Lay Summaries for Volume 27, Issue 1

This page includes the lay summaries for the 2013 Special feature on The Ecology of Stress.

Special Feature: The Ecology of Stress

Other articles

 

Special Feature: The Ecology of Stress

Contending with Chronic Stress: it kills humans and laboratory rats, but not wild animals.

Rudy Boonstra  Snowshoe hare feeding.

Stress, like the weather, is a common feature of human existence and discussion. We live in a stressful world. It is clear that we have a tremendous capacity to undermine our own and each other’s wellbeing. When we talk about being stressed, we are not talking about the momentary unpleasant events that surprise and annoy us. Rather, we are talking about being in long-term situations over which we may have little or no control that affect us deeply (e.g. dead-end jobs with unpleasant bosses, difficult marital relationships, living in a war-torn country). All these are able to rob us of sleep, undermine our health, reduce our memory, and shorten our lives. We tend to think that the rest of vertebrate life on the planet experiences reality in much the same way. Clearly our surrogates in the laboratory (lab rodent and primates models) do; we control their experiences. But do animals in the not-so-peaceable kingdom of nature see their world the same way we do and react in the same way when chronic adversity descends? This paper argues that we have it all wrong. First, animals in nature do experience periods of long-term stress and uncertainty because of lack of food, severe weather, too many or highly efficient predators, and so on. However, only some species are chronically stressed by these factors – showing changes in their physiology, reproduction and condition; others deal with a stressor and then go back to the business of living. Nevertheless, in the first group, their hormonal stress response continues to function remarkably well. The difference between the two groups may be related to their life history. Second, though the biomedical literature and most of the literature on natural populations regard chronic stress-induced changes as pathological, I argue that these changes are adaptive and ultimately promote an animal’s survival and reproductive success.

Image caption: Snowshoe hare feeding. Photograph courtesy of Michael Sheriff.

 

Special Feature: The Ecology of Stress

Beyond hormones: evaluating stress in natural populations of vertebrates.

Creagh W. Breuner, Brendan Delehanty and Rudy Boonstra Phascogale calura, the semelparous marsupial from Australia.  Artwork by John Gould.

Stress hormones have big effects on physiology, behavior and survival. However, most stress-focused research has been biomedical, and so is difficult to apply to vertebrates living in their natural environments. Additionally, the majority of studies (in both laboratory and wild animals) focus primarily on the steroid hormones secreted during stress, the glucocorticoids. Glucocorticoids circulating in the blood are bound by large glycoproteins (corticosteroid binding globulins) that are thought to limit access of hormones to tissues. In spite of decades of research, there is still doubt as to the function of these glycoproteins. In this paper, we take a two pronged approach to the field of comparative stress research. First, we review the recent evidence demonstrating that binding globulins in the blood can regulate hormone access to tissues. Studies over the last 2 years have clarified the function of corticosteroid binding globulins, and it is now fairly clear that only free, unbound hormone in the blood can enter tissues. However, the reservoir of hormone in the blood is also biologically relevant, as changes in binding affinity of the globulin can alter hormone access to tissues. Hence, we integrate the ‘free hormone hypothesis’ with the ‘reservoir hormone hypothesis’ to clarify thinking in the field. Second, we discuss some of the most promising physiological measures that will elucidate the downstream effects of stress in wild populations. The majority of studies have focused on blood hormone level, without taking into account the physiological effect of that hormone. To clarify the effects of stress we need to measure its actual effects on reproduction and survival. In large measure we are still in the natural history phase of this field in wild vertebrates. The accumulation of fundamental knowledge is necessary to extract general principals. Our review outlines what is needed to lay the foundation for these principals.

Image caption: Phascogale calura, the semelparous marsupial from Australia. Artwork by John Gould.

 

Special Feature: The Ecology of Stress

Ecological Processes and the Ecology of Stress: the Impacts of Abiotic Environmental Factors

John C. Wingfield Hurricane Irene, an example of extreme weather (abiotic). Plants and animals suffer devastating losses in such events. Photo by J.C. Wingfield.

One of the major issues of our time concerns coping with perturbations of the environment. Many of these are biological in nature and involve competitors, disease or invasive species . Others are abiotic and include environmental degradation, urbanization, global climate change, storms, earthquakes, tsunamis and so on. This special issue addresses many aspects of how organisms cope with such unpredictable and often capricious events. In this paper I consider some underlying mechanisms of how organisms, particularly vertebrate animals, must organize their life cycles around the predictable environment – seasons, day and night, low tide/high tide etc. – and also deal with unpredictable perturbations. Responses to the predictable environment mean that organisms must use environmental cues to predict future events (the annual change in day length is a good example of a predictive environmental cue). In contrast dealing with unpredictable perturbations requires organisms to respond during and after the event. This means that the mechanisms underlying responses to the predictable and unpredictable environments are fundamentally different. Here I review the types of abiotic factors that can perturb the environment. The photograph shows one example of an extreme environmental event – a hurricane (this one was Irene from August 2011). Extreme events such as this can have devastating effects on both fauna and flora. As it seems clear that extreme events are increasing in frequency and intensity it is important that we understand what makes some organisms more robust and resilient in the face of major perturbations and others not.

Image caption: Hurricane Irene, an example of extreme weather (abiotic). Plants and animals suffer devastating losses in such events. Photo by J.C. Wingfield

 

Special Feature: The Ecology of Stress

The fatter, the better?—Seabirds stay relatively lean when stress is low

Jannik Schultner, Alexander S. Kitaysky, Jorg Welcker and Scott Hatch  Black-legged kittiwake with chick.

For all organisms, energy is essential for daily life. One way to avoid running out of energy is to store it, usually in the form of fat. However, heavy bodies can be a nuisance — when fleeing from predators, for example, or when moving across vast oceans. We were interested in finding out how wild seabirds manage their energy stores under varying foraging conditions. Would they increase their energy stores when conditions are favorable? Or would they stay lean because of important negative side effects connected to being overweight?

We studied black-legged kittiwakes, small gulls that breed in several colonies throughout the North Pacific. At one of the colonies, we provided kittiwakes with as much food as they could want. We captured birds to assess their stress hormones (measuring nutritional stress) and recorded body mass and size (indicating how fat the birds were). Then we examined the birds’ energy stores at different levels of stress. We found that at low stress, birds (including those with unlimited food) stayed relatively lean — they did not increase their fat deposition above a preferred level. Kittiwakes tended to increase their fat deposition as stress levels increased, but very high levels of nutritional stress led to rapid weight loss.

