Lay summaries for Volume 27, Issue 4 of Functional Ecology

 

Special feature: Mechanisms of Plant Competition.

Responses to global climate change: insights from organismal physiology.

Standard Papers

 

Special Feature: Mechanisms of Plant Competition

Mechanisms of plant competition for nutrients, water and light

Joseph M. Craine and Ray Dybzinski Root system of a North American perennial grass, Schizachyrium scoparium, a good competitor for nitrogen.

For millions of years, plants have struggled with one another to acquire the basic resources needed to grow and reproduce. Competition for nutrients, water, and light, sustained over millions of generations, has shaped plants in ways we are only beginning to understand. The unique properties of each type of resource have led to unique sets of adaptations associated with competition for these resources. Competition for light and nutrients has selected for the ability to preempt the supply of these resources from competitors. Competition for light selects for plants that grow taller faster and hold more leaves than is optimal, denying light to potential competitors below. Competition for nutrients selects for plants that produce more roots than is optimal, which ensures that nutrients come in contact with their roots before competitors. These adaptations might not allow them to maximize their productivity in the absence of competition, but they do ensure success in a competitive environment. That is, competition may select for better competitors, but not necessarily for plants that are more efficient or more productive.

Competition for water, as far as we know, selects for similar adaptations as does surviving drought: withstanding immense tensions on internal water caused by drying soils. In all, understanding how plants compete for nutrients, water, and light is a key to understanding the diversity of life on earth and how resources are utilized by plants. As some resources are likely to become scarcer in the future, understanding how plants compete for resources holds promise for improving agriculture and maintaining the ecosystem services provided by plants worldwide.

Image caption: Root system of a North American perennial grass, Schizachyrium scoparium, a good competitor for nitrogen.

 

Special Feature: Mechanisms of Plant Competition

How plants compete with their neighbours.

Ronald Pierik, Liesje Mommer and Laurentius ACJ VoesenekHigh density field of plants. Image courtesy of the authors.

The vast majority of plants grow in high densities and compete for resources. Aboveground, plants compete for light, whereas below-ground competition occurs for the various mineral nutrients and water. Species can differ in their growth rates and resource acquisition. In addition plants can be plastic in their resource acquisition in response to competition. In order for plants to optimize their behavior in response to neighbouring competitors, these neighbours first need to be detected.

Plants can detect their neighbours aboveground through changes in light quality and quantity. Particularly the red:far-red light ratio is reliably reduced due to far-red reflection by neighbouring leaves and red light absorption for photosynthesis. This neighbour detection occurs even before competition sets in, thus preparing plants optimally for the struggle for resources. Additional aboveground signals to detect neighbours include volatile organic compounds that are produced and released by neighbouring plants and even touching of neighbouring leaf tips. In response to these signals plants invest more in vertical elongation of their shoots as part of the so-called Shade Avoidance Syndrome, which positions the photosynthesizing leaves higher in the vegetation, i.e. closer to the light.

Belowground, neighbouring plant roots alter water and nutrient availability and these changes can serve as cues to detect competitors. The root system architecture of most species is highly plastic and can be modified to localize the majority of the roots in nutrient-rich zones so as to maximize nutrient capture. In addition to taking up water and nutrients, roots also excrete a rich variety of water-soluble exudates. These exudates may have direct affects on neighbouring plants. They may for example be toxic and reduce growth of nearby competitors. These exudates will also affect the community of microbes that live in the soil zone directly around the roots systems (called the rhizosphere), which may have indirect effects on interactions with competing neighbour plants.

Our review argues that understanding the mechanistic aspects of plant neighbour detection and plasticity is key to exploring ecological interactions, generating hypotheses and understanding constraints in plant competition.

Image captionHigh density field of plants. Image courtesy of the authors.

 

Special Feature: Mechanisms of Plant Competition

Biophysical Effects on Plant Competition and Co-existence.

Karl Niklas and Sean HammondCatastrophic snow loading on Arbor Vitae. Photo provided by Edward D. Cobb, Department of Plant Biology, Cornell University

Physical forces and chemical processes, such as the weight of snow or the rate of metabolism, invariably affect plant growth and the ability to cope with other neighboring organisms. The effects of these forces and processes on growth and competitiveness can be described mathematically in ways that can help ecologists understand how plants interact with one another, both competitively and cooperatively. Importantly, each mathematical description contains parameters that are physical constants (such as the acceleration of gravity), which no organism can alter. However, each description also contains parameters that can be changed as an organism moves, changes orientation, or grows in size (such as the surface area a plant projects toward the oncoming flow of wind or water). In addition, these mathematical descriptions involve the exchange of atmospheric gases (carbon dioxide and oxygen) and energy (primarily heat). Computer models show how these mathematical descriptions of mass and energy can help us to understand complex ecological phenomena such as species competition and co-existence, and community turnover in species composition. Collectively, these tools show that a critically important factor in understanding plant competition is how plants three-dimensionally display their surfaces to the physical environment, both above and below ground. For terrestrial and aquatic flowering plants, this involves the display of leaves. For aquatic algae, this involves the display of filaments or frond-like organs. An equally important factor is how this display of surface area changes over time as a plant increases in size. By considering a few fundamental physical laws and processes, much (but not all) of the ecology of plants can be explained and in some cases even predicted.

Image caption: Catastrophic snow loading on Arbor Vitae. Photo provided by Edward D. Cobb, Department of Plant Biology, Cornell University

 

 

Special Feature: Mechanisms of Plant Competition

Microbial mediation of plant competition and community structure.

