Lay Summaries for Volume 26, Issue 6

This page includes the lay summaries for the 2012 Special feature on Invasions and Infections.

Special Feature: Invasions and Infections

 

Special Feature: Invasions and Infections

Networking: a community approach to invaders and their parasites.

Helen Roy and Lori-Jayne Lawson Handley Invasive alien harlequin ladybird, Harmonia axyridis. Image courtesy of Helen Roy

Species that establish outside of their native range, following introduction by humans, and threaten native biodiversity are called invasive alien species (IAS). The population growth rate of an IAS is determined by: resources, natural enemies (predators and parasites) and environmental factors such as climate. There are many ideas relating to IAS and their interactions with natural enemies, but most focus on interactions between two species and do not acknowledge the context of the complex communities in which these species exist. It is possible that oversimplified studies could result in misleading conclusions because of the importance of interactions with parasites, which could have knock-on effects for species with no obvious connection with the original invader.

In this paper we begin by defining the main theories and processes relating to parasite-host interactions in the context of invasion biology. We go on to describe recent advances in analytical techniques which provide ways of investigating the complexity of natural enemy interactions within invaded ecological communities. We explore one particular approach, “ecological networks analysis (ENA)”, which arguably provides the most exhaustive method for attempting to assess community interactions. We also highlight the potential for using new molecular and modelling methods to underpin ENA. The invasive alien harlequin ladybird, Harmonia axyridis, which is native to Asia but now distributed globally, provides a case-study for examining the potential of ENA to reveal the dominant mechanisms in determining invasion success. The harlequin ladybird is a very recent invader (first record in Britain in 2004) and presents a unique opportunity to chart its developing relationship with parasites old and new, and the effects this might have on native ladybirds.There are clearly many advantages of adopting a community approach, and particularly coupling ENA with new molecular and modelling tools, to assist in improving our understanding of interactions between invaders and their parasites.

Image caption: Invasive alien harlequin ladybird, Harmonia axyridis. Image courtesy of Helen Roy

 

Special Feature: Invasions and Infections

Invading with biological weapons.

Alex Strauss, Andy White and Mike BootsRed squirrel

During the Age of Exploration, European Conquistadors invaded the Americas and introduced diseases such as smallpox that decimated Native American populations Resistance to these diseases was not the only advantage that the Europeans had over Native Americans, but it certainly contributed to the speed and ubiquity of the conquest. The invading Europeans inadvertently brought biological weapons – their diseases – with them, and concurrently the emergence of disease within the native populations mediated the Europeans’ invasion. Parallel scenarios also unfold in natural communities. We describe the best studied example of these disease mediated invasions – the invasion of grey squirrels into the UK. While there is little doubt that the grey has a superior ability to compete for food, and this competitive advantage has played a large role in replacement. However, evidence suggests that a virus, squirrelpox, was also introduced along with the greys. This virus, related to the parapoxviruses, is deadly to the reds while the greys seem not to suffer unduly from infection. We describe how mathematical models have been used to show that the disease is critical to the invasion. By reviewing the literature we conclude that (1) disease mediated invasions are a widespread phenomenon and suggest that many remain unreported and studied. We finish by discussing the conservation and management implications of this neglected phenomenon.

Image caption: Red squirrel

 

Special Feature: Invasions and Infections

 

Indirect effects of parasites in invasions.

Alison M. Dunn, Mark E. Torchin, Melanie J. Hatcher, Peter M. Kotanen, Dana M. Blumenthal, James E. Byers, Courtney A.C. Coon, Victor M. Frankel,, Robert D. Holt, Ruth A. Hufbauer, Andrew R. Kanarek, Kristina A. Schierenbeck, Lorne M. Wolfe and Sarah E. PerkinsInvasive species disrupt communities in all habitats including freshwaters where invaders affect community structure and biodiversity. Parasites play key roles in invasions. They not only affect their host directly (e.g. increasing mortality), also affect their hosts indirectly by altering competition, herbivory and predation between different species.

Introduced species can disrupt biological communities and alter biodiversity worldwide when they spread extensively in novel habitats. Biological invasions are an increasing problem in this changing world, and ecologists are now considering the factors – environmental, man-made and biological – that influence their outcome. Recent research shows that parasites are a key factor influencing the success and impact of biological invasions by plants and animals. They can facilitate or limit invasions, and positively or negatively affect both native and invasive species. Parasites not only affect their host directly (by increasing mortality or reducing growth and reproduction), they also affect their hosts indirectly by altering interactions such as competition, herbivory and predation between different species. These indirect effects may occur because the parasite causes a reduction in host numbers or because the parasite causes changes in the host life history, morphology or behaviour

We investigate the importance of these indirect interactions for invasion success, and the extent to which these effects ramify throughout communities and influence ecosystems undergoing biological invasion. A review of plant and animal invasions reveals the importance of parasites in mediating both competitive and consumer-resource interactions between native and invasive species. These interactions are influenced both by parasites already present in the native range and parasites that have hitch-hiked with invasive hosts, and also by loss of parasites in invading host populations. The indirect effects of parasitic infection are important at a range of biological scales from within an individual host to the whole ecosystem.

