal scales, and the ecological and evolutionary implications of this variation, has now been undertaken, as a defined field, for a decade. Known as ‘macrophysiology’ the field has contributed substantially to our understanding of the world. A notable example is the demonstration that geographic variation in the ability of animals to tolerate high temperature is much less pronounced than geographic variation in responses to low temperature. Moreover, it appears that environmental temperatures are close to tolerances in tropical and subtropical areas. In consequence, climate change may be especially problematic for species from the tropical and mid-latitudes. These concerns have now made their way into conservation policy.

This review provides a range of other examples of progress realized by the field, and then shows how macrophysiology can further help to address some of the most pressing modern challenges of environmental change. It also shows just how similar evolved responses to the environment are in plants and animals, though the benefits of comparing these groups have yet to be fully realized. We identify 10 key challenges for macrophysiology. These include better understanding of geographic variation in organismal characteristics, understanding how small scale climate variation affects plants and animals, working out how multiple interacting factors might influence populations, and exploring how modification of landscapes by urban areas and agriculture has affected plant and animal environmental responses.

We provide a comprehensive overview of the current standing of the field and its future prospects, giving any newcomer immediate access to its conceptual and methodological foundations. Easy access to the full range of work in the field is also provided along with rich graphic illustrations of its relationships to other areas of biology and the insights it has delivered.

Image caption: Tiliqua rugosa (sleepy lizard) in a South Australian agricultural landscape. Macrophysiology provides a means to understand life’s responses to such impacts at broad scales.
You can read the article in full here.

 

Experimental reduction of hematocrit affects reproductive performance in European starlings

Raime B. Fronstin, Julian K. Christians and Tony D. WilliamsEuropean starling. Image provided by authors.

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Hemoglobin plays a critical role in the transport of oxygen from the lungs to the tissues. It is therefore widely assumed that hemoglobin levels and hematocrit (the proportion of blood volume occupied by red blood cells) are positively related to aerobic performance. Indeed, in birds, hemoglobin and hematocrit increase during aerobically demanding periods such as migration. However, hematocrit and hemoglobin levels generally go down during egg production in female birds, and this decrease can be similar in magnitude to the increases associated with activities such as migration. Furthermore, these decreases can persist beyond egg production, into incubation and chick-rearing. Why would females decrease their aerobic capacity just before they have to gather food for their offspring? One hypothesis is that the decrease in hematocrit is caused by the increased estrogen levels needed to produce eggs. This could potentially mean that estrogen creates a non-resource based trade-off (i.e. one that does not depend on competition for a limiting resource) between egg production and chick-rearing. Such a trade-off would be novel and intriguing, but its existence hinges on the assumption that decreased hematocrit will adversely affect reproductive performance. To address this question, we treated free-living female European starlings (Sturnus vulgaris) with phenylhydrazine (PHZ), which destroys red blood cells. Females treated with PHZ after the removal of their first clutch took longer than controls to produce a replacement clutch, but there were no other effects on measures of reproductive performance. Females treated with PHZ during incubation had chicks that were lighter at hatch. Furthermore, in one of two years, females treated with PHZ during incubation produced fewer and smaller fledglings. The year in which PHZ affected fledgling size and number was a particularly difficult year (low annual productivity) based on other data. Our results suggest that the decrease in hematocrit that routinely occurs during egg production may exert costs on subsequent stages of reproduction, but that these costs may be exacerbated when ecological conditions are poor.

Image caption: European starling. Image provided by authors.
You can read the article in full here.

 

Climate-induced collapse of a tropical predator-prey community

Beata Ujvari, Gregory Brown, Richard Shine and Thomas MadsenBeata Ujvari and Thomas Madsen catching water pythons in the “good old days” when the snakes were common.

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If climate change threatens wildlife populations, will that be by gradual shifts in mean condition such as rainfall and/or temperature, or by increased frequency of extreme weather events? Based on long-term data (from 1991 to 2014), the aim of the present study was to analyze and compare the responses of predator (water pythons, Liasis fuscus ) and prey (dusky rats, Rattus colletti ) to extreme climatic events versus normal, albeit highly variable, climatic conditions in the Australian wet-dry tropics. From 1991 to 2005, water pythons and dusky rats showed significant climate-driven fluctuations in numbers, and annual fluctuations in rat numbers generated a corresponding variation in female python reproductive output (recruitment). Our analyses showed that annual variation in recruitment was the main factor in driving the annual variation in water python numbers. The fluctuations in rat and python numbers recorded from1991 to 2005 were, however, trivial compared to the impact of two massive but brief (24 h) deluges in 2007 and 2011. The two extreme weather events resulted in massive and rapid flooding of the dusky rat’s habitat which drowned virtually the whole rat population. As dusky rats constitute the water python’s main prey, the two floods have resulted in the pythons experiencing an unprecedented famine in 7 out of the last 8 years. The virtual lack of prey has resulted in a significant reduction in python feeding rates, reproductive output, growth rates, condition, survival, body length and importantly in python numbers, from a high of 3173 snakes recorded in 1992 to 96 in 2013. Our results demonstrate that attempts to predict animal responses to climate change, even if based on long-term studies, may be doomed to failure. Consequently, biologists may need to confront the uncomfortable truth that increased frequency of brief unpredictable bouts of extreme weather can influence populations far more than gradual deviations in mean climatic conditions.

Image caption: Beata Ujvari and Thomas Madsen catching water pythons in the “good old days” when the snakes were common.
You can read the article in full here.

 

How tadpole competition affects frog guts and feeding

Sarah S. Bouchard, Chelsea R. Jenney O’Leary, Lindsay J. Wargelin, Julie F. Charbonnier, and Karen M. WarkentinRed-eyed treefrog, Agalychnis callidryas, metamorph.  Photo credited to Karen M. Warkentin.

