Viral transmission and infection prevalence in a cannibalistic host-pathogen system.

Cannibalism, while prevalent in the natural world, is often viewed as detrimental to a cannibal's health, especially when they consume pathogen-infected conspecifics. The argument stems from the idea that cannibalizing infected individuals increases the chance of coming into contact with a pathogen and subsequently becoming infected. Using an insect pest, the fall armyworm (Spodoptera frugiperda), that readily cannibalizes at the larval stage and its lethal pathogen, we experimentally examined how cannibalism affects viral transmission at both an individual and population level. Prior to death, the pathogen in the system stops the larval host from growing, resulting in infected individuals being smaller than healthy individuals. This leads to size-structured cannibalism of infected individuals with the larger healthy larvae consuming the smaller infected larvae, which is commonly observed. At the individual level, we show that the probability of cannibalism is relatively high for both infected and uninfected individuals especially when the cannibal is larger than the victim. However, the probability of the cannibal becoming infected given that a pathogen-infected individual has been cannibalized is relatively low. On a population level, when cannibalism is allowed to occur transmission rates decline. Additionally, by cannibalizing infected larvae, cannibals lower the infection risk for non-cannibals. Thus, cannibalism can decrease infection prevalence and, therefore, may not be as deleterious as once thought. Under certain circumstances, cannibalizing infected individuals, from the uninfected host's perspective, may even be advantageous, as one obtains a meal and decreases competition for resources with little chance of becoming infected.

Carryover effects drive competitive dominance in spatially structured environments.

Understanding how changes to the quality of habitat patches affect the distribution of species across the whole landscape is critical in our human-dominated world and changing climate. Although patterns of species' abundances across a landscape are clearly influenced by dispersal among habitats and local species interactions, little is known about how the identity and origin of dispersers affect these patterns. Because traits of individuals are altered by experiences in their natal habitat, differences in the natal habitat of dispersers can carry over when individuals disperse to new habitats and alter their fitness and interactions with other species. We manipulated the presence or absence of such carried-over natal habitat effects for up to eight generations to examine their influence on two interacting species across multiple dispersal rates and different habitat compositions. We found that experimentally accounting for the natal habitat of dispersers significantly influenced competitive outcomes at all spatial scales and increased total community biomass within a landscape. However, the direction and magnitude of the impact of natal habitat effects was dependent upon landscape type and dispersal rate. Interestingly, effects of natal habitats increased the difference between species performance across the landscape, suggesting that natal habitat effects could alter competitive interactions to promote spatial coexistence. Given that heterogeneity in habitat quality is ubiquitous in nature, natal habitat effects are likely important drivers of spatial community structure and could promote variation in species performance, which may help facilitate spatial coexistence. The results have important implications for conservation and invasive species management.

Top predators determine how biodiversity is partitioned across time and space.

Natural ecosystems are shaped along two fundamental axes, space and time, but how biodiversity is partitioned along both axes is not well understood. Here, we show that the relationship between temporal and spatial biodiversity patterns can vary predictably according to habitat characteristics. By quantifying seasonal and annual changes in larval dragonfly communities across a natural predation gradient we demonstrate that variation in the identity of top predator species is associated with systematic differences in spatio-temporal ß-diversity patterns, leading to consistent differences in relative partitioning of biodiversity between time and space across habitats. As the size of top predators increased (from invertebrates to fish) habitats showed lower species turnover across sites and years, but relatively larger seasonal turnover within a site, which ultimately shifted the relative partitioning of biodiversity across time and space. These results extend community assembly theory by identifying common mechanisms that link spatial and temporal patterns of ß-diversity.

Cannibalism and Infectious Disease: Friends or Foes?

Cannibalism occurs in a majority of both carnivorous and noncarnivorous animal taxa from invertebrates to mammals. Similarly, infectious parasites are ubiquitous in nature. Thus, interactions between cannibalism and disease occur regularly. While some adaptive benefits of cannibalism are clear, the prevailing view is that the risk of parasite transmission due to cannibalism would increase disease spread and, thus, limit the evolutionary extent of cannibalism throughout the animal kingdom. In contrast, surprisingly little attention has been paid to the other half of the interaction between cannibalism and disease, that is, how cannibalism affects parasites. Here we examine the interaction between cannibalism and parasites and show how advances across independent lines of research suggest that cannibalism can also reduce the prevalence of parasites and, thus, infection risk for cannibals. Cannibalism does this by both directly killing parasites in infected victims and by reducing the number of susceptible hosts, often enhanced by the stage-structured nature of cannibalism and infection. While the well-established view that disease should limit cannibalism has held sway, we present theory and examples from a synthesis of the literature showing how cannibalism may also limit disease and highlight key areas where conceptual and empirical work is needed to resolve this debate.

