Invasive species are one of the leading threats to global biodiversity, but until recently, little attention has been paid to how evolution contributes to invasive species effects despite important applications to conservation and our general understanding of evolution in the wild. Invasive species often experience strong selection upon introduction, which may result in adaptive evolution that could facilitate their successful integration into food webs and their effects on native species. Recent species invasions in North America offer a strong opportunity to address fundamental questions in evolutionary ecology as well as advance our understanding of invasive species effects. Bythotrephes longimanus (the spiny water flea) is a predatory zooplankton with a conspicuous tail spine that invaded the Great Lakes region during the 1980s and may be having large negative effects on fisheries. Previous field studies show the morphology and life history of Bythotrephes strongly vary spatially and temporally, but the cause is not known. Evolution by natural selection and phenotypic plasticity are potential sources of Bythotrephes trait variation, but heretofore, these sources of variation have not been investigated.My dissertation research investigated ecological and evolutionary factors that influence morphological and life history variation in Bythotrephes. Using Bythotrephes collected from Lake Michigan, I found moderate-to-high genetic variation in distal spine and body length and maternal effects in both traits. Further, experiments revealed that spine length, body size, and clutch size respond plastically to temperature but not to fish predator cues, with higher temperature inducing mothers to have smaller clutches of larger offspring (longer absolute distal spine and body length) that were better defended against predation. Although Bythotrephes use temperature as the proximate cue of plasticity, it is likely that the trait changes represent adaptations to varying fish predation risk which correlates with water temperature. I also found temporally and spatially variable selection on distal spine length consistent with seasonal changes in gape-limitation of fish predators and spatial heterogeneity of fish, respectively. Yet, despite net selection for increased distal spine length, I observed little evidence of an evolutionary response to selection based on comparisons of historic and contemporary wild-captured individuals and retrieved spines from sediment cores. In a companion study of Canadian Shield lakes, I identified gape-limited fish predators as agents of selection on Bythotrephes distal spine length. Specifically, I found selection for increased distal spine length in lakes dominated by a gape-limited fish predator and no significant selection in lakes dominated by a non-gape-limited fish predator. A large difference (20%) in average distal spine length between lakes of each predator type was consistent with the direction of selection, suggesting potential local adaptation of distal spine length to gape-limited fish predation.The results of my dissertation indicate Bythotrephes respond to fish predation through multiple mechanisms, including phenotypic plasticity and evolutionary responses to selection. These responses to predation likely promote Bythotrephes success as an invasive species, and may also underlie negative effects on important Great Lakes fisheries through food web interactions. More generally, the results of my dissertation suggest the effects of invasive species may occur not just through their ecological interactions, but also through evolutionary and phenotypically plastic trait modifications. As invasive species continue to affect biodiversity worldwide, understanding the mechanisms behind invasive species effects is critical.
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