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The transformation of an aquatic, herbivorous tadpole into a terrestrial, carnivorous frog is a striking example of metamorphosis, involving coordinated changes in many body parts. Legs grow, the tail is absorbed, and the tiny tadpole mouth becomes the broad mouth of a frog. The digestive system, particularly the intestine, is also dramatically remodeled for the frog’s new diet, shrinking in length by about 80%. During the aquatic tadpole stage, digestive systems also vary. In ponds teeming with tadpoles, where individual tadpoles compete for food, they develop very long guts that help them extract more nutrients from their food. Despite this, they grow slowly and emerge from the water as small frogs. In contrast, when tadpoles are few and far between and food is abundant, they invest less in intestines, but are still able to grow fast and leave the water as large juvenile frogs. In the red-eyed treefrogs of Central American rainforests, small frogs that emerge from competitive pond environments grow quickly after metamorphosis, whereas large ones from resource-rich ponds grow slowly once on land. In this research we asked why, specifically testing the hypothesis that initial differences in tadpole guts, despite their radical shortening during metamorphosis, might partially persist to affect frog feeding and growth on land. We raised tadpoles in large outdoor tanks with either high or low levels of competition. Low tadpole competition resulted in big, fat frogs that took nearly two weeks after metamorphosis to begin feeding regularly. High tadpole competition produced very small, skinny frogs that were one third the size of the larger frogs. These small frogs began feeding at a high level even before they finished absorbing their tadpole tails. Despite large differences in body size, small frogs had the same length guts as large frogs. This means that differences in tadpole guts do persist after metamorphosis, despite the extensive remodeling that takes place.

Their relatively large guts allow very small frogs to eat the same amount of food as large frogs without sacrificing digestibility. Together, long guts and high feeding rates help explain why small juvenile frogs grow faster than large ones.

Image caption: Red-eyed treefrog, Agalychnis callidryas, metamorph. Photo credited to Karen M. Warkentin.
You can read the article in full here.

 

Immune response in breeding elephant seals

Hannah E. Peck, Daniel P. Costa and Daniel E. CrockerFemale northern elephant seal with suckling pup. Photo provided by authors.

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Mounting an immune response to infection requires substantial energy. During reproduction, animals must balance the energy requirements of investing in their offspring with the energy required for immune responses that will insure their health and survival. Some animals, known as capital breeders, leave their food resources during reproduction and used stored body reserves to supply these energy costs. Little is known about the impact of capital breeding strategies on the ability to resist infections in mammals. Northern elephant seals forage in the marine environment, breed in dense terrestrial colonies, and exhibit high rates of milk production while fasting completely from food. Their body reserves strongly influence how much milk they give their pups. Mobilizing fat for milk production requires elevation of cortisol, a stress hormone that frequently suppresses the immune system in other animals. We characterized their ability to resist infection by measuring a suite of immune markers in 197 blood samples from elephant seals at the beginning and end of their breeding and moult haul-outs on land. We explored impacts of breeding, body condition, and plasma cortisol on the ability to fight infections while on shore.

Immune system responses were greater and more varied among individuals during breeding. Body mass and fat reserves had positive associations with the ability to fight infections. A marker for the ability to resist infection by parasites was lower in animals with higher cortisol. These data show that coming together in dense breeding colonies increases the risk of infection and that committing energy to milk production while fasting reduces the ability of mothers to fight these infections. Elephant seal mothers need to balance the energy required to stay healthy and survive with the energy needed to make milk and produce healthy pups.

Image caption: Female northern elephant seal with suckling pup. Photo provided by authors.
You can read the article in full here.

 

Structure and functioning of intertidal food webs along a shorebird flyway

Teresa Catry, Pedro M. Lourenço, Ricardo J. Lopes, Camilo Carneiro, José A. Alves, Joana Costa, Hamid Rguibi-Idrissi, Stuart Bearhop, Theunis Piersma and José P. GranadeiroShorebirds gather in large numbers at tidal flats of Bijagós archipelago, Guinea-Bissau.

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Shorebirds are long distance migrants which during the non-breeding season depend on an intercontinental network of extremely diverse coastal wetlands at temperate, sub-tropical and tropical latitudes. In these wetlands, shorebirds gather in huge numbers and mostly feed on macroinvertebrates (e.g. worms, crustaceans and molluscs), playing a key role as predators in local food webs. These food webs include the whole network of trophic interactions among all co-existing organisms, as well as the transfer of nutrients and energy (trophic pathways) from its basal sources (e.g. plants) to primary consumers (e.g. macroinvertebrates) and their predators (e.g. shorebirds). Structure and dynamics of food webs depend on an extensive range of biotic and abiotic conditions which might vary considerably among areas.

In our study we characterized food webs using a set of community metrics based on stable isotopes. Stable isotopes (such as carbon δ13C and nitrogen δ15N) can inform the trophic ecology of consumers (i.e. what eats what), given that isotopic composition in tissues of consumers is directly linked with their diet. We compared the structure and functioning of food webs in four tidal ecosystems along a shorebird flyway: Tejo estuary (Portugal), Sidi Moussa (Morocco), Banc d’Arguin (Mauritania) and Bijagós archipelago (Guinea-Bissau). Our results suggest that food web structure is shaped by the number of trophic pathways of organic matter transfer. Indeed, the food web of Banc d’Arguin was characterized by lower trophic diversity and higher functional redundancy (higher number of community members with similar trophic roles) than the other sites, which might be related to the lack of inputs from both freshwater and nutrient-rich offshore oceanic waters.

There were also differences in the organization of shorebird communities among the study areas, which were largely coincident with the patterns found for the whole food webs. Shorebird communities of Banc d’Arguin and Bijagós archipelago showed comparatively low inter-specific niche overlap, which might result from species differently exploiting available resources. As tropical systems typically offer comparatively lower harvestable prey biomass for shorebirds, niche partitioning can be a strategy to reduce inter-specific competition. While our results highlight the trophic plasticity of species inhabiting areas with distinct environmental conditions, they also suggest that shorebirds’ community structure might serve as a proxy to describe the overall structure of tidal food webs.

Image caption: Shorebirds gather in large numbers at tidal flats of Bijagós archipelago, Guinea-Bissau.
You can read the article in full here.

 

Assessing the ability of flies to adapt to heat

Sandra Hangartner and Ary A. HoffmannDrosophila melanogaster. Photo provided by authors.

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Average temperatures are now predicted to increase by 2-4°C by the end of the century and extreme temperatures are expected to increase even further. This can threaten the survival of species if their upper thermal limits are repeatedly exceeded. Whether animals have the capacity to evolve to adapt to increasing temperatures is widely debated but still poorly understood. One of the limitations of work carried out to date is that resistance to extreme temperatures has been measured in different ways, but the relationships between components of resistance measured by these different methods is unclear.

The fruit fly, Drosophila melanogaster, has been extensively used to study thermal resistance. Previous studies have produced inconsistent results on its evolutionary capacity to increase upper thermal tolerances. We have therefore re-examined this issue using the powerful approach of selection experiments. These experiments also allow us to test the extent to which resistance can be altered and whether different components of resistance are connected or independent.

