No abstract

Numerous studies have demonstrated that dispersal is dependent on both disperser phenotype and the local environment. However, there is substantial variability in the observed strength and direction of phenotype‐ and environment‐dependent dispersal. This has been hypothesized to be the result of interactive effects among the multiple phenotypic and environmental factors that influence dispersal. Here, our goal was to test the hypothesis that these interactions are responsible for generating variation in dispersal behaviour. We achieved these goals by conducting a large, 2‐year, mark–release–recapture study of the backswimmer Notonecta undulata in an array of 36 semi‐natural ponds. We measured the effects of multiple phenotypic (sex and body size) and environmental (population density and sex ratio) factors, on both dispersal probability and dispersal distance. We found support for the hypothesis that interactive effects influence dispersal and produce variability in phenotype‐ and environment‐dependent dispersal: dispersal probability was dependent on the three‐way interaction between sex, body mass and population density. Small males displayed strong, positive density dependence in their dispersal behaviour, while large males and females overall did not respond strongly to density. Small notonectids, regardless of sex, were more likely to disperse, but this effect was strongest at high population densities. Finally, the distance dispersed by backswimmers was a negative function of population density, a pattern which we hypothesize could be related to: (a) individuals from high and low density patches having different dispersal strategies, or (b) the effect of density on dispersal capacity. These results suggest that phenotype‐by‐environment interactions strongly influence dispersal. Since phenotype‐ and environment‐dependent dispersal has different consequences for ecological and evolutionary dynamics (e.g. metapopulation persistence and local adaptation) than random dispersal, interactive effects may have wide‐reaching impacts on populations and communities. We therefore argue that more investment should be made into estimating the effects of multiple, interacting factors on dispersal and determining whether similar interactive effects are acting across systems.

Performance trade‐offs between competition and colonization can be an important mechanism facilitating regional coexistence of competitors. However, empirical evidence for this trade‐off is mixed, raising questions about the extent to which it shapes diverse ecological communities. Here, we outline a framework that can be used to improve empirical tests of the competition‐colonization trade‐off. We argue that tests of the competition‐colonization trade‐off have been diverted into unproductive paths when dispersal mode and/or competition type have been inadequately defined. To generate comparative predictions of associations between dispersal and competitive performance, we develop a conceptual trait‐based framework that clarifies how dispersal mode and type of competitor shape this trade‐off at the stage of dispersal and establishment in a variety of systems. Our framework suggests that competition‐colonization trade‐offs may be less common for passively dispersing organisms when competitive dominants are those best able to withstand resource depletion (competitive response), and for active dispersers when traits for dispersal performance are positively associated with resource pre‐emption (competitive effect). The framework presented here is designed to provide common ground for researchers working in different systems in order to prompt more effective assessment of this performance trade‐off and its role in shaping community structure. By delineating key system properties that mediate the trade‐off between competitive and colonization performance and their relationship to individual‐level traits, researchers in disparate systems can structure their predictions about this trade‐off more effectively and compare across systems more clearly.

Predators affect prey through direct consumption as well as by inducing prey to defensively alter their phenotypes, including behavioral phenotypes, to maximize survival under predation risk. Closely related sympatric prey species with shared natural enemies may resolve behavioral trade-offs under predation risk differently. In a laboratory experiment, we investigated two co-occurring semiaquatic backswimmer congeners, which exhibit differences in their degree of habitat specialization across a gradient of habitat permanence. Notonecta irrorata Uhler primarily occur in ephemeral ponds, whereas Notonecta undulata Say are habitat generalists that are commonly found in both permanent and ephemeral ponds. We tested whether the two species differed in antipredator responses to both visual and chemical cues of a shared predator, the giant water bug, in a fully factorial design. The generalist species, N. undulata, exhibited reductions in activity in the presence of predator chemical cues only, whereas the specialist species, N. irrorata, remained consistently active across predator cue treatments. Our work shows that there are species-specific differences in how prey assess or respond to predation risk. The varying propensities of these backswimmer congeners to behaviorally respond to a shared predator, and differences in their behavior when exposed to different predation risk cues may be linked to underlying divergence in their life-history strategies.