Lay summaries for Volume 30, Issue 8 of Functional Ecology

 

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Special Feature: The functional role of silicon in plant biology

Animal Physiological Ecology

Behavioural Ecology

Evolutionary Ecology

Community Ecology

Ecosystem Ecology

Special Feature: The functional role of silicon in plant biology - read the editorial here

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).
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Evolution of phytolith function in plants

Caroline A.E. Strömberg, Verónica S. Di Stilio and Zhaoliang SongTracing the evolution of phytolith function.

Phytoliths ("plant stones") are microscopic silica bodies that form inside and around cells in plants, for example in the leaves. Although not all plant species have phytoliths, they are widespread among the major groups of land plants alive today, and some of our most important crops (e.g. grasses such as rice) contain a lot of them. For this reason, researchers have long puzzled over what the uses of phytoliths are. Laboratory and field experiments on modern plants suggest that phytoliths may be useful to support the plant body, like a silica skeleton, and help the plant to grow faster without having to invest energy into synthesising special molecules (such as lignin) for support. Alternatively, they may defend against insect or mammalian herbivores by making it harder to access nutrients in the leaves and by wearing down the mouthparts or teeth of the herbivores chewing on them, similar to the action of sand paper. For example, it has long been assumed that grasses became so silica-rich to defend against grass-eating horses and other large mammals during the last 20 million years as grassland vegetation spread on many continents.

But did phytoliths really evolve to serve those functions? Here we test whether they did by reconstructing when and in what plant groups silica accumulation evolved and comparing this history with when, during Earth history, phytoliths may have been most useful to plants. For example, did plants start accumulating silica 350 million years ago when low atmospheric CO2 levels made it harder for them to synthesise lignin for support? Or when insects and large dinosaurs started feeding on land plants? We also look specifically at grasses to test whether they evolved to defend against horses and other large mammals. Our review shows that in most cases, we do not have enough information to say that phytoliths evolved for support or defence. One exception is grasses, which, we conclude, did not increase their phytolith accumulation to defend against large, herbivorous mammals—but perhaps to fend off insect or small mammal grazers.

Image caption: Tracing the evolution of phytolith function.
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Silicon Valley in Soils

Jean-Thomas Cornelis and Bruno DelvauxSoil processes drive the biological silicon feedback loop .

A gold mine can be found in soils worldwide - silicon (Si)! A medicine for plants and a nutrient for micro-algae which build up phytoplankton and fix carbon in oceans, it moves from rock to plant, through soil, towards the ocean, and back again to rock - the Si cycle! Until life appeared on Earth, Si was imprisoned in rock minerals, abundant but trapped. Thanks to water and plant roots, minerals can dissolve, releasing nutrients and Si in young soils. From there, plants take up nutrients like calcium to make strong cell walls, but also Si, which gives them vigor, resistance to water stress and infectious diseases. By extracting Si, plants grow better, contribute to the weathering of rocks and soil formation. Plants use Si to form silica bodies (phytoliths), made of biogenic opal - real gems! Dead leaves return phytoliths to soil, where they are more soluble than the original rock minerals. They dissolve and provide aqueous Si which can be taken up by plant roots and/or transferred to groundwater, and later to rivers and oceans.

By pumping Si from rock minerals and phytoliths, plants increase Si mobility on Earth, yet they do it at vastly different rates depending on plant species. Overall, they accelerate the transfer of Si from land to ocean, and further the evolution of soils. Soils can be strongly altered, as evidenced by deep, red, highly weathered soils in the tropics. There, the mobile Si pool is almost exclusively linked to plant and soil phytoliths because the reserve of rock minerals is exhausted. Thus the soil evolution from early to advanced weathering is a good marker of soil diversity. It parallels a large and progressive shifting of the mobile Si source from rock to plant and demonstrates the critical role of plants as the link between Si in the mineral and living worlds in terrestrial ecosystems, taking place in the soil-plant system. Its amplitude varies depending on plant and soil diversities, but it is crucial for the transfer of Si from land to ocean through the continuum of soil–groundwater–river–ocean, a flow beneficial on land and essential in the ocean.