This is an important finding, as it is often simply assumed that fatter is better in the animal kingdom. Kittiwakes demonstrate how energy stores are balanced when life circumstances change for wild seabirds.

Image caption: Black-legged kittiwake with chick.

 

Special Feature: The Ecology of Stress

Predator-induced stress and the ecology of fear

Michael Clinchy, Michael J. Sheriff and Liana Zanette  Raccoon raiding a songbird nest.

Everyone can imagine their terror at fleeing for their life from a lion or a tiger, which is why predator-induced stress has been used to exemplify the concept of stress for close to a century. But because such terror has been assumed to be acute and transitory, predator-induced stress has not been much studied by either comparative physiologists or population ecologists, until relatively recently.

The focus in biomedical research has always been on chronic stress in humans, which results from "sustained psychological stress – linked to mere thoughts” rather than "acute physical crises" (like surviving a predator attack) or “chronic physical challenges" (such as a shortage of food). Population ecologists have traditionally focused solely on the acute physical crisis of surviving a direct predator attack rather than whether the risk of such an attack may have a sustained effect on other demographic processes (e.g. the birth rate).

Field experiments have now demonstrated that exposure to predators or predator cues can have sustained effects that extend to affecting birth and survival in free-living animals, and a subset of these have documented associated physiological stress effects. These and similar results have prompted some authors to speak of an "ecology of fear", but others object that "the cognitive and emotional aspects of avoiding predation remain unknown".

Recent biomedical studies on animals in the lab have demonstrated that exposure to predators or predator cues can induce “sustained psychological stress” that is directly comparable to chronic stress in humans, and this has now in fact become one of the most common stressors used in studies of the animal model of post-traumatic stress disorder.

We review these recent findings and suggest ways the lab techniques developed to measure the "neural circuitry of fear" could be adapted for use on free-living animals in the field, in order to: (1) test whether predator risk induces "sustained psychological stress" in wild animals, comparable to chronic stress in humans; and (2) directly investigate "the cognitive and emotional aspects of avoiding predation" and hence the "ecology of fear".

Image Caption: Raccoon raiding a songbird nest

 

Special Feature: The Ecology of Stress

The ecology of stress: effects of the social environment

Scott Creel, Ben Dantzer, Wolfgang Goymann & Dustin R. Rubenstein African wild dog. Image courtesy of Scott Collins

Animals often respond to physical or psychological stress by increasing the secretion of glucocorticoids. Glucocorticoids are steroid hormones with a wide range of actions that are produced by the adrenal gland and released into the bloodstream. In the short term, glucocorticoids mobilize energy from stored reserves, but in the long term, high glucocorticoid levels can have harmful effects including suppression of reproduction or immune function. Here we review several decades of research in the wild that has examined the causes and consequences of variation in glucocorticoid levels. We first consider environmental effects on glucocorticoid secretion in non-social species, and then consider social species. This distinction is valuable because variation in several aspects of social interaction can have strong effects on glucocorticoid secretion. In non-social species, high population density or frequent territorial intrusions are associated with increased glucocorticoid secretion in many species, including mammals, birds, fish and reptiles. In social species, glucocorticoid levels are commonly related to social status – sometimes with higher glucocorticoid levels in low-ranking individuals, and sometimes with higher glucocorticoid levels in dominant individuals. The relationship between dominance and glucocorticoid levels varies among species, populations and years, in a manner that depends on the stability of the social hierarchy, environmental conditions, the type of breeding system, and the manner in which high rank is obtained and maintained. The manner in which social status and other environmental variables interact to affect glucocorticoid secretion remains under-studied. Other areas for future research include: (1) a more complete description of the physiology of the stress response. Glucocorticoid secretion is controlled by a complex set of feedback loops involving the adrenal gland, the pituitary gland, and the hypothalamus. A better understanding of the process controlling secretion of glucocorticoids will help to explain variation in glucocorticoid concentrations. (2) direct tests of the relationship between glucocorticoid levels, survival and reproduction. Although it is widely suggested that glucocorticoid responses can affect survival and reproduction, and therefore can alter the dynamics of populations, the effect of glucocorticoid responses on population dynamics remains essentially unstudied.

Image caption: African wild dogs. Image courtesy of Scott Collins.

 

Special Feature: The Ecology of Stress

The Stress of Mothers May Impact Their Offspring.

Oliver Love, Patrick McGowan and Michael Sheriff Baby snowshoe hare

Medical scientists have long supposed that if your mother is stressed during pregnancy, her stress may influence your ability to respond to stressful situations. However, we know comparatively little about why this occurs since studies in the laboratory do not account for how and why wild animals react to stressful events in nature (for example, wild animals have evolved to deal with the stress of trying to avoid being eaten, but laboratory animals are never faced with a predator). Here we examine a number of wild animals to determine how and why maternal stress might not be negative but may actually be a positive thing, preparing offspring to cope with the expected (because the mother is stressed) harshness of the future world. Scientists have now discovered that many natural and man-made events can cause pregnant females to become stressed (such as food shortages, highways, tourists, etc.), raising the level of stress hormones (such as cortisol) in these potential mothers. In turn, offspring in the womb (or in the eggs of birds and fish) are exposed to these stress hormones which can cause many complex, often permanent, and seemingly negative effects, such as low birth weight, hypertension, or anxiety. However, closer inspection of these changes indicates that in the natural world (as opposed to the lab) many effects play positive, adaptive roles, allowing individuals to better cope with their environment. Nonetheless, when maternal stress occurs in unpredictable environments (especially when the stressful event that affected the mother no longer exists in the offspring’s environment), these offspring are usually poorly equipped to cope with their current environment. In this case, maternal stress can lead to long-term negative effects on individuals, and the populations and communities they are a part of. This study challenges the long-held idea that maternal stress is consistently negative, and shows that maternal stress may prepare offspring to cope and survive in a harsh environment.

Image caption: Baby snowshoe hares rely heavily on the stress levels of their mothers (during pregnancy) to help them survive the recurring population crash every 10-years. Photo: Jeffery Werner.

 

Special Feature: The Ecology of Stress

Life history and the ecology of stress: Do glucocorticoid hormones influence life-history variation in animals?

Erica J. Crespi, Tony D. Williams, Tim S. Jessop and Brendan Delehanty  Wood frog at different life stages.  Picture provided by authors.