Angela Hodge and Alastair H. FitterExternal hyphae and spores of an arbuscular mycorrhizal fungus around a root. Image courtesy of the authors.

There is growing but largely circumstantial evidence that micro-organisms can change the outcome of plant competition; direct evidence remains surprisingly scarce. Plants release substances from their roots that influence the microbial community in the soil around their roots creating a zone called the ‘rhizosphere’. Because different plants may release distinct cocktails of chemical substances from their roots into their rhizosphere, which will encourage certain microorganisms over others, they may support very different rhizosphere communities.

The best evidence that microbes can influence plant interactions comes from reasonably well characterised microbial groups that have close symbiotic relationships with plants, such as the arbuscular mycorrhizal fungi and nitrogen-fixing bacteria. However, because the majority of soil micro-organisms cannot be grown in the laboratory, making it difficult to devise manipulative experiments, there has been little progress in understanding how they may directly influence plant interactions. Consequently, most researchers have employed indirect approaches such as ‘plant-soil feedback’ studies, where soil, often sterilised, is conditioned by growing one plant species for a short time to cultivate a rhizosphere microbial community, after which a range of species is grown in the same soil and the impact upon plant growth followed. The consensus from these studies is that plant growth is more constrained by organisms present in their own rhizospheres than that of other plant species. This negative feedback is generally assumed to be due to a build up of microbial pathogens constraining plant growth in their ‘own’ soil, although pathogen populations are seldom measured. These ‘plant-soil feedback’ studies are also often characterised by short duration and small numbers of plant species.

Recent advances in new technologies now enable the mysteries of the soil microbial ‘black box’ to be unlocked and, if embraced, could open up new possibilities for rhizosphere manipulation and enhanced plant productivity as well as new insights into natural plant community drivers.

Image caption: external hyphae and spores of an arbuscular mycorrhizal fungus around a root. Image courtesy of the authors.

 

 

Special Feature: Responses to global climate change: insights from organismal physiology

Phenotypic plasticity and evolutionary demographic responses to climate change: taking theory out to the field.

Luis-Miguel Chevin, Sinéad Collins and François Lefèvre  This forest of introduced cedars was used to study the variation in plasticity of radial growth in response to drought by one of the authors (F. Lefèvre).

When facing strong environmental challenges such as those imposed by climate change, species may avoid extinctions by three main mechanisms: dispersal to their preferred habitat (or niche), genetic evolution in response to natural selection, or adjustment of the characters of the organisms at the individual level. The latter is termed phenotypic plasticity, and has received relatively little attention from ecologists and evolutionary biologists until recently. In this paper, we review the theoretical literature on how phenotypic plasticity and genetic evolution interact with population growth in changing environments. We describe models of plasticity, and how they have been used to generate quantitative predictions in a variety of contexts relevant to climate change. These include adaptation to a new environment following an abrupt change or introduction to a new habitat; responses to a sustained trend of change such as global warming; and adaptation to local conditions in the face of migration bringing locally maladaptive genes. We then assess to what extent these predictions can be tested with natural populations, and identify key difficulties in doing so, as well as possible ways to overcome them. Finally, we identify new questions that should be investigated in theoretical models. These questions arise from the examination of two emblematic cases of physiological effects of current climate change: resistance to drought and temperature extremes in trees, and responses to CO2 elevation in marine microscopic algae, the major carbon sink on earth. Our main conclusion is that proper understanding and prediction of the consequences of climate change for biological diversity requires taking into account the plastic responses of individual organisms, the evolution of populations, and their interacting effects.

Image caption: This forest of introduced cedars was used to study the variation in plasticity of radial growth in response to drought by one of the authors (F. Lefèvre).

 

Special Feature: Mechanisms of Plant Competition

Plants give up rather than fight when facing strong competition.

Stephen P. BonserPhoto credit: Angela Moles.

Competition among neighbours for limited resources is one of the main adversities most individuals must deal with in natural communities. Being denied resources by neighbours has a suite of negative effects on individuals including reduced survival, slower growth, and fewer offspring. For over 40 years, scientists studying competition in plant communities have assumed that plants faced with severe competition should respond by being better competitors. In other words, plants under competition should delay reproduction in favour of more effectively acquiring resources. I tested this prediction by examining previously published studies on how plants grow and reproduce when grown in competitive conditions. Contrary to the long standing assumption on how plants are believed to respond to competition, I found that increasing the severity of competition was related to increased rather than decreased allocation to reproduction. This finding was the same for both short lived weedy annual species, and longer lived perennial herbs. It appears that severe competition is a harbinger of death in plant communities, and plants employ a strategy of increased reproduction prior to dying under severe competition. Greater allocation to reproduction in weedy annual plants under competition demonstrates that these plants have adapted to life in competitive environments. This finding challenges the long standing assumption in ecology that competition is unimportant in highly disturbed weed-dominated communities. Greater allocation to reproduction under strong competition in longer-lived perennial herbs suggests that these plants tend to give up rather than fight when faced with impending mortality. Plants with a strong ability to compete against their neighbours do evolve but, somewhat paradoxically, they likely do so under low or modest competition. The research I present here presents new ways of thinking about how plants adapt in their struggle for existence.

Image caption: Photo credited to Angela Moles.

 

Special Feature: Mechanisms of Plant Competition

Plant competition, temporal niches and implications for productivity and adaptability to climate change in water-limited environments.

Susanne Schwinning and Colleen K. Kelly Two most-closely related species of Bursera in the Mexican tropical dry forest (gray stem: Buresera heteresthes; brown stem: Bursera instabilis). Despite being nearly identical as adults, this species pair, and many others like them, coexist through alternating recruitment in time.

