Trait variation across biological scales shapes community structure and ecosystem function

Phylogenetically-conserved candidate genes unify biodiversity-ecosystem function relationships and eco-evolutionary dynamics across biological scales

The intra- and interspecific facets of biodiversity have traditionally been analysed separately, limiting our understanding of how evolution has shaped biodiversity, how biodiversity (as a whole) alters ecological dynamics and hence eco-evolutionary feedbacks at the community scale. Here, we propose using candidate genes phylogenetically-conserved across species and sustaining functional traits as an inclusive biodiversity unit transcending the intra- and interspecific boundaries. This framework merges knowledge from functional genomics and functional ecology, and we first provide guidelines and a concrete example for identifying phylogenetically-conserved candidate genes (PCCGs) within communities and for measuring biodiversity from PCCGs. We then explain how biodiversity measured at PCCGs can be linked to ecosystem functions, which unifies recent observations that both intra- and interspecific biodiversity are important for ecosystem functions. We then highlight the eco-evolutionary processes shaping PCCG diversity patterns and argue that their respective role can be inferred from concepts derived from population genetics. Finally, we explain how PCCGs may shift the field of eco-evolutionary dynamics from a focal-species approach to a more realistic focal-community approach. This framework provides a novel perspective to investigate the global ecosystem consequences of diversity loss across biological scales, and how these ecological changes further alter biodiversity evolution.

The effect of host community functional traits on plant disease risk varies along an elevational gradient

Quantifying the relative impact of environmental conditions and host community structure on disease is one of the greatest challenges of the 21st century, as both climate and biodiversity are changing at unprecedented rates. Both increasing temperature and shifting host communities toward more fast-paced life-history strategies are predicted to increase disease, yet their independent and interactive effects on disease in natural communities remain unknown. Here, we address this challenge by surveying foliar disease symptoms in 220, 0.5 m-diameter herbaceous plant communities along a 1100-m elevational gradient. We find that increasing temperature associated with lower elevation can increase disease by (1) relaxing constraints on parasite growth and reproduction, (2) determining which host species are present in a given location, and (3) strengthening the positive effect of host community pace-of-life on disease. These results provide the first field evidence, under natural conditions, that environmental gradients can alter how host community structure affects disease.

Climate change is causing shifts in the ecology and biodiversity of different world regions at unprecedented rates. Global warming is also linked with changes in the risk for certain infectious diseases in humans, but also in animals and plants. There are several possible mechanisms for this. For one thing, changing weather patterns may affect how pathogens grow and reproduce. For another, the distribution ranges of animal and plant hosts of certain disease-causing pathogens are changing because of global warming. This means that the distributions of pathogens are also changing, and so is the severity of the diseases that they cause.

Increasing temperatures may also influence the physiological traits that make host species suitable for pathogens. This is because the traits that allow species to survive or adapt to changes in their environment may also make them better at hosting and transmitting the pathogens that cause disease. For example, in plant communities, rising temperatures could favor species with faster growth rates, quicker reproduction and high dispersal, and these traits are often associated with more efficient spread of disease.

Despite a lot of research into the effects of climate, it remains unclear how temperature, pathogen growth and reproduction, and host species’ traits and distributions combine and interact to alter infectious disease risk, especially in wild plant communities. To investigate this, Halliday, Jalo and Laine studied an area in southeast Switzerland where natural temperature and biodiversity change gradually through the region. The aim was to explore how relationships between plant biodiversity, pathogens and disease risk change with temperature, and to understand whether environmental or biological factors influence infectious disease risk more.

Halliday, Jalo and Laine measured the levels of fungal diseases found in the leaves of plant communities spanning 1,100 meters of elevation, showing that higher temperatures increase disease risk both directly and indirectly. Directly, higher temperatures increased pathogen growth and reproduction, and indirectly, they influenced which plants were present and therefore able to act as disease hosts. The results also indicated that temperature can affect how the traits of plants drive the transmission rates of fungal pathogens. Important predictors of disease risk were traits relating to the growth rate of host plants, which tended to increase in areas with low elevation where the surface of the soil was warm.