Animals display striking variation in how fast they develop and grow, numbers of offspring produced and how long they live. Such life-history variation and the movement between these different phases of life require major changes in how animals invest their energy. The glucocorticoid hormones (GCs) are thought to have an important role in coordinating transitions between life-history events or trade-offs among life history traits in vertebrates. This is because GCs play a fundamental role in an animal’s regulation of energy balance, and as hormones that circulate throughout the body, can regulate function in multiple physiological systems and the brain at the same time. Here we review and evaluate the evidence for relationships between glucocorticoid hormones and life-history stage transitions, alongside within- and among-species variation in life-history traits, across all major vertebrate groups including fish, amphibians, reptiles, birds and mammals. The literature indicates that there is good support for consistent relationships between glucocorticoid hormones and early life-history transitions, but not so for hormone relationships with life history traits involving reproduction and survival. We expect that patterns may emerge only when 1) a greater diversity of animals with different life histories are studied in a systematic way, 2) a multivariate approach is taken to account for other internal (e. g. metabolic rate) and external factors (eg. environmental predictability) affecting life-history evolution, and 3) the effects of GCs on more than one life history trait occurring throughout the life span are studied. We also suggest 1) better integration of proximate and ultimate measures across environments to better evaluate relationships between GCs and life-history, 2) the measurement of additional factors that modulate the effects of GCs on life history traits (e.g., GC receptor and binding protein levels), and 3) incorporation of relationships between GCs and survival or reproduction in quantitative population demographic and evolutionary models, to better understand the importance of GC variation on fitness and the evolution of life history strategies in vertebrates.

Image caption: Wood frog at different life stages. Picture provided by authors.

 

Special Feature: The Ecology of Stress

Coping with stress: Some species survive by breaking the rules

Lanna M. Desantis, Brendan Delehanty, Jason T. Weir and Rudy Boonstra  To capture flying squirrels in winter, we provide natural cotton bedding and plastic covers to keep them warm and dry. Live-traps are fastened to platforms mounted on trees since flying squirrels are highly arboreal.  Photo: Glenn Abuja.

Dealing with stress is key to coping with the difficulties of life. For wild animals, stress may arise from conflict with dominant individuals, predators, or severe weather events. For humans, stress is tied to lack of control over one’s environment (e.g. social conflict, periodic starvation, severe climate). The vertebrate body has several ways to cope with stress, but the primary one is the release of cortisol and/or corticosterone from the adrenal glands. This hormone is incredibly potent biologically, by which we mean it affects about 10% of gene expression and, if not regulated, can suppress reproduction, the immune system, growth, and appetite. Most higher vertebrates have two ways to regulate this hormone. First, a feedback system to the brain turns off its production. Second, a blood carrier protein – corticosteroid-binding globulin – prevents the hormone from being free and biologically active. We found that 93% of species use this latter mechanism, leading to about 90% of the hormone being bound to this protein most of the time. However, we discovered that northern and southern flying squirrels have virtually none of their hormone bound to this protein and therefore most of their hormone is free. Our research reports on this discovery, discusses the only other group known to have this trait (four species of New World monkeys), and discusses how these two groups continue to live and function given our understanding of what high levels of stress hormones do. We carried out a comparative analysis of all vertebrates to shed light on the origin of this carrier protein. The earliest vertebrates such as lamprey and many fish did not have it, but the protein then evolved in lungfish (amongst the most advanced of fish) and was subsequently inherited by amphibians, reptiles, birds and mammals. Thus it is clear that most higher vertebrates require hormone binding to this critical protein, but it is not a prerequisite for existence; flying squirrels and some monkeys thrive without it.

Image caption: To capture flying squirrels in winter, we provide natural cotton bedding and plastic covers to keep them warm and dry. Live-traps are fastened to platforms mounted on trees since flying squirrels are highly arboreal.

 

Special Feature: The Ecology of Stress

Why do the stress hormone responses of animals vary?

Tim S. Jessop, Romy Woodford, Matthew R. E. Symonds A monitor lizard Varanus komodoensis, Image courtesy of Achmad Ariefiandy)

For animals in nature, stressors represent ever-present ecological (e.g. predation risk) and environmental challenges (e.g. storms/droughts). Stressors greatly influence whether an individual breeds and ultimately whether they live or die. In a similar way to how the body attempts to reduce the impacts of disease by mounting an immune response, animals try also to reduce impacts of stressors by increasing levels of glucocorticoid stress hormones. These hormones are thought to be key physiological attributes that help determine how animals respond and adapt to stressors. Yet, individual animals, and more broadly species, are likely to confront quite different levels or regimes of stressors. Some species live in inherently more stressful environments than others (e.g. hot arid deserts compared to lush temperate rainforests). Additionally, species vary in attributes, such as body size or metabolic rate, which influence their sensitivity to stressors. It is logical to argue therefore that animals should vary in their stress responses. However as yet we do not understand well what broad-scale environmental or species-specific attributes causes differences in glucocorticoid stress responses of animals.

We tested if environmental factors and species-specific attributes influenced glucocorticoid stress hormone responses of two major vertebrate groups, reptiles and birds. Our results indicated that variation in the glucocorticoid stress response of these animals can, in part, be explained by species’ body mass, latitude and measures of environmental quality. More importantly, different variables were associated with differences in the glucocorticoid stress response between the two groups. These results suggest that differences in glucocorticoid stress responses among species represents, in part, an evolutionary response to large-scale environmental variation and specific attributes of the species. It will be important to now develop a better understanding of how this environmentally-mediated variation in stress response could influence how particular animals adapt to rapid environmental change. Conceptually, a physiological mismatching between a species stress response and a rapidly changing environment could have important fitness consequences that influence population persistence.

Image caption: A monitor lizard (Varanus komodoensis). Image courtesy of Achmad Ariefiandy.

 

Why were sauropod dinosaurs so large?

David M. Wilkinson and Graeme D. Ruxton Horsetails one of the several groups of plants that are assumed to have been important in sauropod diets (other groups include cycads, ginkgo and conifers). Picture provided by  D. Wilkinson

The long necked sauropod dinosaurs were the largest land animals ever to walk the Earth – but why were they so large? One possibility is that it somehow involved the nature of the plant food they eat. In 2002 three plant ecologists from the University of Cape Town, in South Africa, suggested that the key factor could have been the chemistry of the plants at the time - which was likely to have meant that these dinosaurs found it difficult to get enough nitrogen for making proteins. Since then many biologists interested in dinosaurs have suggested that analysis of modern plants, closely related to the ones these dinosaurs probably eat, has shown that this idea is wrong. However this new study argues that this idea has been prematurely discarded because people have confused two different issues in thinking about this problem (in brief they have confused how much energy is in the plant with how much nitrogen is in the plant). So this South African idea is still a contender for explaining the size of the large sauropod dinosaurs.