Water is the primary factor limiting the growth and productivity of land plants, and fluctuations in plant-available water are ubiquitous in most terrestrial environments, due to variable and unpredictable rainfall. Evolution has produced numerous strategies of compromise between the conflicting goals of maximizing growth and reproduction when water is available and minimizing the risk of mortality when it is not. Because no species is able to pre-empt all opportunities for water and nutrient uptake, many plant species can coexist. However, the mechanisms responsible for making this stable, competitive coexistence possible are often hidden and difficult to study experimentally.

Species that have very different form, physiology and life cycle often differ in how fast they respond to the beginning of the rainy season, or how soon they switch from active growth to dormancy. In this way, adult plants can reduce competition between them, but this is not an option for closely-related species that are nearly identical in growth strategy. These species often differ in sensitivity to environmental stress levels during the vulnerable seedling stage, so that some species recruit more often in relatively stress-free years, while more tolerant species recruit in more average years.

Understanding and predicting how plant communities will respond to contemporary climate change remains a challenge to science, but one that can be guided by addressing the fundamental ways in which fluctuations in plant-available water interact with competition, between either adults or seedlings.

Image caption: Two most-closely related species of Bursera in the Mexican tropical dry forest (gray stem: Buresera heteresthes; brown stem: Bursera instabilis). Despite being nearly identical as adults, this species pair, and many others like them, coexist through alternating recruitment in time.

 

Special Feature: Mechanisms of Plant Competition

Kin Recognition And Competition In Plants.

Susan A. Dudley, Guillermo P. Murphy and Amanda L. File Photo provided by authors.

Our lab has shown that plants can recognize their relatives. A plant in a pot with its siblings grows differently than a plant with strangers. These changes are plant behaviour – plants do behave, they just behave very slowly. While the ability of plants to recognize siblings is surprising enough, it leads to the question of why they behave differently with strangers than siblings. We argue that plants are being more aggressively competitive with strangers, and more cooperative with siblings. The question that this review paper addresses is whether we can demonstrate scientifically that plants are behaving competitively or cooperatively. What methods might researchers use, what kinds of plant traits could they measure, what problems do we foresee? A major challenge is that plants are often interacting belowground through their roots, which are difficult to measure. We suggest that plant responses to different competitors could tell us more about how they compete. While this is a pure science question, it has some practical implications: if plants behave differently with siblings or strangers, could they compete more with weeds, and cooperate with other crops?

Image caption: Photo provided by authors

 

Special Feature: Mechanisms of Plant Competition

A functional-comparative approach to facilitation and its context-dependence

Bradley J. Butterfield and Ragan M. Callaway  Facilitation of Potentilla, Erigeron and Aquilegia sp. by Silene acaulis in the Rocky Mountains, and Atriplex confertifolia by Ephedra torreyana in the Great Basin Desert.  Do similar functional traits determine facilitation in these very different environments?

Plants can have both positive and negative effects on one another. A great deal of research has been conducted on the mechanisms and outcomes of competition, such as how neighboring plants suppress growth and reproduction of competitors by reducing light or nutrients. Some of this research has also asked whether specific traits contribute consistently to the ability to compete well, generally traits that determine how plants respond to and influence their local environment. What is less well understood is how such traits influence the outcome of positive interactions among plants. Positive interactions, or facilitation, are a consequence of plants modifying their microenvironment through positive feedbacks (e.g. enhanced soil fertility, herbivore deterrence, reduced temperature extremes) or increasing the availability of a limiting resource for other species (e.g. facilitation of grasses by deep-rooted trees that reduce evaporation from shallow soils through shading). Why certain species benefit from neighbors while others do not, and why some species are good nurse plants and others are not, is likely strongly influenced by their functional traits. However, we know very little about which traits determine the outcome of facilitative interactions, or whether a suite of similar traits are important across the broad array of environments in which facilitation occurs.

In this study, we reviewed the literature on how functional traits respond to a range of environmental factors, and related these responses to the facilitative effects of neighbors. We used this comparison to predict why certain species may benefit from neighboring plants and others may not. We focused on three frequently measured traits: specific leaf area (the ratio of leaf area to dry mass), height and seed mass. We found that predicted facilitative responses of plants fell into two general categories, depending on whether neighbors buffered dynamic fluctuations in environmental stresses, or ameliorated persistent stresses. While these predictions need further testing, the trait-based patterns presented here provide useful generalizations for how plants may influence biodiversity through positive interactions, as well as identifying commonalities in the mechanisms of positive interactions across seemingly disparate environments.

You can also listen to Alan Knapp's podcast interview with Brad Butterfield via Soundcloud:

Image caption: Facilitation of Potentilla, Erigeron and Aquilegia sp. by Silene acaulis in the Rocky Mountains, and Atriplex confertifolia by Ephedra torreyana in the Great Basin Desert. Do similar functional traits determine facilitation in these very different environments?

 

Special Feature: Mechanisms of Plant Competition

Plant ecology’s guilty little secret: understanding the dynamics of plant competition..

Clare J. Trinder, Rob W. Brooker and David RobinsonRibwort plantain and cocksfoot grass grown in competition.