Image caption: Soil processes drive the biological silicon feedback loop .
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The Ecology of Herbivore-induced Silicon Defences in Grasses

Susan E. Hartley and Jane L. DeGabrielVole in gass. Picture provided by authors.

Grasses have been thought of as plants that could not defend themselves against the animals (herbivores) that try and eat them, but it is now known that they contain silicon, a sharp and abrasive substance that makes grasses difficult and unpleasant for herbivores to eat. This paper summarizes what we currently understand about how this defence works in grasses, and highlights the areas which are still uncertain and so need new studies to provide additional knowledge about silicon defences.

We know that the production of these defences by grasses is affected by things like climate (temperature and rainfall), the type of soil the grass is growing in and the water available to the plant. The age of the plant and whether it has suffered previous herbivory are also important in determining the amount of silicon a plant contains. The type of damage to leaves also matters: clipping leaves with scissors produces less silicon defence in leaves than damage by real herbivores eating the leaves. We know less about how silicon defences work in large-scale field studies than we do about defences in laboratory and greenhouse experiments, but at least in the case of herbivory by voles, the amount and timing of silicon defence production are similar in all these conditions.

Different types of herbivore are affected differently by silicon – it may be that large animals are less bothered by these defences than smaller ones, but we have too few studies to be certain of this. The effects of silicon on some types of herbivore, such as kangaroos and wallabies, have not been studied at all. Our paper highlights these sorts of gaps and suggests new research areas to address what still remains unclear about the role of silicon as a plant defence, particularly in the landscape, where the effects of silicon are harder to measure.

Image caption: Vole in gass. Picture provided by authors.
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Silicon in aquatic vegetation

Jonas Schoelynck & Eric Struyf Researchers from Antwerp, Lund and Maun on their 2012 expedition to study the silicon cycle in the Okavango Delta, the largest inland delta in the World. Courtesy of D.J. Conley.

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Most people think about sand, computers or glass when they hear about silicon (Si). However, silicon intrudes much more deeply into our daily lives. It is found in drinks (e.g. beer), in crops (e.g. rice), in cosmetics (e.g. shampoo)… it is basically everywhere. This is not surprising since silicon is the second most abundant element on the planet. It is a major constituent of rocks and soils and it only slowly dissolves from this massive Si pool. This dissolved silicon is transported through rivers to the coasts, where single-celled organisms feed on it, and take large amounts of CO2 from the atmosphere during their growth. This intimate link with carbon strongly influences the climate on Earth. We now know that most of the dissolved silicon is taken up by plants during this transport from land to water. Plants store it as ‘phytoliths’, tiny silica ‘plant stones’. The painful cuts when pulling sharp grass are often caused by tiny silicified ‘razor blades’ on the grass leaf surface. In contrast to the physical and chemical dissolution from rocks, this biological control on the silicon cycle has only recently been acknowledged by science. Improving our understanding of this bio-control is important: Si is an essential nutrient that influences the health of many ecosystems. Up to now, only a few researchers have studied the effects of aquatic vegetation on the silicon cycle. Still, the knowledge available shows that aquatic vegetation can store significant amounts of the element. It alleviates stress by fast current velocities, nutrient limitation and herbivory, and potentially aids in the photosynthetic process. Si also determines decomposition processes of decaying water plants. This is especially important in large rivers and wetlands (such as the Okavango Delta in Botswana, see picture) where the majority of the silica is stored in the organic matter of the sediments. This review provides an overview of the state-of-the-art of knowledge on silicon in aquatic vegetation.

Image caption: Researchers from Antwerp, Lund and Maun on their 2012 expedition to study the silicon cycle in the Okavango Delta, the largest inland delta in the World. Courtesy of D.J. Conley.
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.

 

Collated studies show consistent responses to silicon fertiliser in stressed plants

Julia Cooke and Michelle R LeishmanTracing the evolution of phytolith function.