In addition the new study makes a first attempt (which has to be rather tentative because of a shortage of relevant data) to calculate in more detail the implications of this idea. It suggests that it may have been to the advantage of young sauropods trying to get enough nitrogen to have a metabolism rather like modern mammals, but that this would have been impossible for the adults because of the danger of such large animals overheating. Alternatively - or in addition - it would also have been potentially beneficial for the young to be carnivorous, as this would also have helped them access more nitrogen. The large adults plausibly used their size to help process large amounts of plant food to access enough scarce nitrogen, as suggested in the original 2002 study. However this would potentially have caused them to have to take in more energy than they needed. A mammal (and possibly also small sauropods) would get rid of this surplus as heat, but this would not be possible for a really large dinosaur. Potentially they laid down fat reserves instead.

Horsetails one of the several groups of plants that are assumed to have been important in sauropod diets (other groups include cycads, ginkgo and conifers). Picture provided by D. Wilkinson.

 

Natural selection favours more active lizards that expend less energy.

Jean-François Le Galliard, Matthieu Paquet, Matthieu Cisel & Laetitia Montes-Poloni A juvenile common lizard rests during a basking pause in the sunshine

Repeatable behavioural differences among individuals, also called personalities or temperaments, have now been demonstrated convincingly in numerous animal species including mammals, birds, fishes and even reptiles. More recently, it was proposed that these consistent behavioural differences are generally associated with a “slow-fast continuum” where energetically expensive but short-lived behavioural tactics are located at the fast end of the spectrum. For example, more exploratory individuals should grow faster at the expanse of their longevity (i.e., a growth-survival trade-off) and could have selective advantages over less exploratory individuals if they also expend energy at faster rates or are able to run at faster speeds. Unfortunately, none of the earlier ecological studies had tested for the relationship between growth-survival and personality, metabolic and performance traits of individuals. We examined for the first time these issues through a field study of individual variation in and selection on behavioural activity, metabolism at rest, and maximal sprint speed and endurance capacities of juvenile common lizards. We measured these four traits in a birth cohort of approximately two-hundred newborn lizards as well as one year later after a natural episode of fast body growth and strong mortality inside enclosed populations. Our behavioural data indicate clearly that young lizards display significant and consistent individual differences in locomotor activity. However, the observed correlations between activity, metabolism and maximal locomotor performances were generally weak and our measure of behavioural activity was not involved in a growth-survival trade-off. Instead, our analysis of survival indicates a compromise between activity and resting metabolic rates, such that natural selection favours more active lizards that expend less basal energy. These results indicate that concurrent variation in personality and energetics can contribute to explain significant demographic variation within species.

Image caption: A juvenile common lizard rests during a basking pause in the sunshine. This animal alternates basking and active foraging activities during sunny days, and relies on locomotor performances such as sprinting and endurance to capture its preys when actively foraging. © CNRS Photothèque, Cyril Frésillon.

 

You are what you eat: effects of dietary nitrogen on reproduction of the cabbage white butterfly.

Natasha Tigreros Cabbage white butterflies mating.

Animals and plants grow and reproduce surrounded by nutritional variation, where food is often scarce or key nutrients are lacking. Developing under nutritional constraints can be particularly challenging for animals like butterflies, with discrete larval and adult stages. It has been proposed that larval nutrition may influence adult reproduction through two different pathways. First, by altering larval development: time spent in the larval stage and maximum body size. Second, nutritional constraints can result in latent effects: effects originated during the larval stage but that only become apparent in the adult.

In this study I investigated how larval nutrition influences male fitness of the cabbage white butterfly. To do this, I manipulated the amount of nitrogen in the larval diet using a semi-synthetic diet. Nitrogen is a limiting element for animals that feed on plants, such as the larvae of the cabbage white butterfly. Males reared with different dietary nitrogen concentrations were marked with individual IDs and released in a big outdoor insectary which also contained female butterflies, nectaring plants and oviposition plants. In this insectary I was able to keep track of the reproductive success of males, under close to natural conditions.

The results illustrate that dietary nitrogen influenced male reproductive success by altering larval development as well as by latent effects. For example, males with low nitrogen diets had long developmental times and small pupal size. Those males with a long development had a short lifespan as adults and mated with few females; those males with small pupal size produced small nuptial gifts. A nuptial gift is a nutrient donation that males transfer to the female at each mating and that the female assimilates to produce more eggs. Finally, this study found that larval dietary nitrogen appears to affect adult reproduction through latent effects. Males on low nitrogen diets had less colourful wings and produced fewer sperm.

 

You are what you eat: effects of dietary nitrogen on reproduction of the cabbage white butterfly.

Natasha Tigreros Cabbage white butterflies mating.

Animals and plants grow and reproduce surrounded by nutritional variation, where food is often scarce or key nutrients are lacking. Developing under nutritional constraints can be particularly challenging for animals like butterflies, with discrete larval and adult stages. It has been proposed that larval nutrition may influence adult reproduction through two different pathways. First, by altering larval development: time spent in the larval stage and maximum body size. Second, nutritional constraints can result in latent effects: effects originated during the larval stage but that only become apparent in the adult.

In this study I investigated how larval nutrition influences male fitness of the cabbage white butterfly. To do this, I manipulated the amount of nitrogen in the larval diet using a semi-synthetic diet. Nitrogen is a limiting element for animals that feed on plants, such as the larvae of the cabbage white butterfly. Males reared with different dietary nitrogen concentrations were marked with individual IDs and released in a big outdoor insectary which also contained female butterflies, nectaring plants and oviposition plants. In this insectary I was able to keep track of the reproductive success of males, under close to natural conditions.

The results illustrate that dietary nitrogen influenced male reproductive success by altering larval development as well as by latent effects. For example, males with low nitrogen diets had long developmental times and small pupal size. Those males with a long development had a short lifespan as adults and mated with few females; those males with small pupal size produced small nuptial gifts. A nuptial gift is a nutrient donation that males transfer to the female at each mating and that the female assimilates to produce more eggs. Finally, this study found that larval dietary nitrogen appears to affect adult reproduction through latent effects. Males on low nitrogen diets had less colourful wings and produced fewer sperm.

 

Positive phenotypic correlations among life history traits remain in the absence of differential resource ingestion.

Adriana M. Olijnyk and William A. Nelson Daphnia pulicaria, showing one asexual egg in the brood pouch. Nelson Lab, Queen’s University.