Habitat-mediated carry-over effects lead to context-dependent outcomes of species interactions.

When individuals disperse, their performance in newly colonized habitats can be influenced by the conditions they experienced in the past, leading to environmental carry-over effects. While carry-over effects are ubiquitous in animal and plant systems, their impact on species interactions and coexistence are largely ignored in traditional coexistence theory. Here we used a combination of modelling and experiments with two competing species to examine when and how such environmental carry-over effects influence community dynamics and competitive exclusions. We found that variation in the natal habitat quality of colonizing individuals created carry-over effects which altered competitive coefficients, fecundity and mortality rates, and extinction probabilities of both species. As a consequence, the dynamics of competitive exclusion within and across habitat types was contingent on the natal habitat of colonizing individuals, indicating that spatial carry-over effects can fundamentally alter the dynamics and outcome of interspecific competition. Interestingly, carry-over effects persistently influenced dynamics in systems with interspecific competition for the entire duration of the experiment while carry-over effects were transient and only influenced initial dynamics in single-species populations. Thus carry-over effects can be enhanced by species interactions, suggesting that their long-term effects may often not be accurately predicted by single-species studies. Given that carry-over effects are ubiquitous in heterogeneous landscapes, our results provide a novel mechanism that could help explain variation in the structure of natural communities.

Resolving the roles of body size and species identity in driving functional diversity.

Efforts to characterize food webs have generated two influential approaches that reduce the complexity of natural communities. The traditional approach groups individuals based on their species identity, while recently developed approaches group individuals based on their body size. While each approach has provided important insights, they have largely been used in parallel in different systems. Consequently, it remains unclear how body size and species identity interact, hampering our ability to develop a more holistic framework that integrates both approaches. We address this conceptual gap by developing a framework which describes how both approaches are related to each other, revealing that both approaches share common but untested assumptions about how variation across size classes or species influences differences in ecological interactions among consumers. Using freshwater mesocosms with dragonfly larvae as predators, we then experimentally demonstrate that while body size strongly determined how predators affected communities, these size effects were species specific and frequently nonlinear, violating a key assumption underlying both size- and species-based approaches. Consequently, neither purely species- nor size-based approaches were adequate to predict functional differences among predators. Instead, functional differences emerged from the synergistic effects of body size and species identity. This clearly demonstrates the need to integrate size- and species-based approaches to predict functional diversity within communities.

Linking phenological shifts to species interactions through size-mediated priority effects.

Interannual variation in seasonal weather patterns causes shifts in the relative timing of phenological events of species within communities, but we currently lack a mechanistic understanding of how these phenological shifts affect species interactions. Identifying these mechanisms is critical to predicting how interannual variation affects populations and communities. Species' phenologies, particularly the timing of offspring arrival, play an important role in the annual cycles of community assembly. We hypothesize that shifts in relative arrival of offspring can alter interspecific interactions through a mechanism called size-mediated priority effects (SMPE), in which individuals that arrive earlier can grow to achieve a body size advantage over those that arrive later. In this study, we used an experimental approach to isolate and quantify the importance of SMPE for species interactions. Specifically, we simulated shifts in relative arrival of the nymphs of two dragonfly species to determine the consequences for their interactions as intraguild predators. We found that shifts in relative arrival altered not only predation strength but also the nature of predator-prey interactions. When arrival differences were great, SMPE allowed the early arriver to prey intensely upon the late arriver, causing exclusion of the late arriver from nearly all habitats. As arrival differences decreased, the early arriver's size advantage also decreased. When arrival differences were smallest, there was mutual predation, and the two species coexisted in similar abundances across habitats. Importantly, we also found a nonlinear scaling relationship between shifts in relative arrival and predation strength. Specifically, small shifts in relative arrival caused large changes in predation strength while subsequent changes had relatively minor effects. These results demonstrate that SMPE can alter not only the outcome of interactions but also the demographic rates of species and the structure of communities. Elucidating the mechanisms that link phenological shifts to species interactions is crucial for understanding the dynamics of seasonal communities as well as for predicting the effects of climate change on these communities.