The study of plant competition is controversial. Although it has been studied for over a hundred years, researchers still find it hard to agree on a general set of rules that predict which particular plant will win in different habitats; under different environmental conditions (hot or cold; dry or wet) or when soil nutrients are plentiful or scarce. We suggest that these difficulties arise from the way that we have been studying plant competition. Early competition experiments focussed on agricultural crops and the impact of weed species on final yields. The approach of measuring final yield (as the weight of plant material or its seed output) persists today, even when studying wild plants in the natural environment. From our own work measuring competition between two common grassland plants (ribwort plantain and cocksfoot grass), we found we could interpret our results by studying their growth across the whole summer. We found that at earlier stages of their development plantain was the stronger competitor but later on cocksfoot overtook it and become the winner in this contest for soil nitrogen – a key plant nutrient. From this we realised that the length of time that a plant competition experiment runs for could completely change the outcome of the experiment. In our case, an earlier harvesting date would have shown plantain as the winner, but a later harvest showed cocksfoot as the superior competitor. With this in mind, we urge other researchers to consider plant competition as dynamic, changing through time, rather than a static process with an easily defined end point which tells us all we need to know about the competitive interactions between the plants. We also discuss whether it is appropriate simply to measure the weight of plants at the end of an experiment. We suggest that we should be developing methods to allow us to measure the uptake of resources that plants compete for directly, such as soil nutrients, as plant weight is likely to reflect a range of factors in addition to its ability to take up nutrients before its competitors are able to.

Ribwort plantain and cocksfoot grass grown in competition.

 

Special Feature: Responses to global climate change: insights from organismal physiology

 

Special Feature: Responses to global climate change: insights from organismal physiology

Difficulties in adapting to high temperatures.

Ary A. Hoffmann, Steven L. Chown, Susana Clusella-Trullas  Photo: A species of vinegar fly, Drosophila birchii, found in the wet tropics of Australia and New Guinea with an intermediate upper thermal limit when compared to other Drosophila species. This species and others from the tropics are exposed to high average temperatures but do not necessarily experience extreme temperatures like insects living in arid environments.

Cold-blooded animals including insects and lizards cannot survive and reproduce once temperatures become too high. These upper temperature limits tend to vary among species and also for different activities of animals. Typically limits are lower for reproduction than for survival, and higher for species that occur in deserts than those that are found in cool climate rainforests.

Here we ask whether upper limits are constrained in different ways. Can species evolve to change their upper limits, or are upper temperature limits relatively fixed in evolutionary time? To what extent can upper limits of different groups of species be affected by the environment? Answers to these questions are particularly pertinent today as global climate continues to warm. Average temperatures are now expected to increase by 2-4°C and extreme temperatures are expected to increase even further. This can threaten the survival of species if their upper limits are repeatedly exceeded.

To test for evolutionary constraints, we compare the upper limits across insects and lizards and consider whether related species tend to have more similar limits than unrelated species. If related species are more similar than unrelated species, this can point to a constraint. We suggest that this is likely to be the case, with limits pre-dating recent speciation events. Studies that have investigated genetic variation in upper limits within species also suggest that limits cannot be easily shifted through evolution via natural selection. This contrasts markedly with lower temperature limits that tend to vary much more among species and can be more easily changed by natural selection. We also point out that upper limits can be much less easily modified by the environment - such as through hardening - than lower limits.

As a result of these patterns, we suspect that many species could be facing extinction when the maximum temperatures in environments start to increase. Species will then need to move to cooler conditions if they are to persist. We suggest that this threat in both insects and lizards is greatest at mid-latitude locations rather than at the equator or at high latitudes.

Photo: A species of vinegar fly, Drosophila birchii, found in the wet tropics of Australia and New Guinea with an intermediate upper thermal limit when compared to other Drosophila species. This species and others from the tropics are exposed to high average temperatures but do not necessarily experience extreme temperatures like insects living in arid environments.

 

Special Feature: Responses to global climate change: Insights from organismal physiology

Keeping cool, fed and hydrated under environmental change

Michael R. Kearney, Stephen J. Simpson, David Raubenheimer and Sebastiaan A. L. M. Kooijman  Egernia cunninghami photo by lostandcold at flickrsrc=

Having enough food and water, and staying within comfortable temperatures, are basic life requirements for all animals. If we can accurately predict the degree to which animals can regulate their heat, water and nutritional levels in different environments, we can begin to understand how environmental change will affect biodiversity. In this paper we show how principles from the fields of thermodynamics and animal behaviour can be integrated to understand how animals jointly balance their heat, water and nutrient budgets.

Thermodynamics is the field of physics that concerns the flows of energy and matter. In animals, the flows of heat, water and food are intimately connected. We show how principles developed within the field of ‘biophysical ecology’ for considering heat flow can be connected with principles developed in ‘metabolic theory’ for understanding flow of energy and matter associated with food. In particular, we apply one of the most comprehensive and long-standing metabolic theories, the ‘Dynamic Energy Budget’ theory, to this problem and show how it can be integrated with ‘biophysical ecology’ to work out, jointly, the heat, water and food budget. The set of environmental requirements for an organism to survive and reproduce are called its ‘niche’ and in this article we propose the term ‘thermodynamic niche’ to cover core heat, water and food requirements.

A key issue in determining thermodynamic niches is that animals don’t passively experience their environments. Rather, they use behaviour to try and find their thermodynamic niches. We show how the principles of the Geometric Framework of Nutrition can be used to understand the consequences of different behavioural choices for what to eat and where and when to forage. We demonstrate the approach using the example of an herbivorous lizard, illustrating the potentially strong tradeoffs that exist in balancing heat, water and food budgets, even in ‘dry-skinned’ animals. Such ‘thermodynamic niche’ models will enable physiological ecologists to make predictions of the responses of animals to future environmental change.