To undertake selection, we exposed the flies to high temperatures in each generation and then selected the most heat resistant flies. We also kept flies as control populations which were not exposed to any heat stress. After more than ten generations of strong selection, we tested for differences between the selected and control populations. We found that the selected flies had evolved higher heat resistance in all components measured, but the increase in resistance amounted to no more than 0.5°C. We also showed that these changes did not depend on the way in which resistance was measured.

These results suggest that while D. melanogaster flies have some evolutionary potential to become more resistant to heat, there are limits to this potential. The level of evolution detected here may be insufficient to keep up with temperature increases predicted under climate change. If evolution of upper thermal tolerance is also similarly constrained in other species, then those species that live at temperatures near their upper tolerance limit may be at a particularly high risk of extinction following climate change.

Image caption: Drosophila melanogaster. Photo provided by authors.
You can read the article in full here.

 

Individual plasticity of fish metabolic rate

Tommy Norin, Hans Malte and Timothy D. ClarkJuvenile barramundi. Photo by Timothy Clark.

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Metabolic rate can vary substantially between individual fish of the same species, size, and sex, with some individuals metabolising energy 2-3 times faster than others. This between-individual variation can have considerable ecological and evolutionary consequences as it is associated with variations in growth and other important life history traits. However, almost nothing is known of how this diversity in metabolism is impacted by the dynamically changing conditions of the natural environment. We first measured how individual barramundi, a tropical fish inhabiting estuaries and tidal creeks in Australia, differed in their resting and maximum metabolic rates, as well as in their metabolic scope for various energy-demanding tasks (termed aerobic scope) under acclimation conditions (35 ppt salinity, 29oC, 100% air saturation). Subsequently, we determined how differences in these metabolic attributes were related to the individual fish’s metabolic response (i.e. their change in metabolic rate) when faced with environmental changes in salinity, temperature, and oxygen availability. We found a close relationship between the metabolic attributes of the individual fish at their acclimation conditions and how much the fish changed their metabolic rates when environmental conditions were altered. Individuals that had elevated metabolic attributes under acclimation conditions showed little change in resting or maximum metabolic rate and aerobic scope in response to low salinity (10 ppt) and high temperature (35oC), but maximum metabolic rate and aerobic scope were greatly depressed by low oxygen availability (hypoxia; 45% air saturation). In contrast, individual barramundi with low metabolism under acclimation conditions displayed a substantial increase in resting and maximum metabolic rates, as well as in aerobic scope, in response to high temperature and (to a lesser extent) low salinity, but hypoxia had very little effect. These findings highlight the diversity of physiological responses to environmental change within a population by showing how individual fish can remain metabolically insensitive to one environmental stressor at the cost of being highly sensitive to another. This suggests that possession of certain physiological traits may be advantageous in dynamically changing habitats, depending on the combination of environmental challenges being faced.

Image caption: Juvenile barramundi. Photo by Timothy Clark.
You can read the article in full here.

 

Why do lizard dewlaps glow?

Leo J. Fleishman, Brianna Ogas, David Steinberg and Manuel LealA glowing dewlap. Photo Credit: Manuel Leal. The lizard Anolis lineatopus inhabits shaded forests throughout the island of Jamaica.  Territorial males extend a colorful throat fan – the dewlap – in visual displays that attract females and repel male rivals. The dewlap is translucent: it transmits and diffuses light striking its back surface.  In this picture the sun is located behind the animal, and the sunlight transmitted through the dewlap makes it appear to glow.  The translucent properties of the dewlap make its colors more vivid and easier to see.

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Many animals use impressive color patterns to communicate with other members of their species. Since most animals do not produce light themselves, these colored surfaces are only visible because they reflect natural light. In bright sunlight, reflected colors often appear intense and brilliant. In heavily shaded habitats, where most of the sunlight is blocked, the same colors often appear much duller, and different colors can be difficult to tell apart.

Anolis lizards are well known for their use of a colorful, expandable throat fan, called the dewlap, to attract mates and repel rivals. There are hundreds of species of anoles. They are active only during the day, but many species live in heavily shaded forest habitats where colors can be difficult to see. The dewlap is a very thin structure, and for this reason, some of the light that strikes its surface transmits through it, becoming colored and spreading in all directions as it does. If strongly lit from behind – for example when the sun is on the opposite side of the dewlap from the viewer – the dewlap appears to glow brilliantly, because the light passing through it is made colored and diffuse by the pigments in the dewlap. A color pattern of this type is referred to as translucent.

We explored the reasons why some species of anoles have evolved translucent dewlaps. Those with the most translucent dewlaps live in shaded forest habitats, where light levels are relatively low. When an anole opens its dewlap much of the light striking the back surface (opposite the viewer) passes through the dewlap. This adds to the light reflected back from the front surface (facing the viewer) and greatly increases the total light intensity reaching the viewer’s eyes. This makes the dewlap color much easier to detect, and to distinguish from other colored objects in the habitat. Mates and rivals can, therefore, quickly detect and identify the species of the displaying animal. This is the first study to demonstrate the evolutionary advantage of possessing a translucent display organ that utilizes diffuse transmitted light to increase its visibility.

Image caption: A glowing dewlap. Photo Credit: Manuel Leal. The lizard Anolis lineatopus inhabits shaded forests throughout the island of Jamaica. Territorial males extend a colorful throat fan – the dewlap – in visual displays that attract females and repel male rivals. The dewlap is translucent: it transmits and diffuses light striking its back surface. In this picture the sun is located behind the animal, and the sunlight transmitted through the dewlap makes it appear to glow. The translucent properties of the dewlap make its colors more vivid and easier to see.
You can read the article in full here or watch a video on this reserach below:

 

How should the number of leaves along branches in a plant canopy change with leaf size?

Yingxin Huang, Martin J. Lechowicz, Charles A. Price, Lei Li, Ying Wang and Daowei ZhouSongnen Grassland. Photo provided by authors.

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The three dimensional array of leaves and branches that comprise a plant canopy functions primarily as an integrated system for the capture of solar energy and the uptake of carbon dioxide in photosynthesis. The placement of leaves along branches, influenced by both leaf size and number, affects the whole plant architecture and resource economics in numerous ways. If a species has large leaves, they must be spaced farther apart to avoid self-shading. Conversely, a species with small leaves can pack more leaves onto a branch before self-shading is an issue. Given two species differing in the size of their leaves, can we predict how many leaves they will have relative to the amount of branch – their leafing intensity?

