The effects of environmental variation on the phenotypes expressed by organisms have gradually gained interest in biology. This interest has been sparked by the realization that environment-dependent phenotypic expression (i.e. phenotype plasticity) can improve an organism's fitness when different environments are encountered. The challenge for researchers is to determine the importance of phenotypic plasticity to the various fields of biology.

A major goal in community ecology is to understand predator-prey interactions within natural communities. Recently, ecologists have focused their attention towards the inducible defenses of prey with the realization that prey are not simply passive participants but instead express a variety of inducible defenses. This dissertation explores the ecology and evolution of inducible defenses using freshwater snails and their predators as a model system. My specific objectives were to identify the adaptive significance of induced responses against predators, to explore the importance of development for understanding inducible defenses, and to address the phenotypic and fitness consequences of spatial and temporal variation in predation risk on prey species.

Snails were extremely flexible in their ability to respond to different predator environments. It was evident in each of my experiments that snails were able to alter a unique suite of traits with different predators and integrate their phenotypic responses to environments that contained multiple predator species. Moreover, phenotypic trade-offs resulting from internal resource competition among traits appear to be the underlying mechanism for the expression of inducible defenses. I also found induced defenses can come at the cost of reduced growth, delayed reproduction, or reduced fecundity. However, these costs were dependent on the identity of the predator. By incorporating a developmental perspective, I was able to document that snails have wide developmental windows for formation of defenses but narrow windows for iv the reversal of defenses. Moreover, the lag time associated with the formation of some defenses can limit the benefits of the defense. Lastly, responses to predators early in development can constrain future responses to different predators and have dramatic impacts on fitness.