Silicon fertiliser is used in agriculture because it can help plants lessen the effects of environmental stresses including salinity, drought, heavy metals and nutrient deficiency. Hundreds of studies have looked at impacts of giving silicon to stressed plants, but usually for only one type of stress in one species. These studies have contributed to increasing agricultural yield, but also to understanding the mechanisms of stress relief involving silicon. However, it is hard to see if there are patterns in plant responses to silicon fertiliser across so many studies with different stresses, species and responses measured. We collated data from as many of these studies as we could to check for consistent plant responses. We tested if plant family explained differences among studies, because some families, such as the grasses, accumulate more silicon than others. We also investigated if plant responses varied with stress type as stress relief, facilitated by silicon, can occur through different mechanisms.

We compiled the data from 145 mainly agricultural studies. Selecting the most commonly reported plant responses, we used data from 125 papers in a meta-analysis, a statistical method that allows the collation of studies in an objective, robust way. We showed that adding silicon to stressed plants consistently increased biomass, photosynthetic rate and chlorophyll production. Silicon addition also significantly reduced markers of oxidative stress caused by environmental stress, but this could not be attributed to increased anti-oxidant production as we did not find a consistent increase here. Providing silicon significantly reduced the uptake of arsenic and sodium in roots and cadmium in shoots and roots, and increased potassium accumulation in shoots. We found no evidence that environmental stress induces increased uptake of silicon as is well documented following herbivory in some grasses. There was evidence that stress type and plant family are important in explaining the variation shown in some responses, but there are not enough data to check for confounding interactions between these two factors.

The importance of silicon in helping plants manage environmental stress in natural systems has barely been studied. Our consistent findings suggest an underappreciated role for silicon in environmental stress relief in plant ecology. More studies are needed to understand how and why plant responses to silicon vary, and ecological studies are needed to quantify how important this nutrient is for plant fitness in nature.

Image caption: Photo provided by authors.
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Animal Physiological Ecology

How do metabolic rate and food-deprivation affect sociability in fish?

Shaun S. Killen, Cheng Fu, Qingyi Wu, Yu-Xiang Wang and Shi-Jian FuImage provided by authors.

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Many animal species spend at least part of their time living in groups. With many eyes searching, group membership can allow animals to consistently find food. A potential drawback is that some group members can take more than their fair share of found food. Animals must weigh these benefits and costs when determining how closely they will associate with groups.

A number of factors might affect an animal’s level of sociability. Previous studies in fish have shown that a few days of fasting can cause individuals to stray from groups to decrease competition for food. In the same way, individuals with a higher metabolic rate could be less social to maximise food intake to satisfy their heightened energy demand. Unknown is how prolonged food-deprivation affects sociability. It is very common for wild fishes to experience weeks of food-deprivation during seasonal changes in food availability. The effects of longer-term food deprivation on sociability could differ drastically from the effects of shorter-term hunger.

We examined these issues in juvenile qingbo carp Spinibarbus sinensis. In the laboratory, individuals were either food-deprived for 21 days (to simulate a bout of seasonal food-deprivation), or fed a maintenance ration. Fish from each diet treatment were measured for metabolic rate and tested for sociability twice: once in the presence of a well-fed control shoal of fish and once with a food-deprived shoal.

Over the course of a 30 minute trial, fish that had been on a maintenance ration ventured further away from shoals, while food-deprived fish remained close to the shoal. This is unlike fish that have been fasted for a few days, which tend to decrease association with shoals. Prolonged food-deprivation may cause individuals to put such a high priority on food-acquisition that they need to remain with their group to help alert them to predators while they continuously forage.

Among well-fed fish, those with a higher metabolic rate were least sociable, especially when exposed to food-deprived shoals. This probably minimises competition, allowing them to satisfy an increased energetic demand while foraging. Overall, these results suggest that energy demand and food-deprivation – a challenge common for many ectothermic species – can affect individual sociability as well as the attractiveness of groups to members of their species.

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

 

Survival benefits of winter dormancy: warmer climates are associated with shorter hibernation seasons and reduced survival in rodents

Christopher Turbill and Samantha PriorCommon dormouse (Muscardinus avellanarius). Image provided by authors.