We call for an interdisciplinary approach integrating community ecology, evolution and immunology to better understand these interactions in order to predict and manage the outcomes of biological invasions.

Image caption: Invasive species disrupt communities in all habitats including freshwaters where invaders affect community structure and biodiversity. Parasites play key roles in invasions. They not only affect their host directly (e.g. increasing mortality), also affect their hosts indirectly by altering competition, herbivory and predation between different species. Image provided by authors.

 

Special Feature: Invasions and Infections

Disease emergence and invasions.

Melanie Hatcher, Jamie Dick and Alison Dunn In Europe, the native white-clawed crayfish is endangered by an emerging disease, crayfish plague, which is carried by invasive signal crayfish.

Emerging diseases and biological invasions are two of the great challenges we face in an era of rapid global change. Mysterious new diseases afflicting people, animals and plants are being reported all the time and many of these emerging infectious diseases arise when parasites and microbes "jump" host, becoming capable of infecting and spreading through populations of new host species. In the last decade, among many other examples, we have seen the emergence of bird and swine flu, which had never been reported in humans before, and of SARs, which originated in bats and has caused deadly outbreaks in human populations across the globe. Other diseases emerge when they become more common or severe in their original populations, often as a result of environmental change. For instance, a new fungal disease of frogs is threatening many populations with extinction. This was spread around the globe by man, but increase in the disease is also linked to climate change. Both of these types of emerging disease share characteristics with biological invaders: species that colonize new territories or even move to new continents. Such biological invaders can cause huge disruption in new areas, for example, because they eat native animals. Cats, rats and snakes introduced to islands, for instance, have eaten native birds and mammals to extinction. Invading herbivores (for instance, rabbits) and plant species (such as Japanese knotweed) also cause tremendous changes in natural, managed and man-made environments. We find that emerging diseases and invaders have many ecological and evolutionary factors in common. Many emerging infectious diseases are, in fact, invaders themselves: the new territory they colonize is the new host species. Invaders and emerging diseases have to complete a sequence of similar stages to become successful. Also, we discuss similarities and differences in the response of emerging diseases and invaders to environmental change. For instance, climate change might affect disease and invader spread in similar ways. We point out that these similarities might help us tackle both new diseases and invading species. For example, if we can understand how many individuals of the disease or the invader it takes for them to establish, then we can perhaps stop them from becoming problems.

Image caption: In Europe, the native white-clawed crayfish is endangered by an emerging disease, crayfish plague, which is carried by invasive signal crayfish. Image owned and provided by authors.

 

Special Feature: Invasions and Infections

The effects of invasion on parasite dynamics and communities.

S. Telfer and K. Bown Red squirrel with grey squirrel.

Human activities can result in the transportation of species around the globe. Species introduced into new areas are referred to as invasive when they spread from the original point of introduction. Such invasions can alter ecosystems through a variety of processes. For example, the invasive species may reduce the abundance of native species (species originating in the area) by competing with them for food.

In this paper we discuss the impact that invasions can have on the prevalence of parasites and pathogens (disease causing agents such as viruses and bacteria), and explore what factors influence this impact. We consider parasites and pathogens that are themselves introduced into new areas, but also how the introduction of plant and animal species may influence the prevalence of parasites already present in the area. By altering the parasite and pathogen community in an area, invasions can change disease risks for native species, livestock and humans. For example, in the United Kingdom a virus carried by introduced grey squirrels has been highlighted as a major cause of death in the native red squirrel.

For a parasite to successfully invade it requires a population of competent hosts (species which are susceptible to infection and allow the completion of the parasite life cycle). In the USA, the presence of competent bird host species and mosquito vector species has enabled the invasion of West Nile Virus, a virus that can infect humans. Invading animal and plant species can cause native parasites to either increase or decrease in abundance. If the invader is a competent host for a native parasite, that parasite may become more common. However, if the invader is not a competent host, or it adversely affects the abundance of a native competent host through, for example, predation, the parasite may become less common.

Image caption: Transmission of squirrelpox virus from invasive grey squirrels has resulted in the decline of red squirrels in the UK. Copyright Peter Trimming and licensed for reuse under the Creative Commons Attribution-Share Alike 2.0 Generic Licence.

 

Special Feature: Invasions and Infections

Are parasites the key to invasion success in social insects?