Working from established principles, we show algebraically that the relationship between leaf size and leafing intensity depends on the density of branch tissues. The allocation of biomass to leaves versus branches interacts with the density of branch tissue to determine the size and number of leaves on branches within a given volume of a plant canopy. When the relative biomass investments in leaves vs branch tissues as well as the density of branch tissues are constant, then leafing intensity will differ in simple inverse proportion to leaf size. That is, a species with leaves 10% bigger than another species will have a 10% smaller leafing intensity. Conversely, any change in the density of branch tissue will affect the relationship between leaf size and leafing intensity. Leafing intensity will still be smaller in the species with bigger leaves, but the leafing intensity will not diminish proportionately to changes in leaf size. We affirmed these predictions in comparisons of wildflowers growing in the Songnen Grasslands of northwestern China. These results emphasize the importance of considering the complex tradeoffs among traits that influence adaptive evolution of the structure of plant canopies.

Image caption: Songnen Grassland. Photo provided by authors.
You can read the article in full here.

 

Leaf traits of vascular epiphytes shift with height above the forest floor

Gunnar Petter, Katrin Wagner, Wolfgang Wanek, Eduardo Javier Sánchez Delgado, Gerhard Zotz, Juliano Sarmento Cabral, Holger KreftResearch crane at the San Lorenzo Canopy Crane Site in Panama (left). Prosthechea sp. (Orchidaceae) in flower; leaves of Serpocaulon wagneri (Polypodiaceae) and Stenospermation angustifolium (Araceae; right, from top to bottom). (Photos by G. Petter).

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Leaves control the water and carbon balance of plants. Leaf traits can thus strongly influence growth and survival of plants under different environmental conditions. In dry areas, for instance, plants with thick and leathery leaves are often more successful, illustrating a linkage between leaf traits and the environment. Environmental gradients offer great opportunities to study how environmental conditions affect leaf traits, and which traits are functionally important. Within forests, environmental conditions change gradually with height above the forest floor, from the dark and humid understory to the sunnier and drier outer canopy. Vascular epiphytes, which are plants growing non-parasitically on trees, are exposed to all these different conditions, making them particularly useful for studying how such vertical gradients affect the vertical distribution of leaf traits within and across species. A few studies have compared epiphyte traits at different heights or different positions within trees, but none so far has systematically examined changes in leaf traits along the entire height gradient.

In this study, we analyzed the relationship between several leaf traits of epiphytes and height in a Panamanian lowland rainforest. We accessed the canopy with a research crane. For most traits, the average trait value changed with height, but the pattern of change differed between traits. For instance, while the average leaf thickness increased gradually with height, the average specific leaf area (leaf area/leaf dry mass) decreased strongly only in the first meters above the forest floor. Such trends were similar within the major taxonomic groups of epiphytes (orchids, ferns, bromeliads, aroids), but mean trait values between these groups sometimes differed substantially. Interestingly, trait shifts were also observed within species, indicating that individuals of the same species can respond to environmental changes even over only a few meters of height. We also found that species with higher leaf trait variability occurred at a greater range of heights within the forest. In summary, we observed a link between leaf traits and the vertical environmental gradient within forests at community and species level, contributing to our understanding of the vertical distribution and trait composition of epiphyte communities.

Image caption: Research crane at the San Lorenzo Canopy Crane Site in Panama (left). Prosthechea sp. (Orchidaceae) in flower; leaves of Serpocaulon wagneri (Polypodiaceae) and Stenospermation angustifolium (Araceae; right, from top to bottom). (Photos by G. Petter).
You can read the article in full here.

Review

Plants respond to herbivory by producing more prickles, thorns, and spines

Kasey E. Barton Hawaiian prickly poppy. Photo provided by author.

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Anti-herbivore plant defense is highly variable, changing within plants as they grow, and in response to their environments. Induced defenses occur when plants respond to herbivory by increasing defense traits. While considerable research has examined how plant defensive chemistry changes after herbivory, often leading to more poisonous and better defended plants, relatively little is known about herbivore-induced changes in physical defenses.

Plant physical defense traits include external structures that deter herbivores such as hairs, spines, thorns, and prickles, as well as internal cellular structures, such as calcium oxalate crystals and lignin, that make leaves tough to digest. Like defensive chemicals, physical defense traits may increase following herbivory, making plants better defended against future attacks. However, because induction of physical defense traits requires the growth of new tissues (with more prickles or tougher leaves), it is slower than chemical induction, leading scientists to predict that physical trait induction is likely to be less common and of lower magnitude than chemical induction.

In this study, we performed a meta-analysis to examine general patterns in the induction of physical defense traits. All studies that have been published on physical trait induction were identified, and the data for defense traits in undamaged control vs. damaged treatment groups were extracted and analyzed together.

The results reveal that physical trait induction is common and widespread. In contrast to the prediction, the magnitude of physical trait induction (52%) was not lower than chemical trait induction (43%), and was actually slightly higher. Interestingly, not all physical defense traits are inducible to the same degree. Non-glandular hairs showed the greatest induction (mean 82%), while prickles (40%), spines (26%) and thorns (54%) showed weaker, but still significant increases. Leaf toughness did not respond to damage, perhaps because leaf toughness is more important for eco-physiological functions.

This research highlights some general patterns in plant induced responses to herbivory. However, it also reveals important gaps in our knowledge. For example, very few studies actually examine whether the induction of more prickles, thorns, and spines actually improve defense against future attacks. Therefore, the ecological and evolutionary significance of these responses remain unclear.

Image caption: Hawaiian prickly poppy. Photo provided by author.
You can read the article in full here.

 

Evolution of nutrient acquisition: when space matters

Sébastien Barot, Stefan Bornhofen, Simon Boudsocq, Xavier Raynaud and Nicolas Loeuille Grass layer of the savanna in Hwange National Park, Zimbabwe. Photo provided by authors.