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Doing nothing can have important benefits. Many animals spend the temperate winter season in a state of dormancy. In mammals, seasonal dormancy (hibernation) is facilitated by employing periods of deep torpor, when metabolic energy expenditure is reduced to a trickle. Torpor, in combination with pre-winter fattening or food storage, permits even mouse-sized hibernators to forego all external foraging activity for a large proportion of the year.

The principal cost of activity is an increase in risk of mortality, especially from predation. Hibernation may appear to make mammals vulnerable to predation but in fact mortality rates while hibernating are five times lower compared to the active season. Enhanced survival over winter allows hibernating mammals to exhibit relatively slow life-histories (slow rates of growth and reproduction) compared to their non-hibernating counterparts.

To understand the ecological function of seasonal dormancy in mammals, we analysed published data to test for associations among local thermal climate, duration of hibernation and annual survival rate in hibernating rodents. Annual temperature is known to be linked to a negative relationship between activity and survival among lizard populations. We hypothesised that local thermal conditions might underlie an analogous pattern in annual survival rates of hibernating rodents.

We found that mean annual temperature is negatively associated with hibernation duration and annual survival rate in hibernating rodents (but not in a representative sample of non-hibernating rodents). A straight-forward explanation is the known positive effect of dormancy on survival (i.e. in colder climates, a greater proportion of the year is spent in the relative safety of hibernation). Thus seasonal dormancy has a positive effect on annual survival even in mammals. Our results suggest that shortening of winter hibernation owing to global warming will reduce annual survival rates (by 5% per 1 °C warming). Reproductive output might increase under longer growing seasons, but this compensatory effect is likely to be constrained. Our study highlights an important yet unappreciated mechanism leading to impacts of global climate change on animal populations in temperate climates.

Image caption: Common dormouse (Muscardinus avellanarius). Image provided by authors.
You can read the article in full here.

Behavioural Ecology

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.

 

Hidden Markov Models identify varied foraging tactics in a marine top predator

Alison V Towner, Vianey Leos-Barajas, Roland Langrock, Robert S. Schick, Malcolm J Smale, Tami Kaschke, Oliver J. D. Jewell, Yannis P. PapastamatiouPhoto provided by authors.

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Predators can choose to sit and wait or actively patrol for prey, which may impact the ecosystem they inhabit in different ways. The Great White shark Carcharodon carcharias is a well-documented ambush predator, with much attention drawn to its spectacular ambushing of pinniped prey (seals and sea lions) at the surface. However, like many top predators, white sharks may either remain in one location for prey or actively search by patrolling. We used statistical modelling tools to analyse individual white shark tracks, recording their behavior at the scale of meters at a coastal aggregation site in South Africa. White shark movements differed throughout the day and in their responses to chumming- a method used to attract sharks by diving vessels in the area. We were able to discern differences between male and female movement behavior and recorded individual sharks re-using the same movement tactics- indicating preferences for particular movement modes by individuals. This work provided fundamental evidence that white sharks will ambush fur seals whilst either actively patrolling or using the sit and wait strategy, demonstrating that they show more plasticity in their hunting tactics than previously assumed. As with land based predators it is likely that different individuals exert different levels of pressure in an ecosystem, and may choose a variety of tactics to find and capture prey.

Image caption: Photo provided by authors.
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Evolutionary Ecology

Evolution in space: metapopulation structure and life history evolution

Annelies De Roissart, Nicky Wybouw, David Renault, Thomas Van Leeuwen & Dries Bonte Spider mites as a model for experimental evolution (mature female, on bean). Photo credit by Gilles San Martin.

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Organisms live usually –not to say always- in heterogeneous environments. Habitat patches vary typically in size and connectivity, and this variation determines the local population dynamics, the exchange of individual among patches and the resulting synchronization of changes in population size. While there is a large body of theory on how changes in this ‘metapopulation’ structure affect demography and evolutionary dynamics, empirical evidence remains extremely scarce. Experimental metapopulations and experimental evolution are a strong tool to study these dynamics in a controlled manner.