Ghosts of habitats past: environmental carry-over effects drive population dynamics in novel habitat.

The phenotype of adults can be strongly influenced by the environmental conditions experienced during development. Consequently, variation in habitat quality across space and through time also leads to differences in the phenotypes of adults. This could create carry-over effects where differences in the natal habitat quality of colonizers influence population dynamics in new habitats. We tested this hypothesis experimentally by simulating dispersal of Tribolium castaneum from low- or high-quality natal habitat into new patches of low- or high-quality habitat. Differences in the natal habitat quality of colonizers altered population growth trajectories and led to carrying capacities that differed by up to 63% within a habitat type, indicating that patch dynamics are determined by the interaction of past and current habitat quality. Interestingly, even after multiple generations, the natal habitat of colonizers determined differences in adult traits that were related to density-dependent population regulation. These changes in adult phenotype could at least partially explain why carry-over effects continued to alter population dynamics for multiple generations until the end of the experiment. These results highlight the importance of variable habitat quality and carry-over effects for population dynamics.

Does phylogeny matter? Assessing the impact of phylogenetic information in ecological meta-analysis.

Meta-analysis is increasingly used in ecology and evolutionary biology. Yet, in these fields this technique has an important limitation: phylogenetic non-independence exists among taxa, violating the statistical assumptions underlying traditional meta-analytic models. Recently, meta-analytical techniques incorporating phylogenetic information have been developed to address this issue. However, no syntheses have evaluated how often including phylogenetic information changes meta-analytic results. To address this gap, we built phylogenies for and re-analysed 30 published meta-analyses, comparing results for traditional vs. phylogenetic approaches and assessing which characteristics of phylogenies best explained changes in meta-analytic results and relative model fit. Accounting for phylogeny significantly changed estimates of the overall pooled effect size in 47% of datasets for fixed-effects analyses and 7% of datasets for random-effects analyses. Accounting for phylogeny also changed whether those effect sizes were significantly different from zero in 23 and 40% of our datasets (for fixed- and random-effects models, respectively). Across datasets, decreases in pooled effect size magnitudes after incorporating phylogenetic information were associated with larger phylogenies and those with stronger phylogenetic signal. We conclude that incorporating phylogenetic information in ecological meta-analyses is important, and we provide practical recommendations for doing so.

Life history predicts risk of species decline in a stochastic world.

Understanding what traits determine the extinction risk of species has been a long-standing challenge. Natural populations increasingly experience reductions in habitat and population size concurrent with increasing novel environmental variation owing to anthropogenic disturbance and climate change. Recent studies show that a species risk of decline towards extinction is often non-random across species with different life histories. We propose that species with life histories in which all stage-specific vital rates are more evenly important to population growth rate may be less likely to decline towards extinction under these pressures. To test our prediction, we modelled declines in population growth rates under simulated stochastic disturbance to the vital rates of 105 species taken from the literature. Populations with more equally important vital rates, determined using elasticity analysis, declined more slowly across a gradient of increasing simulated environmental variation. Furthermore, higher evenness of elasticity was significantly correlated with a reduced chance of listing as Threatened on the International Union for Conservation of Nature Red List. The relative importance of life-history traits of diverse species can help us infer how natural assemblages will be affected by novel anthropogenic and climatic disturbances.

Carry-over effects of the larval environment on post-metamorphic performance in two hylid frogs.

Life history theory and empirical studies suggest that large size or earlier metamorphosis are suitable proxies for increased lifetime fitness. Thus, across a gradient of larval habitat quality, individuals with similar phenotypes for these traits should exhibit similar post-metamorphic performance. Here we examine this paradigm by testing for differences in post-metamorphic growth and survival independent of metamorphic size in a temperate (spring peeper, Pseudacris crucifer) and tropical (red-eyed treefrog, Agalychnis callidryas) anuran reared under differing larval conditions. For spring peepers, increased food in the larval environment increased post-metamorphic growth efficiency more than predicted by metamorphic phenotype and led to increased mass. Similarly, red-eyed treefrogs reared at low larval density ended the experiment at a higher mass than predicted by metamorphic phenotype. These results show that larval environments can have delayed effects not captured by examining only metamorphic phenotype. These delayed effects for the larval environment link larval and juvenile life history stages and could be important in the population dynamics of organisms with complex life cycles.