The traits of an individual, such as growth rate, age at first reproduction, survivorship and the amount and timing of reproduction, taken together, represent the life history of an organism, which is used to understand its fitness (its contribution to the gene pool of the next generation). These traits, however, are often linked to each other by physiological or genetic mechanisms. For example, an individual that has higher reproduction likely won’t have the energy to grow as quickly compared to an individual with lower reproduction because both traits demand a lot of resources. Mechanisms such as this are thought to generate negative trade-offs, such that an increase in one trait leads to an inevitable decrease in another. However, there are a surprising number of examples of positive correlations between traits, where trade-offs were expected. The leading explanations for positive correlations are that some individuals are genetically predisposed to acquire more resources than others, or that complex tradeoffs among hidden traits can cause positive correlations among others.

The goal of our study was to remove two key processes thought to generate positive correlations--differential ingestion of resources among individuals and genetic variation--and evaluate the resulting life history traits. We used the freshwater zooplankton Daphnia, which is a model organism used in many life history studies. Daphnia reproduce asexually, which allows for the removal of genetic variation while keeping high replication in empirical experiments. Using carefully controlled resource levels, from near starvation to relatively abundant food, we measured all aspects of life history traits in individuals from multiple genotypes of the species Daphnia pulicaria. We found that positive correlations emerged, even after controlling for genetic variation and for differential resource ingestion. Our results reveal an alternative mechanism for the generation of positive correlations, i.e. differences in resource utilization within an individual. Resource utilization includes physiological processes such as maintenance rates and conversion efficiency, which both influence the amount of ingested resources that become available for growth and reproduction. This new mechanism highlights the role and importance of physiological ecology in shaping life history trait correlations.

Image caption: Daphnia pulicaria, showing one asexual egg in the brood pouch. Photo credit: Nelson Lab, Queen’s University.

 

Three decades of field study reveal the adaptive value of intermittent breeding.

Jean-Pierre Baron, Jean-François Le Galliard, Régis Ferrière, Thomas Tully A female meadow viper released together with her two male offspring of the year in the field at Mont Ventoux, France

 

Most species adjust their timing and effort of reproduction to match temporal environmental variation. But in some species of birds, mammals and reptiles, individuals may regularly skip some reproductive opportunities. The occurrence of such intermittent reproduction is puzzling, for missing reproductive seasons not only loses current reproduction but also risks not surviving until the next breeding opportunity. Earlier attempts to explain the evolution of intermittent reproduction have emphasised the role of food constraints when several years are needed to restore reproductive reserves, or on the occurrence of adaptive flexibility when good environmental conditions are required for breeding to occur. But a three decades field study of a critically endangered viper leads us to propose a third hypothesis. The female vipers reproduce on average every two years when females have accumulated enough reserves to reach a critical body condition. This intermittent reproduction is not driven by any environmental fluctuation since breeding frequencies and reproductive effort are constant in time, nor by food constraints since we found that the vipers can use some resources acquired during their pregnancy to cover the costs of gestation. A close analysis of the life cycle rather indicates that intermittent reproduction has probably evolved to maximise maternal growth and storage of reserves during non-breeding years, while during the breeding years the vipers invest in offspring quality and maternal survival. We used a mathematical model based on the vipers’ life history to confirm that this third hypothesis was likely. This new hypothesis could explain the evolution of intermittent reproduction in many other seasonal organisms in which reproduction mixes capital and income energy. Our results demonstrate the value of long-term field studies for the analysis of reproductive strategies in wild animals

Image caption: A female meadow viper released together with her two male offspring of the year in the field at Mont Ventoux, France. The meadow viper has a unique diet of grasshoppers and can feed throughout a gestation period that lasts from early to late summer in this study area. © CNRS, Jean-Pierre Baron.

 

Ageing in a wild sheep population varies between aspects of reproduction.

Adam D. Hayward, Alastair J. Wilson, Jill G. Pilkington, Tim H. Clutton‐Brock, Josephine M. Pemberton, Loeske E. B. Kruuk  Wild sheep mother and lamb. Credit to Arpat Ozgul.

We are all too aware of the consequences of growing old, but what are the causes of changes in performance between ages? We can only truly understand the ageing process once we understand the causes of ageing patterns. Across a population, we may notice that important traits such as fertility decline with age. This could be because each individual undergoes a decline, becoming less fertile as they age. However it could also be because individuals that are less fertile live longer. If this is true, on average the population will appear to be less fertile at older ages because individuals with high fertility have died: there has been “selective disappearance” of high-fertility individuals. Determining how the processes of individual ageing and selective disappearance contribute to ageing patterns will increase our understanding of how ageing has evolved and how wild animals deal with the conflicting demands of living a long life and reproducing rapidly. We studied female reproductive ageing in a wild population of sheep on the islands of St Kilda, 70km NW of Scotland. We investigated ageing in four aspects of reproduction: whether a female reproduced (fertility); whether a female produced twins (twinning); how heavy the lamb was (lamb weight); and whether the mother raised her lamb successfully (lamb survival). We found an improvement in all four traits from young mothers to middle-aged mothers, possibly because young mothers have less experience and are smaller, reducing their chances of reproducing successfully. Three aspects of reproduction (fertility, lamb weight, lamb survival) then declined in old age, largely because individuals reproduced less well as they got older. For these three traits, females that had better performance also lived longer, but substantial declines in individual performance (individual senescence) generated population-level declines at older ages. Surprisingly, however, there was no decline in twinning rates: females were equally able to produce twins at age 10 (very old for a wild sheep) as they were at age 4 (in the prime of life). Additionally, females that produced twins lived shorter lives, suggesting that the large effort of producing two lambs reduced survival ability. Our results show how complex and varied the ageing process is in the wild, and shed light on the link between reproduction and lifespan.

Image caption: Wild sheep mother and lamb. Credit to Arpat Ozgul

 

Does forest colour tell how much carbon dioxide is absorbed by deciduous trees?

Toshie Mizunuma, Matthew Wilkinson, Edward L. Eaton, Maurizio Mencuccini, James I. L. Morison and John GraceTwo camera systems looking at an oak forest from the top of flux tower in Alice Holt, UK with the pictures taken on 19 May in 2010.