We installed replicated experimental sets of patches connected by dispersal (metapopulations) that vary in the spatial and spatiotemporal availability of habitat. The metapopulations were inhabited by spider mites living on bean leaf patches. Spider mites are arthropod herbivores with short generation times that are known to evolve fast in response to, for instance, host plant and pesticides. They are a serious pest in greenhouses. Our experimental metapopulations reflect the dominant metapopulation types in nature: a classical metapopulation consisting of equally sized patches where resources are randomly fluctuating, a patchy metapopulation with stable patches of similar size, and mainland-island metapopulations that are characterized by stable patches that differ in size.

We earlier reported that this variation in metapopulation structure affects the local and metapopulation-level demography (< a href="http://onlinelibrary.wiley.com/doi/10.1111/1365-2656.12400/abstract">De Roissart et al. 2015 in Journal of Animal Ecology) and anticipated additional evolutionary changes in life history, physiology and gene-expression. By following a common garden approach to exclude environmental effects, we demonstrate strong evolutionary divergence in relation to metapopulation structure. Contrary to expectation from metapopulation ecology, no evolution in dispersal was found. Instead, the evolutionary changes could be attributed to local demographic variation, especially the degree of local competition and resource shortage. Patterns of life history evolution and especially changes in the expression of genes associated with several important metabolic pathways suggested that the evolutionary changes could be attributed to a general stress response. Such responses are known to allow organisms to cope with other unfamiliar stressors, and we indeed found that those mites that evolved in the stressed metapopulations performed better on this challenging host. Changes in habitat configuration and the emerging local dynamics thus induce the evolution of general stress responses that may reverse demographic threats due to other non-related environmental changes, a phenomenon known as evolutionary rescue.

Habitat fragmentation and habitat loss are a major component of global change. Our experimental work demonstrates that changes in spatial configuration of the habitat alone can induce evolutionary dynamics of the inhabiting species, which eventually affect responses towards other environmental disturbances.

Image caption: Spider mites as a model for experimental evolution (mature female, on bean). Photo credit by Gilles San Martin.
You can read the article in full here.

 

City slickers: poor performance does not deter Anolis lizards from using artificial substrates in human-modified habitats

Jason J. Kolbe, Andrew C. Battles, and Kevin J. Avilés-RodríguezA male Anolis cristatellus feeding while perched at the top of a brick wall.  Photo by Jason J. Kolbe.

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How do lizards adjust to life in the city? Urbanization transforms natural environments into a mix of buildings, roads, parks and natural habitats. Through this process, humans are creating novel environments for other organisms. For example, we add artificial substrates, such as buildings, fences, posts, and walls, which become part of the structural habitat of a city. Lizards may use these novel substrates as they do trees in natural forests, but their performance ability on these substrates may be altered.

In this study, we tested how lizards run on substrates that differ in inclination and roughness. We compared rough surfaces like the trunks and branches of trees in the forest to the smooth and vertical surfaces typical of posts and walls in the city. Then we investigated whether lizards use artificial substrates when they are available in human-modified areas. Lizards living in natural environments tend to use habitats in which they perform better. In contrast, lizards in human-modified areas do not avoid the artificial substrates on which they perform poorly. Lizards run slow as well as slip and fall on smooth, vertical surfaces, yet they often use posts and walls when available in human-modified areas. Despite their poor performance, lizards with longer limbs run faster and fall less when moving on smooth, vertical surfaces. From this relationship we predict that natural selection will favor lizards with longer limbs when they use artificial substrates in cities.

Human-induced global change such as biological invasions and urbanization may fundamentally alter the ecological relationships found for organisms living in natural environments. This makes predicting the consequences of global change extremely difficult. Moreover, human-altered environments are likely to be strong sources on natural selection for the organisms that can persist there.

Image caption: A male Anolis cristatellus feeding while perched at the top of a brick wall. Photo by Jason J. Kolbe.
You can read the article in full here.

Community Ecology

Soil microbial communities matter for carbon cycling during drought

Kate H. Orwin, Ian A. Dickie, Jamie R. Wood, Karen I. Bonner, Robert J. HoldawayDifferent land uses in the Wairau Valley, taken by Robbie Holdaway.

