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Evolution of Trophic Linkages in an Invaded Food Web
Andrea L.J. Miehls1, Andrew G. McAdam2, Scott D. Peacor1
1 Department of Fisheries and Wildlife, Michigan State University, 480 Wilson Road, East Lansing, Michigan, 48824, USA
2 Department of Integrative Biology, University of Guelph, Guelph, ON, Canada, N1G 2W1
Invasive species are one of the leading threats to global biodiversity. Considerable research has addressed how the ecology of invasive species contributes to their persistence and negative effects on ecosystems, but until recently, little attention has been paid to how evolution affects invasive species persistence and effects. The spiny water flea, Bythotrephes longimanus, is a predatory zooplankton with a conspicuous tail spine that invaded the Great Lakes during the 1980s. Bythotrephes have reached high densities throughout the Great Lakes and may be having large negative effects on fisheries through competition for shared zooplankton prey. Previous field studies show that the morphology of Bythotrephes strongly varies spatially and temporally, but the cause of this variation is not known. We examined the role that evolution may play in the success of the invasion of Bythotrephes and of the consequent impacts with the goal to both understand the role of evolution in food webs in general, and more specifically in the ecology of Bythotrephes. Using Bythotrephes collected from Lake Michigan, we found moderate-to-high genetic variation in distal spine and body length and maternal effects in both traits. Experiments revealed that distal 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 that exhibit greater morphological defense against predation (measured as distal spine length relative to body size). Although Bythotrephes use temperature as the proximate cue of plasticity, trait changes likely represent adaptations to seasonal variation in gape-limitation of fish predators which correlates with water temperature. We also found temporally variable natural selection on distal spine length consistent with seasonal changes in gape-limitation of fish predators. Yet, despite net selection for increased distal spine length, we 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 Bythotrephes in a set of Canadian Shield lakes, we identified gape-limited fish predators as agents of selection on Bythotrephes distal spine length. Specifically, we 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 our study 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. Our study further elucidates how evolution may influence the invasion and role of non-native species.