Line Ugelvig and Sylvia Cremer  Plant covered with aphids at the university campus in Barcelona. The high aphid densities are sustained by the invasive garden ant, Lasius neglectus. Photographs taken by Line V. Ugelvig.

Social insects are known to be formidable invaders. In this review, we summarise current knowledge on how this success is shaped by the interplay of their parasites, social structure and immune systems. A general feature of species introductions is that natural parasites are left behind, leaving the introduced host free to invest fewer resources into immune defence and more into growth and reproduction. Additional factors come into play when the introduced host is a social insect. Not only do they have collective anti-parasite behaviours, which form a social immune system, some also have unique social structures (supercolonies), existing of extensive cooperative networks at the population level. We analyse how processes during introduction and after establishment can affect the degree of genetic diversity at the three levels of population, colony and individual, and how interactions among these shape disease dynamics of invasive social insects.

We suggest that parasite release is as an important factor for social insects as it is for other introduced species. Yet, how it affects resource allocation into immunity versus growth and reproduction can currently not be determined due to a lack of studies in this field. Clearly however, the combination of immune defences at the individual and colony level likely increases robustness against diseases, thereby contributing to their success as invaders. It is plausible that disease dynamics are very different in species that form supercolonies in the introduced range. Free mixing of individuals within the enormous supercolonies cancels out any genetic substructuring, and interacting individuals are thus on average less related than in ‘normal’ social insect societies. This should decrease disease transmission between individuals. However, if too little genetic variation exists at the population level, the free mixing may allow diseases to spread quickly through the population. The number of individuals being introduced and founding the population determines the degree to which genetic diversity is lost. Genetic diversity may later be restored through gene exchange with other introduced populations, however not in supercolonies. The severity of the initial genetic bottleneck may therefore be crucial in determining disease resistance of supercolonies. Development of biocontrol measures should take these idiosyncrasies into account.

Image caption: Plant covered with aphids at the university campus in Barcelona. The high aphid densities are sustained by the invasive garden ant, Lasius neglectus. Photographs taken by Line V. Ugelvig.

 

Special Feature: Invasions and Infections

Invasive species: success through altered immunity to parasites?

Thomas A White and Sarah E Perkins  Helminth parasite in the invasive bank vole.

An invasion occurs when a species arrives in a new environment, where it has previously been absent. Famous examples include cane toads in Australia and the grey squirrel in the UK. A small proportion of invasive species become very successful, the reasons for which are not always obvious. One reason that has been suggested is that invasive species escape their natural enemies (predators, competitors and parasites), and therefore can spend more of their energy in growing and reproducing rather than dealing with enemies. Of particular interest to us is the loss of parasites, which has been observed in many invasive species. Not all individuals are equally infected by parasites, so if a handful of individuals are transported to a new environment, this group may, by chance, not have particular parasite species.

In this article, we explore how parasite loss might affect the immune system of invasive species. We expect that, given the reduced parasite diversity in the invaded range, invasive species should invest less of their resources into energetically expensive aspects of the immune system, and instead invest more energy in dispersal, growth and reproduction. When individuals invade a new environment, they are also likely to lose genetic diversity that may be important in fighting parasites. Again, this is because not all individuals have the same genetic variants (alleles), so if only a handful of individuals are selected, some of the various alleles may not be present. This may be particularly important for immunity, as a large number of alleles are important for recognizing, and dealing with, different types of parasites. To summarize, we expect invasive species to allocate resources away from immunity to other things, and also genetically to be less able to deal with a large diversity of parasites. Therefore, one might expect invasive species to, ultimately, be more at risk from parasites. However, there are many complicating factors influencing immunity in invasive species. We review the available evidence, and find that much more scientific research is needed before we can make broad generalizations.

Image caption: Helminth parasite in the invasive bank vole in the Republic of Ireland

 

Forecasting extinction risk of ectotherms under climate warming: an evolutionary perspective.

Richard J. Walters, Wolf U. Blanckenhorn and David Berger Tropical rainforest.

Forecasts of the ecological impacts of global climate change are essential for conserving biodiversity and managing natural resources. Although it is inherently difficult to predict the fate of any particular species it is possible to make at least qualitative predictions regarding extinction risk in relation to habitat, life style or region. Recently it has been suggested that tropical ectotherms (“cold-blooded” organisms) are more vulnerable to a rise in temperature than temperate and polar species, which are subject to even greater rates of regional climate change. The reason is that species in the relatively stable tropics tend to operate much closer to their upper thermal limits, having evolved more specialised physiology that maximises performance over a narrower range of temperatures. Because the tropics are home to the greatest biodiversity it is imperative that this risk of extinction and the potential for adaptation to climate change are better understood.