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Ecologists have extensively studied plant strategies (sets of characteristics that allow plants to survive and reproduce in particular environments), and a key characteristic of these strategies is the rate at which plants acquire mineral nutrients from soils. This rate should not only strongly influence the success of plant species confronted by competition for resources, but should also have an impact on ecosystem characteristics such as the availability of mineral nutrients or primary production. Meanwhile, evolutionary ecology has paid little attention to the evolution of traits, such as the rate of nutrient uptake, that affect fitness (measured as the number of descendants) only indirectly but are closely linked to ecosystem functioning. Using a theoretical approach, we therefore studied the evolution of the rate of nutrient uptake and the consequences of this evolution for ecosystem characteristics. We built a spatially-explicit simulation model where a limiting nutrient is recycled in local patches by individual plants. The model includes both local competition for the local nutrient source in each patch and regional competition for the colonization of all patches. We show that heterogeneity in nutrient availability and limited capacity to disperse seeds mitigate the effect of competition for the local nutrient resource and allow the evolution of lower rates of nutrient uptake. Our spatially explicit model suggests that evolution in richer ecosystems selects "expensive" strategies (high acquisition, but low conservation of resources) compared to poor ecosystems. Low rates of nutrient acquisition can be considered as a form of altruism because they leave more resource available for other individuals. Our model thus suggests that the influence of spatial processes on the evolution of altruism is pervasive and is linked to key aspects of ecosystem functioning. Our work confirms that the interplay between local and regional competition is critical for the evolution of plant nutrient strategies and its effect on ecosystem properties. Our approach could be used to study the evolution of many traits allowing plants to control nutrient availability, e.g. the capacity to control the mineralization of dead organic matter or nitrification. This should be particularly important in the context of global changes because plant reactions to these changes are both ecological and evolutionary.

Image caption: Grass layer of the savanna in Hwange National Park, Zimbabwe. Photo provided by authors.
You can read the article in full here.

 

Defenders in the Tundra: Plant defense is determined by nutrient availability and elevation

Jonathan R. De Long, Maja K. Sundqvist, Michael J. Gundale, Reiner Giesler and David A. Wardle Photo credit: Paul Kardol.

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Plants use chemical and structural defense compounds to protect themselves from herbivory and harsh environmental conditions. Many of these compounds also influence decomposition rates in dead leaves and thereby control nutrient cycling and availability. A number of theories predict that alleviating nutrient limitation in the soil, namely nitrogen (N) and phosphorus (P), and reducing other environmental stressors such as harsh climatic conditions, will result in decreased production of plant chemical defenses. Basically, plants will need reduced defense against herbivory and environmental stresses when growing conditions are more favorable. In this study, we measured plant defense properties in both fresh and dead plant leaves in a N and P fertilization experiment set up at each of three elevations in Swedish subarctic tundra heath vegetation. We also measured how responses of defense properties at the plant community level to elevation and nutrient addition were driven by variation within species (i.e., different members of the same species) versus variation among species. We hypothesized that N fertilization would reduce plant defense properties and that this reduction would be greater at higher elevations where nutrients are most limited and climatic conditions are least favorable, while the effects of P fertilization would have no effect at any elevation. Broadly in line with our hypotheses, N fertilization reduced most plant defense compounds in both fresh and dead leaves, while P fertilization had few effects. The effects of N fertilization frequently varied with elevation, but in contrast to our hypothesis, these effects were strongest at the lowest (i.e., warmest) elevation. The effect of N fertilization and its interactive effect with elevation were primarily driven by variation within species, rather than by variation between different species. Our findings suggest that as temperatures warm and N availability increases due to global climate change, defense compounds in subarctic heath vegetation will decline particularly within species. These results highlight the need to consider the effects of both nutrient availability and temperature, and their interaction, in driving subarctic plant defense.

Image caption: Photo credit: Paul Kardol.
You can read the article in full here.

 

Parent-offspring co-adaptation in a wild bird

Carsten Lucass, Peter Korsten, Marcel Eens, Wendt MüllerBlue tit brood. Photo credit: Wendt Müller.

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The offspring of many animals depend on their parents’ care (like the provision of food) and have to solicit such care through specific behaviours such as begging, as exhibited by hungry nestlings in birds. Nestlings beg more intensely when they are hungrier, to stimulate their parents to bring more food. When the parents do so, the offspring thus become less hungry and so beg less in turn. Because of the tight interplay between parents and offspring, their behaviours will evolve in order to adapt to each other, leading to ‘co-adaptation’ between parents and offspring. Because behaviours are flexible and parents and offspring continuously respond to each other, it is unlikely that the overall levels of begging and food provisioning are co-adapted; it is more likely that the amounts of change in both the offspring’s begging when they become hungrier and the parents’ provisioning in response to this begging will be co-adapted. Thus, the behavioural responses of parents and offspring may be matched within families. For example, parents that are relatively unresponsive to their offspring’s begging may be better matched with very responsive offspring, because highly responsive parents would overload such responsive offspring with food, rapidly tiring themselves. To test this, we studied wild blue tits. We exchanged same-sized clutches between breeding pairs to disrupt the behavioural match between parents and their offspring. Next, we measured the provisioning responses of parents to changes in food demand of their foster brood (by temporally manipulating their brood size) and the begging responses of the nestlings in relation to different hunger levels. We found the mother’s provisioning and genetic offspring begging were unrelated. However, even though fathers were not raising their own offspring, the provisioning and begging responses of fathers and their genetic offspring (raised by foster parents) were related. Fathers that strongly respond to changes in brood demand have genetic offspring that only show weak begging responses when hungry and vice versa. This is the first study to show the evolutionary interplay between behavioural responses of both parents and offspring. The outcomes are highly relevant for improving theoretical models of parent-offspring co-adaptation.

Image caption: Blue tit brood. Photo credit: Wendt Müller.
You can read the article in full here.

 

The relationship between microhabitat use, allometry, and functional variation in the eyes of Hawaiian Megalagrion damselflies

Jeffrey Scales & Marguerite ButlerThe eyes of  Megalagrion n. nigrolineatum, an example of a Hawaiian damselfly that breeds along pools.

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The eyes are our “windows to the world”, but do they match where we live? One of the ways in which animals generate biodiversity is through the process of adaptation, for example when the beaks of birds matches the food type they specialize on. The eyes, while not as well-studied for ecological adaptation as the jaws for feeding or limbs for locomotoion, are particularly interesting because eyes are under a lot of constraints. Fundamentally, larger eyes can see better because larger eyes with larger photoreceptors can collect more light. To produce a finer image, however, requires more photoreceptors viewing the same image. Nowhere are these constraints more obvious than in the eyes of insects, with their modular organization. How this tradeoff is resolved, and how it interacts with size is a very interesting question for visual evolution, and is especially apparent in forest-dwelling flying insects, where the habitat may be dark and variably lit, and insects face additional constraints related to flight. Here we examine how differences in size and habitat use (where they live within the forest) influence the visual design of the eyes of closely related Hawaiian damselflies. While all of the Hawaiian damselflies rely heavily on vision to capture prey and find mates, they live in slightly different habitats within the forest that differ in lighting and complexity. We find that eye size is tightly linked to body size so that larger damselflies have larger eyes. However, other eye traits related to light sensitivity and visual resolution are associated with the “niches” used by damselflies. Because eye design varies with small differences in habitat, species with small eyes should be able to see as well, or better than even the largest Hawaiian damselfly species. Thus, even small species with relatively small eyes can live in dark habitats. These findings suggest that although body size plays an important role in determining eye size, eyes can be fine-tuned to match their different microhabitats, and there is sufficient variation to exploit even small scale differences.