Image caption: Egernia cunninghami photo by lostandcold at flickr

 

Special Feature: Responses to global climate change: insights from organismal physiology

Phenotypic plasticity and evolutionary demographic responses to climate change: taking theory out to the field.

Luis-Miguel Chevin, Sinéad Collins and François Lefèvre  This forest of introduced cedars was used to study the variation in plasticity of radial growth in response to drought by one of the authors (F. Lefèvre).

When facing strong environmental challenges such as those imposed by climate change, species may avoid extinctions by three main mechanisms: dispersal to their preferred habitat (or niche), genetic evolution in response to natural selection, or adjustment of the characters of the organisms at the individual level. The latter is termed phenotypic plasticity, and has received relatively little attention from ecologists and evolutionary biologists until recently. In this paper, we review the theoretical literature on how phenotypic plasticity and genetic evolution interact with population growth in changing environments. We describe models of plasticity, and how they have been used to generate quantitative predictions in a variety of contexts relevant to climate change. These include adaptation to a new environment following an abrupt change or introduction to a new habitat; responses to a sustained trend of change such as global warming; and adaptation to local conditions in the face of migration bringing locally maladaptive genes. We then assess to what extent these predictions can be tested with natural populations, and identify key difficulties in doing so, as well as possible ways to overcome them. Finally, we identify new questions that should be investigated in theoretical models. These questions arise from the examination of two emblematic cases of physiological effects of current climate change: resistance to drought and temperature extremes in trees, and responses to CO2 elevation in marine microscopic algae, the major carbon sink on earth. Our main conclusion is that proper understanding and prediction of the consequences of climate change for biological diversity requires taking into account the plastic responses of individual organisms, the evolution of populations, and their interacting effects.

Image caption: This forest of introduced cedars was used to study the variation in plasticity of radial growth in response to drought by one of the authors (F. Lefèvre).

 

Special Feature: Responses to global climate change: insights from organismal physiology

Will adaptation rescue marine life from ocean acidification?

Morgan W. Kelly and Gretchen E. Hofmann Graduate Student Paul Matson samples water for pH measurements below sea ice in Antarctica.

Atmospheric carbon dioxide (CO2), produced by the burning of fossil fuels, is being taken up in large quantities by the world’s oceans. Dissolved CO2 produces carbonic acid and decreases ocean pH, a process known as ocean acidification (OA). This process leads to a reduced concentration of carbonate ions, which are used by many marine species to build calcium-based shells and skeletons. Laboratory experiments simulating future ocean conditions have shown negative effects of OA on many species, including effects on growth rates and reproduction. However most of this research has exposed modern populations of organisms to conditions not projected to occur until the next century. And yet we know from previous examples of human-caused environmental change, that natural populations of organisms can sometimes evolve quite quickly in response to a changing environment.

In this paper, we review the status of current scientific research on the potential for adaptation to OA in marine organisms. This body of work is currently quite small, but we argue that data on natural variation in pH, and lessons learned from previous work on adaptation to temperature, can shape the direction of future research. We conclude with a list of recommendations for future research priorities, and the technological tools for accomplishing these goals.

The current process of human-driven acidification is expected to produce changes in ocean chemistry more rapid than any experienced in the past 20 million years. Except for organisms with short generation times, the current rate of change will likely be too great for much adaptation based on new mutations to occur. However, given enough genetic variation in a species, substantial adaptation to environmental change is possible, even over a single generation. As a result, a priority in OA research for the next decade will be to document the extent of such variation and consider the role of evolutionary processes in projections about the effects of OA on modern marine species.

Image caption: Graduate Student Paul Matson samples water for pH measurements below sea ice in Antarctica

 

Standard papers

 

 

Climate warming and ectotherm body size – from individual physiology to community ecology

Jan Ohlberger  European grayling (Thymallus thymallus)Photo taken by Jan Ohlberger

Climate warming can have profound effects on animal species. The most commonly observed responses to climate warming are shifts in species’ geographic distribution and the timing of biological events. Recently, declines in average body size have been reported across a range of species and suggested to represent a universal response to increasing temperatures. Such shifts can have severe consequences for the structure and functioning of ecosystem and thus the services they provide for humans. However, our knowledge about the causes of declining body sizes and how these vary between species and environments is limited.

In this study, I summarize evidence for the effects of rising temperatures on the mean body size and the distribution of sizes within populations. I show how different mechanisms determine organism responses to climate warming in complex ways and at different levels of organization, from individuals to entire food webs. These mechanisms include physiological effects on individual growth and development, changes in ecological interactions such as size-dependent survival or dispersal, and shifts in the species composition of a food web.

While we generally understand how single organisms respond to temperature changes under laboratory conditions, in nature the actual response of a population will depend on complex interactions with potential competitors, predators and prey. Considering the broader ecological context is therefore necessary when trying to understand the underlying causes of declines in mean organism body sizes. A better functional understanding will help to improve our predictions about future changes and the management of ecosystems in the face of a warming climate.

Image caption: European grayling (Thymallus thymallus). Photo taken by Jan Ohlberger.

 

The use of leaf economics and size traits to classify woody and herbaceous vascular plants

Simon Pierce, Guido Brusa, Ilda Vagge and Bruno E. L. Cerabolini Bruno Cerabolini (University of Insubria) and co-workers identify and collect plant material near the Stelvio pass on the border between Italy and Switzerland, as part of a study of the life-history traits of hundreds of species from a range of habitats.