Models of evolutionary responses to environmental change predict extinction risk in relation to a range of factors, including the degree of environmental specialisation. When applied to thermal adaptation under climate warming one key finding is that thermal specialists, i.e. tropical species, are at greatest risk of extinction because they are predicted to have the least genetic variance and therefore capacity to adapt. There are few empirical data to support this assertion directly, but some tropical species do appear to have less genetic variance for other ecological traits. Whether this reflects a restricted range size or environmental specialisation remains unclear.

One evolutionary advantage that tropical species are expected to have is a shorter generation time. We used the metabolic theory of ecology (which proposes that most observed patterns in ecology are governed by fundamental relationships between body size, body temperature and metabolic rate) to predict temperature- and body size-dependent generation times and thus to reassess the extinction risk of tropical species. Providing tropical species can adapt we predict they should have an equal or lower risk of extinction than temperate species. Our sensitivity analysis suggests this result is robust, although uncertainties remain about some underlying assumptions of the model. Here we critically assess these uncertainties to help identify where future empirical research efforts can best improve evo-ecological forecasts.

Image caption: Tropical rainforest. Photograph credited to ‘Free extras .com’ and available free for commercial use.

 

Male great tit song perch selection in response to noise-dependent female feedback

Wouter Halfwerk, Sander Bot and Hans Slabbekoorn

Accepted on: 3rd May 2012

Anthropogenic noise is omnipresent and many animals alter their behaviour in response to the masking impact on their communication signals. Songbirds respond by altering the timing, amplitude or frequency of their songs, presumably to increase detection when confronted with high noise levels. However, whether birds have to rely on social feedback to change their behaviour in noise has so far not been assessed. The use of noise-dependent social feedback could be important in adapting to noisy environments, such as cities or along busy highways.

We studied the role of female noise-dependent feedback to singing males in male-female communication in great tits. Males visit the females at their nest before sunrise at the start of the breeding season and sing continuously until their female emerges and copulates with them. We exposed females inside their nest box to artificial low-frequency traffic noise and monitored the acoustic interactions with their male mates. We also assessed male song behaviour and song post selection to see whether males, who were not exposed to the noise, could restore communication.

Females exposed to noise were found to respond more slowly to the males singing outside the nest box, but the effect of noise on female response behaviour disappeared in just two days. Males with females in noisy nest boxes sang from a perch that was about half the distance from the nest box compared to males from the control group. We also recorded louder songs at the position of the nest box for males from the noise treatment group, which was most likely related to the difference in song post use. Males did not sing songs with different frequencies, despite the fact that male great tits have been previously found to sing high songs in response to direct low-frequency noise exposure.

Our data show that males, while not exposed themselves to noise, still show noise-dependent song perch changes, revealing a critical role for females during within-pair communication. These data may be more broadly applicable as we predict that species that lack social feedback during communication may suffer more from the masking impact of anthropogenic noise.

Image caption: Male great tit singing towards female inside nest box under high levels of anthropogenic noise. Courtesy of Wouter Halfwerk.

 

Staying tuned: grasshoppers from noisy roadside habitats produce courtship signals with elevated frequency components

Ulrike Lampe,Tim Schmoll, Alexandra Franzke and Klaus Reinhold  Male of the acridid grasshopper Chorthippus biguttulus. Image courtesy of Ulrike Lampe.

Human-caused noise represents a major challenge for animals that communicate by sound. Growing cities, new airports and expanding road systems around the world produce elevated background noise levels, which have the potential to degrade or even completely mask sound signals. To ensure successful signal transmission under background noise, animals may use several different strategies. They can produce louder signals, signals with an altered frequency range, elongated signals, or they can time their signals precisely to avoid masking. All of these strategies have been found in several vertebrate species. Yet, we were unable to find studies concerned with the effects of human-caused noise on insect communication.

We examined potential long-term effects of road noise on signal production in the grasshopper Chorthippus biguttulus, an insect with acoustic sexual communication that is well suited for laboratory experimentation. We collected grasshoppers from eight pairs of geographically matched populations; half of them from roadside habitats, the other half from rural - and therefore less noisy - control habitats. Males were housed in the laboratory for 24-72 h before they were recorded under standardized, quiet conditions.

Comparing courtship songs of males from roadside and control habitats, we found that roadside grasshoppers produced signals with higher frequency components in the low-frequency band of their signals (i.e. 6-9 kHz). Low-frequency road noise at major highways is easily loud enough to degrade or mask this part of the grasshopper signal spectrum. Fine-tuning song frequency upwards would allow grasshoppers to shift their signals to more “private” frequency ranges. Furthermore, we found that song frequency increased with increasing time since highway construction, suggesting local adaptations may be responsible for the observed effects. Our results represent the first evidence for long-term effects of human-caused noise on communication by sound in an insect species and illustrates that similar strategies to avoid masking by noise are used by a wide range of animals.