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Land use intensification results in changes in soil conditions (e.g. pH, total soil carbon and nutrient contents), the microbes that inhabit the soil, and the way in which soil carbon and nutrients are cycled between the soil, the atmosphere and plants. Soil microbes are the organisms that are primarily responsible for recycling the carbon and nutrients found in dead matter (leaves, roots animals). They are therefore fundamentally important for determining how much carbon is stored in the soil, and how fertile it is. However, because land use, soil conditions and soil microbial communities all change simultaneously across land use gradients, it is unclear whether the types, biomass, and diversity of soil microbes drive variation in carbon and nutrient cycling independently of land use and underlying soil resources. Further, we have a poor understanding of whether the main drivers of carbon and nutrient cycling are the same under both stable and disturbed conditions. Understanding this is particularly relevant given the projected increase in disturbance frequency (e.g. drought) under climate change. Here, we examined whether the types, biomass, and diversity of soil microbes were able to predict various measures of carbon and nutrient cycling under stable and disturbed (a simulated drought) conditions, after effects of land use and underlying soil conditions were taken into account. The land use gradient consisted of natural forest, planted forest, high- and low-producing grassland, and vineyards. Results showed that although measures of the soil microbial community were frequently correlated with carbon and nutrient cycling under stable conditions, they did not add any further predictive power once land use and soil conditions were accounted for. However, knowledge of the microbial community was essential to explain the response of carbon cycling to drought. This suggests that carbon cycling in the future may be strongly dependent on the characteristics of the microbial community.

Image caption: Different land uses in the Wairau Valley, taken by Robbie Holdaway.
You can read the article in full here.

 

Coexistence resulting from being more different or more similar?

Lin Zhao, Quan-Guo Zhang and Da-Yong ZhangBacterial colonies growing on nutrient agar plates.  Competing strains are distinguishable by their colony colors. Picture by Lin Zhao.

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The conventional wisdom of the ‘competitive exclusion principle’ in ecology stresses the importance of being different for stable species coexistence. It states that two species competing for the same niche (say, using the same resources and the same habitats) cannot coexist; and evolutionary thinking suggests that related species, when competing with each other in their overlapping ranges, will evolve toward greater differences in their niches, which in turn alleviates competition and promotes stable coexistence. The recently emerging neutral theory in ecology, however, highlights the importance of being similar for species coexistence, that is, species that use the same niche but also have the same competitive ability can co-occur for a very long time. However, how neutrality among species could emerge in the first place remains unclear. One possibility is that populations of related species that evolve in isolation show convergence in both niche use and competitive ability, and thus become ecologically equivalent competitors. Such species may form a ‘neutral community’ when they have a chance to colonize the same habitat (secondary contacts).

We carried out an experimental evolution study with laboratory bacterial populations. Bacteria can grow fast and thus their evolutionary changes can be observed in real time. Several pairs of Escherichia coli strains that showed niche differences were used to examine how evolution alters coexistence mechanisms. When bacterial strains were allowed to evolve under competition for over one thousand generations, niche differences among them were maintained. Strains that evolved in isolation showed convergence in niche use, but not in competitive ability. Therefore, our work fails to provide support for the possibility that convergent evolution creates equivalent competitors and leads to the emergence of neutral communities. The origin of neutral communities remains an open question.

Image caption: Bacterial colonies growing on nutrient agar plates. Competing strains are distinguishable by their colony colors. Picture by Lin Zhao.
You can read the article in full here.

 

Do invaders most strongly impact similar species?

Erica. J. Case, Susan Harrison, Howard V. Cornell Image provided by authors.

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Although invasive plants may eliminate some resident species at local scales, they seldom cause region-wide extinctions. One possible explanation is that highly successful invaders tend to be functionally similar to, and therefore compete most strongly with, resident species that are also relatively successful and widespread. High abundance may then buffer these functionally similar residents from complete extinction over large areas despite their stronger competition with the invader.

We examined whether greater declines among abundant species could be explained by greater resemblance to the invader in a diverse, species-rich Californian serpentine grassland invaded by Aegilops triuncialis (barb goatgrass). We calculated the relative change of each resident species’ abundance in invaded plots compared to paired uninvaded plots, and explored whether this change correlated with abundance and/or functional resemblance to Aegilops.