Forests play the important role of fixing carbon dioxide from the atmosphere, and storing the carbon as biomass and soil organic matter. In the northern hemisphere, CO2 concentration fluctuates with the seasonality of photosynthesis of plants. Recent global warming has made the arrival of spring earlier, and leaves appear sooner. How does this influence the carbon cycle in forests? Will trees absorb more CO2 in the future because of earlier springs and later autumns? The observation network FLUXNET monitors the carbon balance at about 500 ecosystems worldwide using instrument towers. These are operated continuously and remotely, and it becomes important now to see how photosynthetic uptake depends on the development cycle of leaves. We have investigated two prototype camera systems to do this.

In an oak-dominated (Quercus robur L.) forest in Hampshire UK, photographs of the forest canopy were taken every half an hour over two years using two different digital camera systems at different viewing angles (an outdoor webcam with a near-horizontal view and a commercial ‘fish-eye’ digital camera with a downward view). Images from cameras were stored in a digital photograph format, JPEG, as numbers of Red, Green and Blue (RGB digital number). The transition of colours from both cameras showed the seasonality of the forest: when budbreak started, the green sharply increased, gradually decreased in summer, and returned to the original level when leaves were shed; the rise of red colour was shown when oak leaves turned yellow in autumn. The timing of the sharp increase in green coincided with the onset of carbon absorption. We modelled the photosynthesis of the forest using the extracted colours to compare with the flux measurements. We used a RGB-derived parameter, Hue, and found that it was well-correlated with actual photosynthesis. Our results suggest that digital cameras can be an important aid in monitoring forests and the colour signals can be a useful proxy for photosynthesis.

Image caption: Two camera systems looking at an oak forest from the top of flux tower in Alice Holt, UK with the pictures taken on 19 May in 2010. Image courtesy of authors.

Want to find out more? You also watch the video abstract on our youtube channel.

 

The timing of abscission affects dispersal distance in a wind-dispersed tropical tree

Kyle D. Maurer, Gil Bohrer, David Medvigy and S. Joseph Wright Luehea seemannii fruit in a tropical forest. Fruit pods hold many seeds in a start orientation, typically facing upward (above the horizontal). Cracks in the distal end of the pod widen during the season allowing the seeds to be released..

Many plant species, termed “wind-dispersing” plants, take advantage of the wind to carry their seeds to suitable growth locations. It has been shown that plants whose seeds have the ability to be carried long distances from the parent tree will be more successful than species whose seeds travel only short distances in exploring the landscape and producing at least a few successful seedlings in suitable locations. For a wind-dispersed seed the best chance to travel a long distance is to get caught in an updraft of wind, where it will be ejected high above the plant canopy where the wind is typically faster and the time wind will be able to carry it is prolonged. We use high-frequency measurements of environmental conditions, including wind, air temperature and humidity, solar radiation and seed release rates to show that a wind-dispersing plant displays characteristics allowing it to selectively release its seeds in targeted conditions. We find that environmental conditions, such as temperature, humidity, wind and light levels affect the rate of seed release and that these effects can be characterized based of the time scale of the environmental phenomena, from the dispersal season (longer term) through the daily cycle, to fast effects that are controlled by rapidly changing environmental conditions, such as wind. At the shortest time-scale, we find updraft strength is the most important determining factor of seed release rate. We use models to show that the instantaneous effects of selecting particular environmental conditions during seed release, and particularly preferring updrafts, accumulate over a long period into large differences in the chances of seeds dispersing long distances. To our knowledge, we are the first study to show this phenomenon in a natural environment. Our findings provide insight into the complex process of plant population spread and survival and, if properly implemented into models, may be used to enhance our ability to accurately predict plant population movement in patchy environments and under climate change.

Image caption: Luehea seemannii fruit in a tropical forest. Fruit pods hold many seeds in a start orientation, typically facing upward (above the horizontal). Cracks in the distal end of the pod widen during the season allowing the seeds to be released.

 

Males protect their sperm by an antimicrobial substance in the ejaculate

Oliver Otti, Aimee P. McTighe and Klaus ReinhardtUpper: Copulating Bedbugs, Lower left: Activity of antibacterial substance measured in an immune assay, Lower right: Male reproductive organs. Pictures courtesy of Richard Naylor and the authors

Reproduction is a complicated process. Males and females invest differently into offspring, leading to an arms race between the sexes. Under such coevolution males have evolved diverse ejaculate substances, which manipulate the outcome of fertilisation in females. Among these antimicrobial substances have been identified, which might have evolved under a different scenario. Males have been proposed to protect their sperm with these substances from sexually transmitted microbes or microbes encountered in the females. To date we lack a test of this ejaculate protection hypothesis where the two basic components are examined in conjunction: 1) microbes damage sperm and 2) antimicrobial ejaculate substances prevent such microbe-induced sperm damage.

For our experiments we used the common bedbug, Cimex lectularius, which mates by traumatic insemination and possesses an antibacterial ejaculate substance. The male injects sperm through the punctured female abdomen into a special organ from where the sperm travel through the female’s blood to the ovaries. We exposed bedbug sperm to microbes either with or without the antibacterial substance and measured the proportion of dead sperm. Exposure to microbes alone induced high sperm mortality, while in combination with antibacterial substance sperm survived as well as a control group of sperm without antibacterial substance and microbes.

We also asked ourselves if male provisioning of antibacterial substance is a nuptial gift, leading to higher offspring numbers in females. We provisioned females weekly with a dose of antibacterial substance and a mating. Counting the eggs we could observe a higher egg number laid over the first few weeks, but also a faster decrease in egg numbers later on in the provisioned females compared to control groups not receiving the antibacterial substance. Sperm protection seems to be the more likely candidate for the evolution of antimicrobial substances in the seminal fluid than a nuptial gift. Although females do not seem to benefit directly from antibacterial substances, the protective effect on sperm might also reduce microbe numbers transmitted during mating. We still need to investigate to what extent the microbe-induced sperm mortality translates into male fitness and in general how microbes affect reproductive traits of both sexes.

Image caption: Upper: Copulating Bedbugs, Lower left: Activity of antibacterial substance measured in an immune assay, Lower right: Male reproductive organs. Pictures courtesy of Richard Naylor and the authors.

 

Implications of flower orientation for hover-feeding hummingbirds.

Nir Sapir and Robert Dudley Anna's Hummingbird, picture by Eyal Shochat,

Hummingbirds comprise a group of about 330 species that are common in many New World ecosystems. The main food source of hummingbirds is nectar and they are important pollinators of various plant species. They typically hover-feed from flowers that may come in different shapes, sizes and orientations. Many of these flowers are oriented downwards rather than horizontal, thereby requiring that hummingbirds feed while hovering with the bill oriented vertically upward. Yet the implications of flower orientation on feeding hummingbirds have not yet been described. Hummingbirds very commonly visit vertically-oriented flowers, so we asked: is feeding from such flowers energetically advantageous? Our specific research questions were how do hummingbirds change their body posture and the way they flap their wings while feeding from flowers that have different orientations, and do they conserve energy or pay a metabolic cost while feeding from vertically oriented flowers?