Image Caption: Male of the acridid grasshopper Chorthippus biguttulus. Image courtesy of Ulrike Lampe.

 

Quantifying the functional responses of vegetation to drought and oxygen stress in temperate ecosystems.

J.C. Douma, V. Bardin, R.P. Bartholomeus and P.M. van Bodegom One of dune slack valleys where species traits were measured. Image credited to V. Bardin.

A lot of research has been devoted to understanding why plant species grow at a particular location and not at other locations. This understanding has progressed substantially by relating the traits of species to the abiotic conditions they grow in. Plant traits are characteristics that express how a plant deals with its environment. How plant traits vary along gradients of soil fertility or disturbance is relatively well understood, but this is less so for water availability.

We investigated how the traits of plants change along a gradient of water, from a limited amount of water (leading to stress by drought) to an excess of water (i.e. waterlogging, leading to stress by a lack of oxygen in the soil). We investigated this for fifteen plant traits: eight leaf traits, two root traits, two seed traits, and three traits related to the allometry of the plants growing in 171 plots in the Netherlands. These plots span a large gradient from drought stressed to oxygen stressed.

We show that species that grow under oxygen stressed conditions do have different traits than species from drought stressed conditions. For example, species growing under oxygen stressed conditions have on average much more porous roots than species from dry conditions. Increased root porosity is an adaptation to transport oxygen from the air down to the roots. Another important adaptation involves the capacity of the seeds to float in water. Species found in wet places more often have seeds that have a high floating capacity than species from dry places. This is an adaptation to disperse seeds to other places via water. The others traits that were studied in relation to water availability showed a much weaker response. Yet, the relationships were much stronger when a suite of traits was analysed simultaneously. This suggests that there are multiple ways to cope with drought and oxygen stress. The relatively weak relationships found between traits and water-related stressors contrasts with the strong control of other environmental drivers (such as disturbance and nutrients) on traits and suggests that these strong constraints imposed by other environmental drivers necessitate varied solutions to cope with water availability.

Image caption: V. Bardin – One of dune slack valleys where species traits were measured

 

The effects of changing temperature and soil water regimes on soil-microbial-plant processes in a high Arctic environment.

H. C. Glanville, P.W. Hill, L.D. Maccarone, P. Golyshin, D.V. Murphy and D.L. Jones Image caption: Measuring soil respiration on a high Arctic moraine, Ny-Ålesund, Svalbard. Picture courtesy of Dan Murphy.

The Arctic is often perceived to be a barren, un-vegetated, snowy landscape. However, there is a lot more to the Arctic than first meets the eye, especially below-ground, where plants and microorganisms are adapted to life under these extreme conditions. The soil contains a large reservoir of organic carbon and while frozen and under snow cover, it is largely protected from microbial decomposition and release of greenhouses gases. Alarmingly, over the past 100 years, the Arctic has warmed at twice the average global rate, which is causing reduced snow cover and thawing of the soil; thus exposing this organic carbon store and making it vulnerable to microbial decomposition. In addition, rain and snow fall is projected to increase in Arctic areas. Predicting how Arctic soils will respond to climate change, in particular to changing temperature and soil water regimes, remains difficult because of our poor understanding of fundamental soil-microbial-plant processes in polar environments.

Using field and laboratory studies, we investigated how vegetation emergence, below-ground microbial communities and nutrient concentrations were affected by snow melt in the Arctic. Our results, in the field, showed vegetation and below-ground microbial communities responded rapidly to snow melt, corresponding with a peak of nutrients being released immediately following snow melt. Our field data suggest that this immediate response is largely driven by temperature change as soil water content remained relatively constant. We also investigated, in the laboratory, how the breakdown of different carbon compounds is influenced by two key environmental variables; temperature and soil water content. We propose that early in the growing season, temperature is likely to be the main regulator of carbon cycling in the Arctic, with soil water content becoming more important as the growing season progresses. This means that with the combined projected increase in temperature, rain and snow fall, there is the potential for the large soil carbon reservoir to be broken down and released from Arctic soils to the atmosphere as greenhouse gases.

Image caption: Measuring soil respiration on a high Arctic moraine, Ny-Ålesund, Svalbard. Picture courtesy of Dan Murphy.

 

The evolution of floral mimicry: identifying traits that visually attract pollinators

 

Jana Jersáková, Andreas Jürgens, Petr Šmilauer and Steven D. Johnson Long-tongued tabanid fly Philoliche aethiopica probing the blue artificial inflorescence.