A vast number of plant species carpet the terrestrial landscape; diversity that is hard to come to terms with by investigating the peculiarities of each species one at a time. However, where plants grow in similar circumstances similar lifestyles and physiologies are apparent. By measuring the traits associated with these lifestyles the bewildering diversity of species can be reduced into a manageably small range of survival strategies that can help us to understand how diversity develops, how communities of species form and how species affect the working of ecosystems. The study published in this edition of Functional Ecology is based on the variability of traits involved in plant competition and resource use, and combines these trait values into a practical tool allowing the quantification of the overall survival strategy. Whilst this has been possible in one form or another for some time, recent advances in our understanding of how traits vary worldwide indicate that a small number of leaf traits are particularly important and representative of the overall strategy, and are shared by a broad swathe of plant life including ferns, gymnosperms and both herbaceous and woody flowering plants. Fortunately these traits are relatively straightforward to measure, and the new method will allow ecologists to rapidly compare the survival strategies of huge numbers of plants growing in their natural environment. In this way, different species within a community can be compared, different communities can be characterized, and even the differences within populations of single species can be measured. The new method, known as CSR classification after the theory of Competitor, Stress-tolerator and Ruderal strategies developed by British ecologist Philip Grime and co-workers, is based on traits measured for wild herbaceous and woody plants from a broad range of habitats spanning the high alpine zone of the Italian Alps (see photo) to the continental climate of Lombardy’s lowlands.

Image caption: Bruno Cerabolini (University of Insubria) and co-workers identify and collect plant material near the Stelvio pass on the border between Italy and Switzerland, as part of a study of the life-history traits of hundreds of species from a range of habitats. This led, in the current issue, to the production of a novel method for quantifying and comparing plant survival strategies (photo: Simon Pierce).

 

A trait-based ecosystem model suggests slow-growing plants are more responsive to rising atmospheric CO2 concentration than fast-growing plants in field conditions

Ashehad A. Ali, Belinda E. Medlyn, Kristine Y. Crous, Peter B. Reich Measuring photosynthesis on Bromus inermis at the BIOCON FACE experiment. Photo: David Ellsworth.

Rising atmospheric CO2 concentration (Ca) affects plant growth, but it does not affect all species equally. In glasshouse studies it is generally found that fast-growing species are the most responsive to high Ca. This finding has often been taken to imply that fast-growing species, such as many weeds, will benefit most strongly from rising Ca.

We submitted this finding to the rigours of mathematical logic, by developing a simple model that simulates plant growth, and using it to examine what kinds of species might be expected to benefit most from high Ca. The model showed that for young plants that are not limited by resource availability, the fast-growing species should benefit most, in agreement with the findings from glasshouse studies.

However, when the model is applied to larger plants growing in field conditions where they are limited by light and nutrient availability, this finding is turned on its head: it is the slow-growing species that benefit most from high Ca.

This result is logical because fast-growing species are already efficient at current levels of Ca. It is the relatively inefficient species that stand to gain the most from rising Ca. The result was supported by data from the BIOCON Free-Air CO2 Enrichment experiment, where plants were grown in the field for several years under high Ca, and the relative increase in growth was largest for species that were slowest-growing at ambient Ca.

This result is important because it demonstrates that we cannot extrapolate directly from glasshouse experiments to the field. Potentially, rising Ca may not favour the fast-growing weedy species; it may in fact give an advantage to slower-growing native species in the long run.

Image caption: Measuring photosynthesis on Bromus inermis at the BIOCON FACE experiment. Photo: David Ellsworth

 

Pollinators, mates and Allee effects: the importance of self-pollination for fecundity in an invasive lily

James G. Rodger, Mark van Kleunen and Steven D. Johnson  Agrius convolvuli pollinating Lilium formosanum. Photo Steve Johnson.

Biologists lack a good understanding of what causes some species to invade natural habitats, after people introduce them to new environments, while others do not spread successfully. Initially, only a few individuals may be present and after dispersal, single individuals are isolated from the original population. Clearly, the chances that invasion takes place are greater if plants in these small populations can reproduce successfully. Small isolated plant populations attract fewer pollinators. Furthermore, pollinators that visit single isolated plants may not be carrying pollen from other plants of the same species. Recent studies have shown that plant species in which individuals are capable of being fertilised by their own pollen are more likely to be invasive than those that need pollen from another individual (the case in around 50% of flowering plants). However, there has not been any direct evidence that this is because self-fertilisation mitigates effects of low population size or isolation on reproduction. We tested this hypothesis in Lilium formosanum, a Taiwanese lily species invasive in South Africa. Although a giant hawkmoth pollinates their flowers, plants are also capable of self fertilisation in the absence of pollinator visits.

We prevented self-fertilisation in Lilium formosanum by emasculating flowers – removing their male pollen-producing parts – in 66 ‘naturally occurring’ invasive populations ranging from 1 to 6000 individuals over three years. Emasculation reduced seed production by two thirds. This shows that Lilium formosanum depended on self-fertilisation for at least this fraction of its seed production, due to infrequent pollination by hawkmoths. However, this was not related to population size. We also artificially isolated emasculated plants 3-702m from two ‘naturally occurring’ populations, and put either a second emasculated plant or an intact plant – one that still had its male parts and could supply pollen – next to each of these emasculated plants. Isolation increased dependence on self-pollination for seed production, but only in the absence of a pollen supply. This shows that moth pollinators visited isolated plants as frequently as non-isolated plants, but were less likely to be carrying the right kind of pollen when they visited isolated plants. Observations of hawkmoth body scales and pollen, deposited on emasculated flowers, supported this conclusion. This is the first study to show that self-fertilisation may facilitate invasion by increasing reproduction especially in isolated plants, and uses new methods to separate effects of pollinator visitation from pollen availability.