We found Aegilops most strongly impacted other annual grasses, which are more abundant than other functional groups. However, we did not find a relationship between regional declines, functional resemblance to the invader, or abundance in general. Additional factors, such as varying environmental conditions, must contribute to the relative scarcity of large-scale extinctions under invasion.

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

Ecosystem Ecology

Top-down becomes bottom-up: consequences of nutrient cycling for trophic cascades between green and brown webs

Kejun Zou, Elisa Thébault, Gérard Lacroix and Sébastien Barot An oxbow lake of Bandama River, Côte d’Ivoire. Photo by Gérard Lacroix.

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Food webs represent trophic (feeding) interactions among species in ecosystems. Top-down trophic cascades in food webs are key to understanding population dynamics and ecosystem functioning. For example, because fish consume zooplankton that feed on algae, the removal of fish can increase the abundance of herbivorous zooplankton and strongly decrease the abundance of algae. Such trophic cascades are widely studied in classical food web theory, where the green (based on primary producers such as plants on land and algae in water that produce organic matter through photosynthesis) and brown food webs (based on decomposers such as bacteria and fungi that break down detrital organic substances) are usually studied separately.

Studies on trophic cascades have so far ignored that nutrient cycling connects green and brown food webs. Nutrients that cannot be assimilated or are lost from organisms can either be returned directly to the nutrient pool (direct cycling), or indirectly through decomposition processes performed by decomposers of the brown food web (indirect cycling). The recycled nutrients support primary producers in the green food web but also decomposers when their growth is limited by mineral nutrients and not by carbon.

We developed a simple food web model to explore the consequences of nutrient cycling for cascading effects between green and brown webs. We found that top-down effects propagate from one web to the other in a bottom-up way to affect ecosystem production. We show that on the one hand, since direct recycling immediately supports the growth of primary producers, predators of decomposers in the brown food web can increase or decrease primary production depending on whether they release more or less nutrients directly than decomposers. On the other hand, top predators of the green food web decrease or increase decomposer production depending on whether decomposers are carbon or nutrient limited. These results could be useful to ecosystem management where human activities strongly impact nutrient fluxes worldwide. For example, they could allow the improvement of agricultural practices through the management of belowground-aboveground interactions, or taking better account of the effects of recycling processes when using biomanipulation techniques for improving water quality.

Image caption: An oxbow lake of Bandama River, Côte d’Ivoire. Photo by Gérard Lacroix.
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How do soil respiration components and their specific respiration change with forest succession?

Wenjuan Huang, Tianfeng Han, Juxiu Liu, Gangsheng Wang and Guoyi ZhouForest succession in subtropical China. Photo credited to Xuli Tang, Qianmei Zhang and Yunting Fang.

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Soil respiration usually includes two parts: heterotrophic respiration (RH) and autotrophic respiration (RA). The former refers to CO2 (or C) release through microbial decomposition of soil organic matter, while the latter primarily denotes respiration by roots themselves and microbial decomposition of carbohydrates derived from live roots. It is still unclear how soil respiration components change during forest development from its early to later stages (‘succession’). Based on Odum’s theory of ecosystem development, it has been hypothesized that the ratio of respiration to biomass (specific respiration) is likely to decrease with forest succession because forests tend to evolve towards less energy-wasting (i.e. a reduced ratio of maintenance to structure). However, this hypothesis has seldom been tested on the specific soil respiration components.

In this study, we practically separated total soil respiration into RH and RA using a trenching method in three successional forests in subtropical China. This method can effectively prevent tree roots from entering a dedicated volume of soil so that RH can be measured. The results showed that RH in the growing season was significantly greater in the old-growth forest than in two early-stage forests. RA in the old-growth forest also tended to be the highest among the three forests, but specific RH and specific RA showed a declining trend with forest succession. Our results highlighted the importance of forest succession in determining the variation of RH and RA. The relatively high efficiency of the old-growth forest may suggest an important mechanism for increasing C storage in subtropical soils.

Image caption: Forest succession in subtropical China. Photo credited to Xuli Tang, Qianmei Zhang and Yunting Fang.
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