The orchid family is renowned for its unusually high frequency of species that provide no nectar reward and employ various mechanisms to deceive pollinators. One strategy, termed Batesian floral mimicry, involves orchids that have an uncanny resemblance to rewarding plants that grow with the rewarding species and share their pollinators, which are literally unable to distinguish a mimic from the model. The floral traits critical for pollinator attraction in this specialized relationship are not yet well understood, although colour, shape and scent have all been implicated.

We presented artificial flowers varying in spectral and shape properties to pollinators under field conditions to identify traits that are required for the evolution of Batesian floral mimicry. As a study system, we used the non-rewarding orchid Disa pulchra which mimics the iris Watsonia lepida which has pink flowers arranged in racemes and which is pollinated by the long-tongued tabanid fly Philoliche aethiopica. At some sites this fly also feeds on nectar-providing Agapanthus campanulatus which has blue flowers arranged in umbels. We asked how similar the floral traits of D. pulchra would need to be to those of local rewarding plants (either W. lepida or A. campanulatus) in order to elicit visits by P. aethiopica. The traits examined were 1) spectral reflectance, 2) colour brightness, 3) inflorescence architecture, 4) flower shape and 5) presence of nectar guides.

Our results demonstrate that colour matching is essential for mimicry as flies intensively visited and probed plastic flowers of colours indistinguishable in a fly vision model from those of the rewarding plants. Inflorescence architecture and flower brightness made little difference to fly attraction, but those that matched the shape and nectar guides of Watsonia flowers were significantly more attractive. Flowers of the three plant species are weakly scented and divergent in scent chemistry, yet flies probed some of the plastic flowers as readily as they did those of the plants, suggesting that scent is not as important as visual cues in this system.

Our experiments help to explain why deceptive orchids in general often seem to match the flowers of rewarding plants more in visual attributes than in scent chemistry.

Image caption: Long-tongued tabanid fly Philoliche aethiopica probing the blue artificial inflorescence. Image courtesy of Jana Jersáková

 

Does the leaf economic spectrum hold within local species pools across varying environmental conditions?

Justin P. Wright and Ariana Sutton-Grier  Pictures of experiment.  Pictures provided by authors.

One of ecology’s challenges is predicting the consequences of species losses or invasions on the way that ecosystems work. Although every species is unique in some respects, recent theory suggests that we might be able to predict the effects of species based on a few basic biological traits. If this is so, we might be able to predict, for example, that a species with a unique value for a trait (e.g. having leaves with a very high photosynthesis rate) would have a bigger impact on an ecosystem than a species with trait values that are similar to species already present in the ecosystem. There has been considerable interest in trying to find general patterns in how different traits are related to each other. For example, recent analysis of global datasets has shown that plants are constrained in how they can allocate resources to their leaves – there appear to be trade-offs between building sturdy leaves with a long lifespan but low efficiency in capturing light, or building flimsy leaves with a short lifespan but a high efficiency. However, it is unknown whether similar patterns occur at more local scales, particularly when you consider the same plant species growing under different conditions. In this study, we examined the effects of varying nitrogen availability and water table depth on the form and function of leaves of over 20 species of wetland plants.

We found that species responded to changes in environmental conditions in different ways – some changed their leaves substantially, while others grew the same type of leaves under all conditions. As a result of these idiosyncratic leaf responses, we found weak evidence for the relationships between different leaf traits that have been observed at global scales. These results suggest that trying to predict what kind of leaves a species will make in different environments is likely to be more complicated than previously thought. It looks like ecologists will need to conduct further research before we can successfully predict how the addition or subtraction of species will affect the way that ecosystems function.

Picture provided by authors

 

A quantitative framework for ovarian dynamics.

Romain Richard and Jerome CasasFemale Nasonia vitripennis ovipositing on a fly pupa. Photograph taken by Peter Koomen.

In many species, ovarian production is a highly dynamic process. Not only is this process influenced by both physiological and ecological factors, but it also largely affects individual reproductive and foraging behaviour. Historically, scientific interest in these issues has led to one of the first successful example of integration between the fields of physiology, ecology, and evolution through the study of insect foraging ecology. Despite the great deal of effort that has been spent understanding how the many factors involved affect each other, and despite the quantitative nature of the problem, few attempts have been made in rigorously quantifying ovarian dynamics, and no model was ever proposed to take into account the flow of nutrients back in the organism due to the resorption of eggs.

In this study we propose a simple modeling approach that can be used to tackle these issues in more depth. Despite the apparent simplicity of the processes included, a review of the literature shows that confusions are commonly encountered, a state of affairs that is reflected by the almost-complete lack of quantitative estimation of ovarian production and resorption rates. We apply this framework on physiological data involving the ovarian dynamics of the jewel wasp Nasonia vitripennis subjected to different conditions of host and food availability. We found that the dynamics between egg production, resorption, and oviposition can generally be modeled in a very simple way.