This study represents the first analysis, in wild plants, of how changing temperatures, the traits of shifting host species, and resident parasite populations interact to impact infectious disease risk. The insights Halliday, Jalo and Laine provided could aid in predicting how global climate change will influence infectious disease risk.

Distributive stress: individually variable responses to hypoxia expand trophic niches in fish

Environmental stress can reshape trophic interactions by excluding predators or rendering prey vulnerable, depending on the relative sensitivity of species to the stressor. Classical models of food web responses to stress predict either complete predator exclusion from stressed areas or complete prey vulnerability if predators are stress tolerant. However, if the consumer response to the stress is individually variable, the result may be a distributive stress model (DSM) whereby predators distribute consumption pressure across a range of prey guilds and their trophic niche is expanded. We test these models in one of the largest hypoxic “Dead Zones” in the world, the northern Gulf of Mexico, by combining geochemical tracers of hypoxia exposure and isotope ratios to assess individual‐level trophic responses. Hypoxia‐exposed fish occupied niche widths that were 14.8% and 400% larger than their normoxic counterparts in two different years, consistent with variable displacement from benthic to pelagic food webs. The degree of isotopic displacement depended on the magnitude of hypoxia exposure. These results are consistent with the DSM and highlight the need to account for individually variable sublethal effects when predicting community responses to environmental stress.

Foraging aggressiveness determines trophic niche in a generalist biological control species

There is a growing evidence that consistent interindividual differences in behavior, that is, behavioral types, can play an important role in key ecological processes such as predator–prey interactions, which in turn can have direct implications on biological control. Behavioral types of generalist predators may affect these interactions through individual differences in predators’ prey preferences and the breadth of predators’ trophic niches. This study examined how the multivariate nature of behavior, namely foraging aggressiveness, activity level, and risk-taking behavior, determines prey selection and trophic niche of the generalist agrobiont spider Philodromus cespitum. In laboratory experiments, we determined the repeatability of these behaviors and the preference between crickets, moths, fruit flies, and collembolans. We found that all three behaviors were moderately to strongly repeatable but there were no correlations between them, thus they did not form a behavioral syndrome. Only foraging aggressiveness influenced the prey selection of philodromid spiders and the more aggressive individuals had wider trophic niches because they incorporated prey that were more difficult to capture in their diet. In addition, more aggressive individuals killed a greater quantity of particular prey types while other prey types were killed at a similar rate by both aggressive and nonaggressive individuals. The differences in philodromids’ foraging aggressiveness, therefore, affected not only the overall prey density but also resulted in different prey community composition. As pest density and composition can both affect crop performance, further research needs to investigate how the interindividual behavioral differences of generalist natural enemies cascade down on the crops.

Ecological Consequences of Intraspecific Variation in Coevolutionary Systems

AbstractThe patterns and outcomes of coevolution are expected to depend on intraspecific trait variation. Various evolutionary factors can change this variation in time. As a result, modeling coevolutionary processes solely in terms of mean trait values may not be sufficient; one may need to study the dynamics of the whole trait distribution. Here, we develop a theoretical framework for studying the effects of evolving intraspecific variation in two-species coevolutionary systems. In particular, we build and study mathematical models of competition, exploiter-victim interactions, and mutualism in which the strength of within- and between-species interactions depends on the difference in continuously varying traits between individuals reproducing asexually. We use analytical approximations based on the invasion analysis and supplement them with numerical results. We find that intraspecific variation can be maintained if stabilizing selection is weak in at least one species. When intraspecific variation is maintained under competition or mutualism, coexistence in a stable equilibrium is promoted when between-species interactions mostly happen between individuals similar in trait values. In contrast, in exploiter-victim systems coexistence typically requires strong interactions between dissimilar exploiters and victims. We show that trait distributions can become multimodal. Our approach and results contribute to the understanding of the ecological consequences of intraspecific variation in coevolutionary systems by exploring its effects on population densities and trait distributions.

