Multiple prey traits, multiple predators: keys to understanding complex species interactions

Abstract

Species interactions generate both natural selection and ecological community structure. Among the more interesting species interactions are those that create adaptive tradeoffs-where phenotypes conferring improved performance in interactions with one species decrease performance with another. Such tradeoffs create divergent natural selection and favor the evolution of strategies such as phenotypic plasticity. Functional tradeoffs may also provide little "enemy-free space" for a focal species in complex communities, resulting in emergent ecological impacts. Emergent community properties arise when community patterns are not predicted based on additive knowledge of pairwise interactions. These emergent impacts typically result from indirect effects, where interactions between two species depend on intermediary species. In this thesis, I examined constraints on the evolution of phenotypic plasticity and addressed its potential significance in community dynamics. I further reviewed concepts and issues related to linking functional and community studies to elucidate mechanisms underlying community structure. I proposed an integrative approach to the subject, emphasizing four major areas of promise, by combining perspectives of ecology and evolution. Freshwater physid snails induce defensive phenotypes in the presence of specialized molluscivorous sunfish. While predator-induced shells reduce predation from shell-crushing fish, I examined whether non-predatory fish induced unnecessary responses. I raised freshwater snails, Physella virgata, with either molluscivorous or non-molluscivorous sunfish species. Regardless of the predator class with which they were raised, snails exhibited two induced responses: reduced growth, and development of rotund shells. Although induced responses are adaptive in the presence of molluscivorous species, responding unnecessarily to non-molluscivores is maladaptive. Both growth reduction and rotund shells entail substantial costs without compensatory benefits in the absence of predatory fish. A pluralistic approach to studying ecological interactions shaping community dynamics should prove profitable in revealing details of complex interactions. I primarily illustrate these principles with case studies involving predation ecology in the freshwater snail-fish-crayfish model system. The system illustrates how predator impacts on prey are mediated by multiple prey traits, correlations between traits, functional tradeoffs in predator defense, interactions between predators, and interactions with other community members. These case studies demonstrate the importance of a multiple-trait and multiple-environment perspective in functional ecological studies.