Our methodology is very general and can also be used for studying the reproductive biology of a wide range of organisms, including plants. An asset of the approach lies in its extensive inferential power that can lead the way to new and refreshing insights that may be of interest to both physiologists dealing with reproduction and energy metabolism, as well as ecologists dealing with reproductive strategies, life history evolution or individual based population dynamics.

Image caption: Female Nasonia vitripennis ovipositing on a fly pupa. Photograph taken by Peter Koomen.

 

Parental diet has strong trans-generational effects on offspring immunity.

Alison M. Triggs & Robert J. Knell Adult Plodia interpunctella.

Over the past few years research has shown that transgenerational effects – when the environment experienced by a parent affects their offspring – can be surprisingly important. In rats, for example, we know that a poor diet for a mother leaves her offspring more likely to develop diabetes. Diet can also have a big effect on immune response (the ability to respond to infections or potentially harmful substances), so we tested whether a poor parental diet can lead to a reduced immune response in the offspring.

We used a moth called the Indian meal moth, Plodia interpunctella, as a model organism for our experiments. A parental generation was reared on either their normal lab diet or on a poor diet with less fats and protein, and their offspring divided into two groups with half reared on each type of diet. Once the offspring were close to pupation we sampled their haemolymph (insect blood) and measured two indicators of immune function: the number of haemocytes (cells which are similar in some ways to white blood cells in vertebrates) and the activity of an enzyme called phenoloxidase (PO) which is important in the insect immune response.

We found parental diet to have unexpectedly strong effects on the immune response of the offspring. In the case of PO activity the effect of the parental diet was as strong as that of the diet eaten by the offspring: even if only one parent had eaten the poor diet, PO activity was sharply reduced whether the parent in question was the mother or the father. Haemocyte count was also reduced when the parents had eaten the poor diet, although not as much as PO activity.

With the current rate of environmental change it is more important than ever to know how environmental stress can change an animal’s or plant’s biology. The results of this experiment show us that we need to consider the environmental stresses placed on the parents of an organism, as well as those that it is currently experiencing, if we want to understand how environmental change will affect immunity and the spread of disease.

Image caption: Adult Plodia interpunctella. Picture courtesy of the authors.

 

Responses to climate change vary within species.

Frank Seebacher, Sebastian Holmes, Nicholas J. Roosen, Morgane Nouvian , Robbie S. Wilson and Ashley J. W. Ward Mosquitofish.

The earth’s climate is naturally variable, and over evolutionary time organisms have adapted to cope with this variability. Human-induced global warming, however, is rapid and presents a serious challenge for animals; they must respond quickly to temperature change or suffer negative consequences. Animals do have the capacity to compensate for changing environmental temperatures within their lifetime, and this “thermal acclimation” may be one of the most effective mechanisms that confer resilience to climate change. An important question therefore is to what extent species can acclimate, particularly considering that species comprise many different and often separated populations. We addressed this question by studying different populations of mosquitofish. This small freshwater fish, originally from the southern USA, has been introduced into many parts of the world over the last century in an (unsuccessful) attempt to combat mosquitos. We show that populations of mosquitofish occurring over a relatively small geographic range in Australia are genetically distinct. We identified two distinct genetic lineages, which may represent different introductions. But to our surprise, there were also pronounced differences between populations within these lineages, both genetically and in the capacity of individuals to acclimate to changing temperatures. This diversity of responses within the species is partly due to genetic differences between populations and their varying evolutionary history, and partly to unique responses of populations to their environmental conditions, although the climate currently experienced by different populations was a poor predictor of acclimation ability. The implications of our findings are that using “species” as the smallest currency for wildlife management and to predict the future impact of climate change may produce misleading results, because it does not capture the true diversity of responses that animals possess in the face of changing climates.

Image Caption: Mosquitofish, image provided by authors.

 

A specialist root herbivore reduces plant resistance and uses an induced plant volatile to aggregate in a density dependent manner.

Christelle A. M. Robert, Matthias Erb, Bruce E. Hibbard, B. Wade French, Claudia Zwahlen and Ted C. J. Turlings  Diabrotica undecimpunctata larvae feedingon a germinating maize kernel.

Plants possess a reactive immune system that helps them to fend off attacking herbivores. The ecological and agricultural importance of this phenomenon is well documented above ground, but little is known about the reaction of below ground tissues to insect herbivores, despite the fact that root feeders are among the most important agricultural pests and are also important in natural ecosystems. In this study, we aimed at understanding how roots of maize plants respond to an attack by the western corn rootworm (Diabrotica virgifera virgifera), an economically important maize pest that is very well adapted to its host plant.