Image caption:Agrius convolvuli pollinating Lilium formosanum. Photo Steve Johnson.

 

Sex appeal is cheap for male threadfin rainbowfish

Andrew Trappett, Catriona H. Condon, Craig White, Phil Matthews & Robbie S. Wilson Threadfin rainbowfish (Iriatherina werneri).  Photo provided by authors.

A basic assumption of evolutionary ecology is that sexiness is costly – otherwise, why wouldn’t everyone be sexy? It’s believed that only the best individuals are able to bear the costs associated with excessive attractiveness; that developing and maintaining an outrageously elongated tail, or heavy horns, or conspicuously-bright scales takes a lot of energy or increases the individual’s risk of being spotted and captured by a predator.

A male who can thrive in spite of his sexy features? Now that’s attractive.

We tested this assumption by measuring the costs and benefits associated with the long, flamboyant fins of male threadfin rainbowfish. Found in streams and lakes across the top-end of Australia, threadfin males develop extensive, trailing fin streamers, presumably at great cost to their swimming abilities. Swimming is an energetically expensive activity, and changes to a fish’s hydrodynamic profile – for example, via a greatly-elongated fin – should increase the energy needed to swim and decrease its overall swimming performance. To see if this was the case, we measured the metabolic rates and sprint swimming speeds of male fish with varying natural fin lengths, and then re-measured males after experimentally shortening their fins.

We found, as expected, that female threadfins preferred males with longer ornaments; but we were surprised to find no evidence that longer fins were hydrodynamically costly to males. Males swam just as fast after their fins were shortened as before, and fin size didn’t affect metabolic rates during swimming.

Ultimately, our results didn’t support the idea that sexiness is a burden. Is this because it isn’t? Not exactly – a threadfin’s ornaments didn’t affect its metabolic rate or swimming speeds, but that doesn’t mean these traits aren’t costly; it could merely mean that we didn’t measure the right thing. Our next step in this research will involve looking at how ornament length relates to a male’s ability to survive in more-natural conditions, where escaping predation depends on more than just swimming fast.

Threadfin rainbowfish (Iriatherina werneri). Photo provided by authors.

 

Coping with unpredictability in early life

Benjamin Homberger, Susanne Jenni-Eiermann, Alexandre Roulin and Lukas Jenni Grey partridge chick around 1 week old. Early life conditions can profoundly affect an individual’s traits later in life. Photograph by Markus Jenny.

Understanding how organisms cope with their environment and why they show different survival strategies is crucial in a changing world. An organism’s characteristics are shaped by the interplay of genes and environmental conditions which ultimately affect physiology, behaviour and fitness. Early developmental conditions (e.g. nutritional supply in prenatal or early life) can profoundly affect an individual’s traits later in life. However, adverse prenatal or early life conditions do not necessarily entail detrimental consequences but can act as developmental cues that allow an organism to adapt to these specific environmental conditions. For example, a mother can react to environmental perturbations (e.g. infections or food shortage) during gestation or egg-laying by transferring messenger chemicals (hormones, maternal antibodies) to her embryos or eggs. Her offspring might then profit in terms of enhanced immunity or decreased energetic demands. But physiological systems such as the immune system or the physiological stress response not only provide essential functions but are also costly. A cost of many physiological systems is the production of free radicals, which can damage cells. Thus, an organism’s physiology has to be well orchestrated and carefully adjusted to the environment it functions in.

We investigated in grey partridges from wild and domesticated origin how they physiologically cope with pre- and postnatal unpredictable food supply by measuring indices of stress physiology, immunity, energy production and resistance to free radical attack. Wild birds had a strong stress response and immune response and had a good resistance to free radical attack which could benefit them in the wild. However, their strong physiological stress response was ill-fitted to the captive environment. Because of their strong reaction to the everyday stress of an unpredictable captive environment, their resistance to free radicals was markedly reduced. On the other hand, wild birds reacted to prenatal unpredictable food supply by lowering their stress response and thereby reducing the production of free radicals. Contrarily, domesticated birds showed relatively lower stress response, immune response and resistance to free radicals. Nor did they adjust their offspring’s physiology to prenatal conditions.

In all birds we found that postnatal food unpredictability caused a lasting boost in the bird’s immunity, improving their ability to fight infections. In essence, our study highlights important differences and coping strategies between wild and domesticated birds and it sheds light on the complex interplay of physiological systems which can profoundly affect an organism’s life-history.

Grey partridge chick around 1 week old. Early life conditions can profoundly affect an individual’s traits later in life. Photograph by Markus Jenny.

 

Foraging behaviour and success in elephant seals

Jason L. Hassrick, Daniel E. Crocker and Daniel P. Costa A female northern elephant seal departs for sea with a GPS transmitter on her head and a time-depth recorder on her back.  Credit: D.P. Costa.