We measured Anna’s Hummingbirds in the laboratory at The University of California at Berkeley. The birds were introduced to artificial flowers that were oriented horizontally, tilted 45º downward, and pointing vertically downward and their metabolism, body posture and wing motions were quantified. We found that when feeding from vertically oriented flowers, hummingbirds held their body in an upright angle and flexed their heads towards their backs. In addition, the stroke amplitude - that is the angle between the birds’ maximal and minimal positions of the wings - increased, and the same was found for the stroke plane angle in relation to the longitudinal body axis, that is the angle between the plane through which the birds flap their wings, and the angle of the body. Metabolic rates also increased by around 10 % when feeding from vertically compared to horizontally oriented flowers. Other wing flapping variables, such as wingbeat frequency, did not vary between flower orientations, and neither did feeding duration. Our results show that hummingbirds are well adapted to feeding from flowers that face downwards, and that in order to do so they modify their body posture and change the way they flap their wings. In addition, the birds pay a substantial energetic cost while feeding from vertically oriented flowers, refuting our hypothesis that feeding from vertically oriented flowers may be energetically beneficial.

Image caption: Anna's Hummingbird by Eyal Shochat

 

The perils of disease increase starvation risk in wintering birds.

Andreas Nord, Sandra Chiriac, Dennis Hasselquist, and Jan-Åke Nilsson Adult male great tit (Parus major Linnaeus).

Birds, like man, must maintain a high and even body temperature to function properly. This is a challenging task, not least in winter when the internal temperature may be some 50-60 °C above that of the surroundings. It is not surprising that this challenge requires high food intake. In fact, a great tit, which is a common garden and forest bird across Europe, must sometimes put on 10 % of their body weight as fat on a daily basis during cold winter days. A human of average weight would have to eat some 200 hamburgers to ingest the same amount of fat. However, the time of peak food demand coincides with the time when food resources are the most difficult to obtain. To overcome such hardships, many animals actively reduce their body temperature at night (nocturnal hypothermia), a process that substantially lowers their energy demands. Yet the use of nocturnal hypothermia is often not enough to avoid the risk of starvation, because the demands from other body functions compete for the same fat reserves. In these situations, birds may have to prioritize overnight survival by reducing the use of other costly functions, such as the immune defense system. In other words, because food availability is limited in winter, it may not be possible to uphold both ample amounts of body fat and an adequate defense against invading pathogens. This was the subject for our study, in which we investigated how a bacterial infection influenced the use of nocturnal hypothermia in wild great tits. Despite the energy saving benefits that accrue from maintaining a low body temperature at night, we found that birds fighting a bacterial infection became febrile (that is, had a higher body temperature) at night. This was probably associated with an increased risk of starvation, because infected birds lost more body mass overnight and consequently were in worse condition in the morning. To regain a decent fat level, these birds were likely forced to be very active in searching for food during the first daylight hours, thereby exposing them to their chief predator, the pygmy owl, which specializes in hunting at this time of the day.

Adult male great tit (Parus major Linnaeus). These birds are common guests in artificial nest boxes across Europe during spring and summer. They also use the very same nest boxes for roosting during cold winter nights, which allows them to save substantial amounts of energy.

 

Plants do not wait for their flowers to be eaten.

Dani Lucas-Barbosa, Joop J. A. van Loon, Rieta Gols, Teris A. van Beek and Marcel Dicke Caterpillars of the Large Cabbage White Butterfly that feed on flowers of Black Mustard plants, but not on the seeds

Insects that feed on plants consume not only leaves, but also flowers of plants. Plants do not passively undergo this attack, but activate defences by changing odour and taste. When plants change their odours, this may influence how other insects perceive the plants, and may indeed affect the behaviour of every insect that interacts with the plant. This includes insect pollinators that feed on sweet nectar and pollen, and in so doing help plants to reproduce, by transferring pollen from one plant to another. In this field study, we have investigated how the exposure of black mustard plants to eggs and caterpillars of the Large Cabbage White Butterfly influences the behaviour of pollinators. We also looked at the consequences for production of seeds. Butterflies lay eggs on leaves of black mustard plants, and newly hatched caterpillars will initially feed on the leaves, but they soon move higher up in the plant, and start feeding on the flowers. When caterpillars were feeding on the flowers, flower-infested plants produced less odour when compared to plants that had not been infested with caterpillars. The behaviour of pollinators was affected by caterpillar infestation, but only when these caterpillars were feeding from the flowers. Remarkably, even before the caterpillars hatched from the eggs, plants infested with eggs produced seeds sooner than plants without eggs. The caterpillars that develop from the eggs prefer to feed on flowers over leaves of black mustard plants, but they do not feed on the seeds. Thus, it is advantageous for the plants to accelerate the production of seeds. By making seeds sooner, plants infested with butterfly eggs produced offspring before the caterpillars could eat the flowers, and thus safeguarded their reproduction when faced with these ravaging caterpillars.

Image caption: Caterpillars of the Large Cabbage White Butterfly that feed on flowers of Black Mustard plants, but not on the seeds. Photographs by Dani Lucas-Barbosa.

 

Variation in larval survival on non-host pollen might enable expansion of host plant spectrum in solitary bees

Mare Haider, Silvia Dorn, Andreas Müller  Copulating pair of the solitary mason bee Osmia cornuta (picture A. Krebs).

Bees collect large amounts of pollen, which they store mixed with nectar in their brood cells as larval food supply. For pollen collection, most solitary bee species are restricted to a certain range of host plants. This restriction might partly be due to unfavourable chemical properties of pollen, which impede its digestion by the larvae of unspecialized bee species. It has recently been shown that the pollen of meadow buttercup (Ranunculus acris) possesses such unfavourable properties. One of the bee species, whose larvae are usually not able to successfully develop on a pure buttercup pollen diet, is the solitary mason bee Osmia cornuta, which is widespread in Central and South Europe.

In our study, we tested whether individuals of O. cornuta differ in their ability to develop on buttercup pollen and whether populations of O. cornuta originating from five different European localities exhibit differences in the proportion of individuals that are capable of developing on buttercup pollen. For that purpose we placed unhatched eggs of O. cornuta into artificial brood cells that were supplied with a pure buttercup pollen diet and observed larval survival and development.