Surprisingly, we found that D. virgifera actually benefits from feeding on attacked plants: second instar (developmental stage) larvae grew better on previously attacked plants, and larvae that fed in medium-sized groups showed improved performance both in the laboratory and the field. Subsequent experiments showed that the improved growth is due to metabolic changes in the roots that are favorable for D. virgifera. In particular, we were able to show that attacked roots have higher levels of free amino acids, and that their defensive system becomes less reactive to subsequent attack.

We also found that D. virgifera larvae find already infested roots by using volatile chemicals that are emitted by roots in response to insect attack. They do so in a density-dependent manner: plants that are already attacked by a high-number of herbivores are avoided, but plants that carry an intermediate number of herbivores are attractive to D. virgifera. Overall, the results suggest that D. virgifera is able to locate the “best” host plants using one particular signal, the sesquiterpene (E)-β-caryophyllene, in a density- and concentration dependent manner. Using this signal enables the herbivore to aggregate on plants in an optimal density.

 

Taken together, our study shows how well adapted D. virgifera is to the induced metabolic changes and signals of maize plants. To us, these findings constitute an important piece of the puzzle that explains the extraordinary success of D. virgifera as a global maize pest.

 

Image caption: Diabrotica undecimpunctata larvae feeding on a germinating maize kernel.

 

When herbivores come back: effects of repeated damage on induced resistance

Nora Underwood  Spodoptera exigua feeding on tomato. Photo credit: Brian Inouye.

Plants are often subject to damage from herbivorous insects which can decrease individual plant growth and influence plant populations. Damage from insects is particularly important in agricultural systems where it can strongly affect yield. Plants can respond to this damage by increasing (or decreasing) their resistance to insects. Most previous studies of how resistance changes after insect attack have focused on responses to single attacks, but the consequences of changes in resistance for plant and herbivore individuals and populations will depend on both the response of a plant to a particular attack at a given moment, and on how plants respond through time to varying levels of damage and varying numbers of attacks. Few previous studies have examined the cumulative level of resistance after multiple attacks, and little is known about how the plant’s response to damage changes with subsequent attacks. This study reports on two experiments addressing the consequences of repeated damage for resistance of tomato plants to a common pest, the beet armyworm. The first experiment documented how plant resistance changed following a single damage event. The second experiment examined whether the plant responded differently to one versus two damage events. Results show that tomato plants significantly increased their resistance to armyworms by one day after damage and remained at this higher resistance level until 15 or 20 days later, suggesting that repeated damage during the response to initial damage is likely. Plants receiving a second bout of damage were able to further increase their resistance level over the level reached in response to a first bout of damage, but the magnitude of response to the second damage event was initially smaller and slower than the response to a single damage event. Results of this study suggest that plants can respond to repeated damage, but that there is some limit on these responses. Such limits on total plant resistance will affect the influence of induced resistance on herbivore populations, and are consistent with assumptions of existing mathematical models of effects of plant resistance on herbivore population growth.

Image caption: Spodoptera exigua feeding on tomato Photo credit: Brian Inouye

 

Long-term water addition has limited effects on plant community structure in native tallgrass prairie

Scott Collins, Sally Koerner, Jennifer Plaut, Jordan Okie, Daniel Brese, Laura Calabrese, Alejandra Carvajal and Ryan Evansen

Debate continues about how changes in precipitation might affect plant communities. Models predict that year-to-year changes in growing season precipitation will have a greater impact on plant communities than increased precipitation variability within a growing season. Yet, most manipulative experiments either vary within-season precipitation variability, or reduce total growing season precipitation. However, in the northern Great Plains, where we conducted our study, growing season precipitation is predicted to increase with climate change. We conducted a 19-year long experiment in which we used an irrigation system to increase total growing season precipitation in upland and lowland areas of a native tallgrass prairie in Kansas, USA. Frequent fires maintain tallgrass prairie vegetation in this region, but increased annual precipitation could favor invasion by shrubs and trees.

Total cover of grassland plants increased over time with irrigation in uplands and lowlands. However, the total number of plant species did not change with irrigation. This is surprising because irrigation has been shown to increase aboveground plant biomass, and species diversity tends to decline in this grassland as biomass increases. The strongest response occurred within the irrigated lowland where after seven years of irrigation one long-lived, clonal grass, switchgrass, increased dramatically in abundance and thereafter became the most abundant species in this treatment. However, big bluestem, another long-lived clonal grass, remained the dominant species in irrigated uplands and control areas. Our results suggest that increased precipitation during the growing season as a result of climate change will have limited effects on plant community composition in frequently burned tallgrass prairie. These systems are "buffered" by strong responses by tall, clonal, long-lived grasses; one of these grasses may be largely replaced by another, but the structure of the community remains unchanged.

Image caption: Photo of study site at Konza Prairie, Kansas, USA, looking along one of the irrigation transects running from upland to lowland prairie. Courtesy of Scott Collins.

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