Differences in prey personality mediate trophic cascades

Functional trait approaches in ecology chiefly assume the mean trait value of a population adequately predicts the outcome of species interactions. Yet this assumption ignores substantial trait variation among individuals within a population, which can have a profound effect on community structure and function. We explored individual trait variation through the lens of animal personality to test whether among-individual variation in prey behavior mediates trophic interactions. We quantified the structure of personalities within a population of generalist grasshoppers and examined, through a number of field and laboratory-based experiments, how personality types could impact tri-trophic interactions in a food chain. Unlike other studies of this nature, we used spatial habitat domains to evaluate how personality types mechanistically map to behaviors relevant in predator-prey dynamics and found shy and bold individuals differed in both their habitat use and foraging strategy under predation risk by a sit-and-wait spider predator. In the field-based mesocosm portion of our study, we found experimental populations of personality types differed in their trophic impact, demonstrating that prey personality can mediate trophic cascades. We found no differences in respiration rates or body size between personality types used in the mesocosm experiment, indicating relative differences in trophic impact were not due to variation in prey physiology but rather variation in behavioral strategies. Our work demonstrates how embracing the complexity of individual trait variation can offer mechanistically richer understanding of the processes underlying trophic interactions.

Biodiversity loss underlies the dilution effect of biodiversity

The dilution effect predicts increasing biodiversity to reduce the risk of infection, but the generality of this effect remains unresolved. Because biodiversity loss generates predictable changes in host community competence, we hypothesised that biodiversity loss might drive the dilution effect. We tested this hypothesis by reanalysing four previously published meta‐analyses that came to contradictory conclusions regarding generality of the dilution effect. In the context of biodiversity loss, our analyses revealed a unifying pattern: dilution effects were inconsistently observed for natural biodiversity gradients, but were commonly observed for biodiversity gradients generated by disturbances causing losses of biodiversity. Incorporating biodiversity loss into tests of generality of the dilution effect further indicated that scale‐dependency may strengthen the dilution effect only when biodiversity gradients are driven by biodiversity loss. Together, these results help to resolve one of the most contentious issues in disease ecology: the generality of the dilution effect.

Abundance and traits link predator ontogeny to prey communities

Function and abundances shape species interactions and thus ecological communities. While communities are often summarized as the mean function of each species, intraspecific variation in traits and thus function is an important driver of community composition. Ontogeny is a common source of intraspecific variation, but while age-related functional changes can alter species interactions, so too can the effects of those functions on the density of the focal organism. For instance, ontogenetic variation can trigger higher levels of cannibalism, reducing abundances and altering interspecific interactions. I manipulate ontogenetic variation in damselfly larvae to show that intraspecific variation can impact communities through two distinct mechanisms. First, within-species differences affect population sizes, and thus indirectly shape communities (indirect effect). In particular, ontogenetic variation resulted in smaller damselfly populations, likely because of increased cannibalism rates, and thus ontogenetically diverse populations had a smaller total effect on their prey. Second, trait variation can affect communities by creating differences in the strength of per capita species interactions (direct effect). In this case, damselfly populations with greater age variation had smaller per capita effects on prey communities. I conclude that ontogeny of a single species can directly and indirectly shape community composition.

Ecological and Evolutionary Stochasticity Shape Natural Selection

The distribution of biodiversity depends on the combined and interactive effects of ecological and evolutionary processes. The joint contribution of these processes has focused almost exclusively on deterministic effects, even though mechanisms that increase the importance of random ecological processes are expected to also increase the importance of random evolutionary processes. Here we manipulate the sizes of old field fragments to generate correlated sampling effects for a focal population (a gall maker) and its enemy community. Traits and communities were more variable in smaller patches. However, because of the preference of some enemies for some trait values (gall sizes), random variation in population mean trait values exacerbated differences in community composition. The random distribution of traits and interactions created predictable but highly variable patterns of natural selection. Our study highlights how stochastic processes can affect ecological and evolutionary processes structuring the strength and direction of selection locally and at larger scales.