Like many mammals, northern elephant seals grow throughout life and breed on land. Unlike other animals, elephant seals are pelagic foragers and find food in the open ocean. Unable to feed on land, elephant seals devote a fixed percentage of stored body reserves to their young. Thus, as they grow, more energy is invested in offspring and mothers must find and catch more food every year that they breed. We examined how a female’s size and age interact with changing environmental conditions to determine how well they forage at sea. Using time-depth recorders to measure how deep and how long seals dived and satellite transmitters to measure how far they travelled, we tested whether diving and movement behaviors determined how much weight females gained. We identified components of behavior that described how much time elephant seals spent under water searching for food, the distance seals had to swim on a foraging trip, and how females dived. To describe how seals dived, we incorporated sequences of dives that change based on whether seals actively forage, are traveling to foraging areas, or are passively drifting through the water. While body size and age are positively correlated, they influence foraging efficiency, which is the amount of prey consumed in a given period of time, in different ways. A female’s size determined how long she could hold her breath under water, which in turn determined foraging efficiency. Age affected diving patterns and older seals were more efficient divers, but the effect was subtle and evident only on long foraging trips. Distances seals traveled were mainly driven by yearly changes in the distribution of prey, which are spread out and patchy in the open ocean. Our study helps to explain how large body size positively influences foraging success at sea. This has important implications for tradeoffs between current and future reproduction, because a female that invests too much energy in a pup this year runs the risk of returning to sea smaller and not being able to dive and store adequate body reserves for the next year she breeds.

A female northern elephant seal departs for sea with a GPS transmitter on her head and a time-depth recorder on her back. Credit: D.P. Costa.

 

Does herbivory influence litter decomposition of contrasted grasses in similar ways?

Sébastien Ibanez, Lionel Bernard, Sylvain Coq, Marco Moretti, Sandra Lavorel & Christiane Gallet Male grasshopper (Chorthippus scalaris). Photo credited to Sébastien Ibanez.

When herbivores eat leaves, plants may alter their chemical composition. For example, herbivory can induce the production of defensive compounds such as phenolics. If these changes are still present after leaf senescence, they can affect litter decomposition. This phenomenon has rarely been studied for grasses, which contain fewer defensive compounds than trees or broad-leaved herbs. Grasses include species having either permanent or induced defences, so we expect that depending on the grass strategy against herbivores, herbivory would differentially alter litter decomposition.

We conducted an experiment in which mountain fescue, Festuca paniculata (permanent defences) and cocksfoot, Dactylis glomerata (induced defences) were consumed by grasshoppers. We studied the speed of litter decomposition and measured the chemical composition of fresh, senescent and decomposed leaves, focusing on their carbon:nitrogen ratio and on phenolics.

Grasshoppers did not modify the carbon:nitrogen ratio of D. glomerata leaves, but they induced the accumulation of phenolics. However, most phenolics were lost during senescence, so that grasshoppers did not influence the litter decomposition rate.

Grasshoppers slightly increased the carbon:nitrogen ratio of F. paniculata, but the litter decomposition rate did not depend on this chemical ratio, contrary to previous findings. Grasshoppers did not induce the accumulation of phenolics in fresh leaves, but they increased the rate at which they disappeared during senescence. Interestingly, this led to a decreased litter decomposition rate, probably because the phenolics of this fescue are substrates for microbes and enhance decomposition.

We conclude that depending on the characteristics of grasses, in particular the way they respond to herbivory, herbivory can have contrasted effects on their litter decomposition, which might lead to complex outcomes at the ecosystem level.

Male grasshopper (Chorthippus scalaris). Photo credited to Sébastien Ibanez.

 

Fruit bats and bat fruits: the evolution of fruit scent in relation to the foraging behaviour of bats in the Old and New World tropics

Robert Hodgkison, Manfred Ayasse, Christopher Häberlein, Stefan Schulz, Akbar Zubaid, Wan Aida Wan Mustapha, Thomas H. Kunz and Elisabeth K. V. Kalko A short-nosed fruit bat (Cynopterus brachyotis) eating a fig (Ficus hispida). Photo by Rob Hodgkison.

Bats are important seed dispersers for many tropical plants. Fruit consumption by bats is believed to have evolved at least twice: once in the Old World tropics (in Africa, Asia, and the Pacific) within the flying fox family (Pteropodidae), and once in the New World tropics (Central and South America) within the leaf-nosed bat family (Phyllostomidae). Bats from both families have a keen sense of smell, which they use to locate fruits. However, it is currently unknown whether bats from both families share a preference for the same types of scent.

To explore this idea, we conducted fieldwork in Malaysia and Panama to collect the natural fruit scents of wild figs. We also performed behavioural experiments on bats to test whether natural fig fruit scents from both regions would induce the bats to feed. The bat species selected for these experiments were the short-nosed fruit bat (Cynopterus brachyotis) in Malaysia, and the Jamaican fruit bat (Artibeus jamaicensis) in Panama.

We discovered that figs, from both Malaysia and Panama, had chemically similar fruit scents--dominated by a group of chemicals known as monoterpenes. Interestingly, these monoterpenes were completely absent from similar, closely-related fig species consumed by birds.

Closely-related fig species, consumed by bats, produced chemically similar fruit scents. However, one group of figs from Malaysia (from the sub-genus Sycomorus) proved to be an exception. The fruit scents of these figs were highly variable, sometimes completely lacking monoterpenes, which suggests that their interactions with bats are likely to be less specialized. In other words, in addition to bats, these Old World figs are likely to be consumed by a broad range of other seed-dispersing animals.

Consistent with this view, Jamaican fruit bats, from Panama, clearly preferred fruit scents with monoterpenes, and were attracted to similar fruit scents from both Panama and Malaysia. Short-nosed fruit bats from Malaysia, by contrast, were only attracted to fruit scents from Malaysia, and rejected chemically-similar fruit scents from Panama. Short-nosed fruit bats also consume Sycomorus figs, which may lack monoterpenes, so in this species, no obvious fruit scent preference could be discerned.

Image Caption: A short-nosed fruit bat (Cynopterus brachyotis) eating a fig (Ficus hispida). Photo by Rob Hodgkison.

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