Image caption: The eyes of Megalagrion n. nigrolineatum, an example of a Hawaiian damselfly that breeds along pools.
You can read the article in full here.

 

Evidence of trophic specialization in cave species challenges the usual prediction of generalist feeding in food-limited environments

Clémentine Francois, Florian Mermillod-Blondin, Florian Malard, Francois Fourel, Christophe Lécuyer, Christophe J. Douady and Laurent SimonSampling of groundwater organisms at Borne aux Cassots cave (France). Photo: Robert Le Pennec.

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The identification of feeding strategies along a specialist-generalist continuum is fundamental to the understanding of many ecological processes associated with food web structures and ecosystem functioning. Theory states that the amount of food available in a given environment influences these feeding strategies. In food-limited environments, generalist species are expected to occur because species may not be able to subsist when feeding only on a reduced set of food sources. However, species living in these poor environments often exhibit peculiar biological traits, such as improved food-finding abilities or low metabolic rates, which may release the constraints due to low food availability, and allow for specialist species even in these harsh habitats.

This paper tested this hypothesis of specialist species in food-limited habitats by focusing on two aquatic cave-dwelling water slaters (isopods within the Proasellus genus), as they live in one of the most food-limited environments on Earth. We used carbon and nitrogen stable isotopes (13C and 15N) to quantify the transfers of carbon and nitrogen from each food source to the organism.

The degree of specialization of these two isopod species was determined in the laboratory by measuring the rate at which they assimilated C and N when being fed separately with one of the 3 food sources available in caves: fine and coarse particulate organic matter (corresponding to fragments of leaves and wood of different sizes) and sedimentary biofilm (corresponding to the conglomeration of microorganisms developed on the surface of sediments). We demonstrated that both species assimilated far more (up to 10 times) C and N from the sedimentary biofilm than from both kinds of particulate organic matter, indicating a high degree of specialization on biofilm.

In parallel, the actual diets of these isopods were estimated for five populations (= five distinct caves) per species. All isopods fed predominantly on the sedimentary biofilm (representing in average 83 % of the diet), regardless of the population or species considered.

Our results showed for the first time in cave species a strong trophic specialization on the sedimentary biofilm. This evidence of specialist species in groundwater challenges the traditional view of the selection of generalist species in food-limited environments.

Image caption: Sampling of groundwater organisms at Borne aux Cassots cave (France). Photo: Robert Le Pennec.
You can read the article in full here.

 

Seed transport by physically active animals: farther than we think?

Casper H.A. van Leeuwen, Rosanne Beukeboom, Bart A. Nolet, Elisabeth S. Bakker, Bart J.A. PolluxCarp in tank. Photo provided by authors.

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Seeds enable plants to spread within and between patches of suitable habitat. Wind and water often transport ripe seeds to new places. However, also animals can be vectors for seeds: many animals feed on nutritious seeds or fruits, but do not completely digest all the ingested food. Numerous plants produce seeds that can still germinate after multiple hours of retention in the digestive system of animals. In case animals defecate these seeds after they have moved to a new location, plants may spread into new suitable habitat.

To estimate the impact of this dispersal mechanism on ecosystems, we need to estimate how far animals might be able to transport seeds. This largely depends on how long it takes before an ingested seed is defecated again. This is often determined experimentally by feeding animals with a known quantity of seeds, followed by monitoring seed retrieval over time. However, up until now, almost all these experiments have been performed with animals resting in cages, while animals dispersing seeds in the wild will be actively moving elsewhere by either swimming, walking or flying. We hypothesized that seed digestion in resting animals would be different than in animals engaged in the physical activity involved in moving to new habitat.

In our study we therefore compared seed retrieval patterns between animals that were resting in a cage, and actively swimming animals. We used common carp as a model species, as fish are increasingly recognized as important seed dispersers in rivers, particularly for plants colonizing upstream habitat. Physically active fish were found to retain ingested seeds for up to two hours longer than resting fish. This implies fish may transport seeds many kilometres further upstream in river systems than previously estimated. We expect that also in other seed dispersing animals physical activity will influence their seed digestion, and emphasize that this should be taken into account when making predictions of the impact of seed dispersal by animals on ecosystems.

Image caption: Carp in tank. Photo provided by authors.
You can read the article in full here.

 

Nitrogen saturation in humid tropical forests after 6 years of nitrogen and phosphorus addition: hypothesis testing

Hao Chen, Geshere A. Gurmesa, Wei Zhang, Xiaomin Zhu, Mianhai Zheng, Qinggong Mao, Tao Zhang, Jiangming MoAn old-growth tropical forest. Photo credit: Yunting Fang.

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Nitrogen (N) deposition is increasing globally causing N-saturation, where N input to forests exceeds plant and microbial demand. It has been hypothesized that this N saturation will cause increasing N leaching, nitrous oxide (N2O) emission and N transformation rates and cause limitation of other elements. However, this hypothesis has commonly been tested in temperate forests, but it is not well tested in N-saturated tropical forests.

To test this hypothesis, we measured soil inorganic N, soil N mineralization and nitrification rate, soil N2O emission rate, and nitrate (NO3-) leaching rate in an N-saturated old-growth tropical forest in south China after 6-years of N and phosphorus (P) addition.

The results showed that N addition indeed caused further N saturation, as indicated by a significant increase in soil inorganic N, N2O emission and nitrate leaching after N addition, however, N addition significantly decreased in situ rates of net N mineralization and nitrification. On the other hand, P addition significantly decreased soil inorganic N concentration, N2O emission and NO3- leaching, but it significantly increased the net rates of N mineralization and nitrification.

Our study showed that long-term N deposition in tropical forest may not induce N-saturation symptoms as observed for temperate forest, and that P addition can alleviate N-saturation in such tropical systems; hence it provides new insight into the N-saturation hypothesis.

Image caption: An old-growth tropical forest. Photo credit: Yunting Fang.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

Arboreality and associated gravitational stress on blood circulation have influenced the evolution of tail length in snakes

Coleman M. Sheehy III, James S. Albert and Harvey B. LillywhiteAn arboreal eyelash viper (Bothriechis schlegelii) resting on a branch in Costa Rica.  Photograph by Coleman M. Sheehy III.