We found that the majority of larvae from all five O. cornuta populations died when feeding on buttercup pollen. However, in every population a few bees existed that successfully reached the adult stage. Although these surviving bees had a longer development time and stayed distinctly smaller than bees reared on a control pollen diet of field mustard (Sinapis arvensis), they were able to successfully reproduce. These results show that the possibility to incorporate buttercup as a new pollen host into the host plant spectrum of O. cornuta exists in each of the five tested populations. In addition, they suggest that the physiological capability the larvae need to digest buttercup pollen might be handed down to the next generation. Therefore, the observed variation in the ability of O. cornuta individuals to utilize non-host pollen might enable this bee species to react to environmental changes that affect flower availability.

Image caption: Copulating pair of the solitary mason bee Osmia cornuta (picture A. Krebs).

 

The environment and space, not phylogeny, determine trait dispersion in a subtropical forest

Xiaojuan Liu, Nathan G. Swenson, Jinlong Zhang and Keping Ma  This subtropical forest in Gutianshan was used to conduct study for the current paper. (Photo by Keping Ma).

A central aim of forest ecologists is to quantify the relative importance of different community assembly mechanisms in diverse tree communities. Recent work in this field has focused on the importance of functional trait similarity and abiotic filtering. That is, do neighbouring trees need to be functionally different to ‘fit together’, or does the dominant effect of the environment mean that all the trees must be rather similar? While important, none of this work has simultaneously: linked these trait dispersion patterns to the underlying abiotic environment, considered dispersal limitation and quantified the degree to which patterns of trait dispersion may be explained simply by shared ancestry.

Here we use data from a subtropical Chinese forest to accomplish this goal. We first examine the trait dispersion (leaf area, specific leaf area, seed mass, wood density, maximum height and five traits together) on local scales by comparing the observed trait dispersion pattern to that expected if these traits were distributed at random. Then we use a variance partitioning approach to examine the degree to which spatial proximity, environmental similarity or the phylogenetic dispersion of the species determine the observed trait dispersion.

The results show that, on local scales, trait dispersion is often non-random. Further the widespread trait clustering observed is largely explained by the environment and space (i.e. similar kinds of trees grow near each other and occupy similar environments), while the phylogenetic dispersion or relatedness of species in a sample explains relatively little. This result further underscores that inferring an assembly mechanism from a pattern of phylogenetic dispersion is tenuous. The work is important because it is the first to partition the variation in tree trait diversity into its spatial, environmental and phylogenetic components and to demonstrate that functional trait data often lack enough phylogenetic signal on local scales to confidently link patterns of trait and phylogenetic dispersion. Ultimately, the findings suggest a strong role for abiotic filtering and dispersal limitation during community assembly on local spatial scales and that shared evolutionary history plays a relatively small role.

Image caption: This subtropical forest in Gutianshan was used to conduct study for the current paper. (Photo by Keping Ma).

 

How does grazing affect ecosystem functioning in a semi-arid steppe?

Dima Chen, Shuxia Zheng, Yumei Shan, Friedhelm Taube, and Yongfei Bai  Grazing experiment (Photo credit: Yongfei Bai)

The Eurasian steppe, the largest uninterrupted biome in the world, has long been subjected to grazing by domestic ungulates (e.g. sheep, goats and cattle) at high levels, leading to widespread deterioration of biodiversity and ecosystem services. While abundant evidence demonstrates that heavy grazing alters plant diversity, community structure and composition, and primary productivity across a wide range of arid and semiarid systems, research on how grazing affects plant and soil biota and thereby ecosystem functioning is scarce. Generally, grazing alters ecosystem function (e.g., soil nitrogen cycling and primary productivity) by modifying several pathways including plants (e.g., plant species diversity and community composition) and soils (e.g., soil nutrients, soil environments, and soil fauna community) in grasslands. There are two pathways of grazing effects: 1) grazing directly affects plant community, soil nutrients, and soil environments; and 2) grazing indirect affects ecosystem functioning via alterations in plant community, soil nutrients, soil environment, and their interactions. However, we still are not clear about the relative importance of plants versus soils to changes in ecosystem function. In the current study, we explored how grazing affects the ecological linkages between plants and soils and thereby ecosystem function using a field experiment maintained for five years with seven levels of grazing intensity in the Inner Mongolian grassland, which is representative of the Eurasian steppe. We show that grazing affects primary productivity by altering soil nitrogen cycling and soil environment but not by changing plant community, soil nutrients, or soil nematode community structure. These findings have important implications for the management of the vast semiarid grasslands in the Eurasian Steppe region.

Grazing experiment (Photo credit: Yongfei Bai)

 

Diversity effects important in complex environments

 

Nico Eisenhauer, Wiebke Schulz, Stefan Scheu and Alexandre Jousset  Pseudomonas fluorescens F113 on King's B agar. Photo: Alexandre Jousset.

The anthropogenic spread of species threatens the biodiversity and functioning of ecosystems worldwide. Thus, it is important to understand which properties of communities make them resistant to the invasion of exotic species. The diversity within communities may be a central factor driving this resistance to invaders, but effects of diversity may depend on the context. Diverse communities exploit resources more efficiently and may thereby reduce invader success, but if only a limited number of resources is available, diversity may play a minor role.

We used bacterial model systems to test the relationship between the diversity of communities and their resistance to invasion. Pseudomonas fluorescens is a highly diverse phylogenetic group and a major component of bacterial communities associated with plant roots. Bacterial strains belonging to this species produce antifungal compounds inhibiting root pathogens and thereby improve plant performance. We tested the effect of functional dissimilarity of Pseudomonas fluorescens communities, i.e. the dissimilarity of different bacterial communities in using different resources, on the success of a bacterial invader in microcosms of varying resource richness.

Invader success significantly decreased as the diversity (functional dissimilarity) of the resident bacterial community increased. However, in the presence of only one or two different resources, diversity played a minor role, while diversity decreased the success of the invader significantly in the presence of 3 and 5 different resources. Our results indicate that diversity effects were not only due to the presence of single dominant bacterial strains, but due to interactions between different genotypes within the community. Moreover, we found that functionally dissimilar bacterial strains efficiently used different resources, which – as a consequence – were not available for the invader. Our findings call for the preservation of functionally dissimilar taxa to enhance resistance of communities to invasive species, particularly in complex environments.

Image caption: Pseudomonas fluorescens F113 on King's B agar. Photo: Alexandre Jousset

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