The Effects Of Environmental Variation On Individual Variation

Despite general consensus among ecologists that variation is ubiquitous and important in nature, ecological experiments have historically focused on changes in mean response assuming a constant environment and interchangeable individuals, potentially masking important mechanisms that underlie ecological dynamics. In this thesis I present statistical and methodological advances for studying variation and results from two experiments that investigate how environmental variation affects variation in individual phenotypes. In chapter one I present simulation-based power analyses for generalized linear mixed effects models that I designed, which detect how natural or manipulative treatments affect variation in responses, such as among- and within-individual variation. My results indicate power to detect differences in variance by treatment was low overall (in most cases >1,000 total observations per treatment needed to achieve 80% power) and heterogeneity in power across ratios of individuals to repeated measures with an optimal ratio that differed by target variance parameter. With these power analyses I hope to inspire novel experimental designs in ecology and evolution investigating the causes and implications of individual-level phenotypic variance. In chapter two I evaluate the effects of variation in predation risk by crayfish (Procambarus sp.) on among- and within-individual variation as well as average anti-predator behavior of freshwater snails (Physa acuta) using novel design that isolates changes in variance from changes in the mean. I found that both the type and magnitude of environmental variation cause changes in mean behavior and variation in individual behavior. Changes in the average response versus changes in how variable individuals are can have important implications for the evolution of behavioral reaction norms, and for the maintenance of variation in populations.M.S