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Gravity is a pervasive force that can severely affect blood circulation in terrestrial animals, and these effects can be particularly pronounced in tall or long organisms such as giraffes and snakes. Upright postures create vertical gradients of gravitational pressures within circulatory vessels that increase with depth. In terrestrial animals, this pressure potentially induces blood pooling and edema in the lower-most tissues and decreases blood volume reaching the head and vital organs.

Since their evolutionary origins about 100 million years ago, snakes have diversified into a wide variety of aquatic, burrowing, terrestrial, and arboreal habitats where they experience various levels of gravitational stress on blood circulation. At the extremes, these stresses range from low to none in fully aquatic species living in essentially “weightless” environments, to relatively high in climbing species, especially arboreal forms specialized for climbing trees. As a result, arboreal snakes exhibit many adaptations for countering the effects of gravity on blood circulation, including relatively tight tissue compartments in the tail. However, patterns of tail length in relation to arboreal habitats and gravity have not been previously studied.

We obtained length data for 226 snake species representing almost all snake families to test the hypothesis that arboreal snakes have longer tails than do non-climbing species. We found that average tail length increased and average body length decreased with increasing use of arboreal habitats and that arboreal snake species had average tail lengths 3–4 times longer than those of non-climbing species. Snakes with longer tails have a higher percentage of elongate blood vessels contained within the relatively tight skin of the tail, which counters blood pooling experienced during climbing. Total body length appears to be constrained in arboreal species, and total body length in adult female arboreal snakes appears to be an evolutionary tradeoff that favors longer tail lengths over maximum production of offspring as arboreal habitat-use increases. Our findings provide evidence that long tails of arboreal snakes function, at least in part, as an adaptation to counter cardiovascular stresses on blood circulation imposed by gravity.

Image caption: An arboreal eyelash viper (Bothriechis schlegelii) resting on a branch in Costa Rica. Photograph by Coleman M. Sheehy III.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

How is wood decomposition affected by nutrients in a tropical forest?

Yao Chen, Emma J. Sayer, Zhian Li, Qifeng Mo, Bi Zou, Yingwen Li, Yongzhen Ding, Xiankai Lu, Jun Wang, Jianwu Tang and Faming WangWood decomposition patterns over time. Photo from Yao Chen.

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Tropical forests represent a major store of carbon, a large proportion of which is contained in woody plant parts. The decomposition of woody debris plays an important role in the forest carbon cycle, so understanding what controls wood decay will help us determine how much carbon may be released from wood decay in future. Previous research suggests that wood decomposition is regulated in part by the availability of nutrients. This topic has recently received much attention because nitrogen inputs from the atmosphere are increasing dramatically in tropical regions as a result of human activities. To investigate how changes in nutrient inputs will affect the decomposition of woody debris in tropical forests, we conducted a fertilization experiment using branch segments from four common tree species in a lowland tropical forest in China .

Fertilization with phosphorus increased decomposition rates whereas fertilization with nitrogen had no significant effect. We observed changes in the patterns of nutrient release from the decaying branches that highlight the importance of the balance of nutrients for decomposer organisms. Our results indicate that the decomposition of wood is primarily constrained by phosphorus availability in this tropical forest. Our study suggests that it may be possible to predict decay rates using the ratio of carbon to phosphorus in tropical woody debris. Additional work with more tree species is needed to determine whether the patterns we observed are more generally applicable.

Image caption: Wood decomposition patterns over time. Photo from Yao Chen.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

Xylem vessel traits tell the timing of leaf emergence and senescence in native and non-native understory species of temperate deciduous forests

Jingjing Yin, Jason D. Fridley, Maria S. Smith and Taryn L. BauerlePhoto of the common garden at Syracuse University, Photo by Jason D. Fridley.

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Non-native understory deciduous woody species tend to advance spring leaf emergence and delay autumn leaf fall compared to native species when growing among natives in temperate forests, whereby they can exploit temporally available resources, such as water, nutrients, sunlight and pollinators. This strategy can potentially enhance growth in non-native species, affect their distribution and abundance, and ultimately cause significant changes to native ecosystems. However, prolonged leaf display also subjects non-native species to possible disruptions in their water conducting cells, due to freezing during the periods of increased frost probability in early spring and late autumn, blocking the water-conducting pathway and disrupting normal water transport capacity. Little is known about how non-native understory species are able to maintain normal water transport for leaf display within this context.

Xylem vessels are long hollow chains of dead cells, and can carry water from roots to leaves in a plant. Earlier spring leaf emergence in temperate deciduous trees has been shown to be related to narrower xylem vessels because narrower vessels are more resistant to freezing-induced disruptions. We examined the xylem vessel traits in different developmental stages of xylem formation, i.e. metaxylem (formed before true wood), earlywood and latewood, across 82 native and non-native understory deciduous woody species common to Eastern U.S. deciduous forests, and monitored their timing of leaf emergence and senescence during 2008-2010 in a common garden.

We found that compared to the natives, non-native species possessed specific xylem vessel traits that facilitated their tolerance to frost and allowed them to extend leaf display in late autumn, such as larger metaxylem vessels, smaller latewood vessel diameter and a higher proportion of solitary vessels within early- and latewood. In the spring, native and non-native species exhibited similar dates of spring budbreak and leaf emergence, linked to their similar xylem vessel size and vessel area percentage (the proportion of xylem cross section occupied by vessels) within metaxylem and earlywood. Within both groups, species with earlier bud and leaf emergence had a higher vessel area percentage within metaxylem and earlywood, suggesting understory species need sufficient water to support their early spring growth, even at the risk of freezing-induced cavitation.

Image caption: Photo of the common garden at Syracuse University, Photo by Jason D. Fridley.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

Plasticity in sexual size dimorphism

Camille Bonneaud, Erin Marnocha, Anthony Herrel, Bieke Vanhooydonck, Duncan J. Irschick and Thomas B. SmithA sagrei Bimini. Image provided by authors..

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Why do male and female animals differ in body size? Differences between males and females in body size, known as sexual size dimorphism (SSD), is one of the most fascinating and mysterious features of animals. Both different species, and different populations within a species, often vary notably in this feature, but the reasons have remained obscure. Many studies have focused on sexual selection being the primary factor driving variation in SSD. Sexual selection involves the selection of features that help to maximize reproductive success, either by attracting mates, or by outcompeting rivals of the same sex. But another possibility is that variation in resource availability, such as food, could explain much of the variation in SSD. For example, consider two populations that vary in food availability – if males and females differ in how rapidly they grow in response to increased food, then variation in food availability could explain among-population variation in SSD. We tested this hypothesis by integrating laboratory and field studies for different populations of the common Bahamian lizard Anolis sagrei. In this species, males are typically larger than females, but how much they differ varies among different populations. The Bahamas offers a wonderful test case because islands vary in size, which usually correlates with variation in food availability. We first performed laboratory studies which showed that the amount of food had a significant impact on male, but not female, growth. Then, in the field, we showed that the total amount of food biomass within an island explained variation in male, but not female, body size. Our data therefore offer a fairly simple explanation for variation within some species in SSD – namely, the amount of food available to them. This is an alternative view to the common explanation of sexual selection driving variation in SSD. Further, these findings provide support for the condition-dependence hypothesis, according to which the larger sex should display greater plasticity in growth in response to environmental conditions.

Image caption: A sagrei Bimini. Image provided by authors..
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

 

Prey density and distribution drive the three-dimensional foraging strategies of the largest filter feeder.

Jeremy A. Goldbogen, Elliott L. Hazen, Ari S. Friedlaender, John Calambokidis, Stacy L. DeRuiter, Alison K. Stimpert, and Brandon L. SouthallPhoto by Ari Friedlaender under NMFS Permit: #14534-2.

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Predators use a suite of foraging strategies to maximize their energetic gain and support their metabolism. Foraging in aquatic vertebrates can be broadly categorized into particulate feeding, where single prey items are seized and ingested, and bulk-filter feeding that involves the capture and processing of large volumes of prey-laden water. Several animal groups have independently evolved a bulk-filter feeding strategy, including cartilaginous fish (e.g. whale sharks and basking sharks) and baleen whales. Many filter feeders exhibit a ram-feeding mode where animals use their forward locomotion to drive water into the mouth where filtration occurs.

Large bulk filter feeders have long been assumed to be indiscriminate "vacuums" of the ocean, slowly filtering water regardless of variation in prey distribution, but here we reveal tremendous plasticity of foraging strategies in the world's largest filter feeder, the blue whale (Balaenoptera musculus), which is strongly a function of prey density and depth. Blue whales exhibit a unique mode of ram feeding called lunge feeding which involves the intermittent engulfment of large volumes of prey-laden water that are commensurate with the whale’s body size. Lunge feeding is a high-drag, high intake filter feeding strategy that requires high prey density for energetically efficient foraging.

We simultaneously measured whale foraging behavior and the characteristics of their sole prey resource, krill. Our analyses found that blue whales exhibit much more acrobatic lunge feeding events when foraging on small, low-density, more patchily distributed krill. In contrast, when foraging on dense, deeper, and larger krill aggregations, blue whales increased lunge frequency and maneuvered less during each lunge. These data demonstrate a previously unrecognized range of adaptable foraging strategies in a large bulk-filter feeder. Because maneuvering and diving require significant amounts of energy, the variation in foraging behavior that we revealed has major implications for optimal foraging and bioenergetic models.

Image caption: Photo by Ari Friedlaender under NMFS Permit: #14534-2.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

Links between metabolic rates and growth depend on food availability.

Sonya K. Auer, Karine Salin, Agata M. Rudolf, Graeme J. Anderson, and Neil B. MetcalfePhoto of brown trout. Copyright Sonya Auer.

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Metabolic rates determine the energetic cost of living and can differ dramatically between individuals. At the very minimum is an individual’s standard (or resting) metabolic rate (SMR) – the energy it must expend on the maintenance of tissues and bodily functions needed to sustain life. Metabolic rates are thought to have important impacts on fitness, but results thus far are equivocal. Some studies find a negative while others find a positive correlation between SMR and different measures of fitness such as growth and survival. These inconsistencies might arise because links between metabolism and fitness depend on environmental conditions. Consideration of an individual’s aerobic scope (AS), in addition to its SMR, might also improve our understanding of the links between energy metabolism and fitness. Aerobic scope (the difference between SMR and maximal metabolic rate – after exhaustive exercise) determines the extent to which an individual can increase its metabolic rate above SMR to finance key functions such as digestion, locomotion, growth and reproduction.

We examined the links between individual variation in both SMR and AS and growth rates of brown trout (Salmo trutta) under different levels of food availability. We measured the SMR and AS of 120 juvenile trout and then fed each fish either a low, intermediate, or unlimited food ration in individual tanks in the laboratory. After two weeks we measured how much they had grown and examined whether the growth rates of individuals differing in their SMR and AS depended on food level. We found that an individual’s SMR was not correlated with its AS but both metabolic traits affected growth. However, their effects depended on each other and also on food level. Growth was faster at higher food levels, but individuals with different SMR and AS performed differently at each food level such that there was no combination of SMR and AS that was associated with the fastest or slowest growth at all food levels. These results demonstrate the importance of AS in explaining growth rates and provide evidence that links between individual variation in metabolism and fitness can depend on environmental conditions, in this case food level.

Image caption: Photo of brown trout. Copyright Sonya Auer.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

 

A cross-seasonal perspective on local adaptation: Metabolic plasticity mediates responses to winter in a thermal-generalist moth

Caroline M. Williams, Wesley D. Chick & Brent J. SinclairImage provided by authors.

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Across latitudinal and altitudinal gradients, environmental conditions vary strongly. To cope with these changing conditions, populations of organisms may be adapted to their local conditions, allowing them to survive and thrive better in their home environment than would populations from other regions. In temperate regions, this local adaptation must serve the organisms across their whole lifecycle, but characteristics that enhance survival and performance in one season may be detrimental in other seasons. Thus, to understand local adaptation we need to look at survival and performance across seasons, but most studies to date have focused only on the summer growing season. We tested for local adaptation to winter conditions in a common species of moth, Hyphantria cunea, which occurs throughout North America in diverse thermal environments. We collected larvae from the northern edge and centre of their geographic range, exposed them to both northern and central winter conditions in the lab, and monitored their survival and performance throughout the winter and into the next spring. We found that indeed the populations were locally adapted to their winter environment, with higher rates of survival and larger size and carbohydrate reserves when overwintered at their home conditions. This suggests that climate change may disrupt populations of this moth from their optimal conditions, and that populations may suffer if winter and growing season temperatures become decoupled.

Image caption: Image provided by authors.
This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. You can find the As Accepted version here.

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