Background/Question/Methods: One of the most devastating human impacts on the world’s ecosystems is the introduction of non-native invasive species which can negatively impact agriculture, industry, human health, and the natural environment. The invasion of non-native predators can have particularly damaging effects on native prey species and the introduction of non-native predators is common in many systems. Native species can undergo rapid adaptation in response to selection imposed by non-native invaders, leading to changes in their morphology, physiology, and behavior. The goal of this study was to examine the response of native Daphnia species to invasion of the spiny water flea, Bythotrephes cederströmii, in Lake Mendota (WI). The impact of Bythotrephes on native Daphnia is of concern because in many lakes, Daphnia serve as primary algae grazers and are an important food source for commercially and recreationally important fish species. Therefore, shifts in zooplankton community composition can have dramatic ecosystem wide impacts. We conducted a series of transfer experiments in the presence and absence of Bythotrephes and fish (Poecilia reticulata) chemical cues to evaluate the escape ability of three Daphnia species (D. ambigua, D. mendotae, and D. pulicaria) from both pre- and post-Bythotrephes invasion time periods. Results/Conclusions: Responses to treatments were relatively consistent across all Daphnia species tested. Fish chemical cues elicited a significantly enhanced escape response compared to the control and Bythotrephes cue treatments (LME: z=-3.2390, df=152, P=0.0036 and z=-2.9090, df=152, P=0.0073 respectively); however, no significant difference in escape ability was detected between Bythotrephes chemical cue and control treatments (LME: z=0.3300, df=152, P=0.7412). Comparison of Daphnia from pre- and post-invasion time periods revealed a constitutive increase in the escape ability of D. pulicaria (LME: F= 12.1594, df= 1, 4, P= 0.0252). We also found that there were general differences in escape ability across species (LME: F=9.1822, df=2,10, P=0.0055). Differences in the escape ability of native Daphnia may have contributed to differences in vulnerability which, in turn, have influenced the community changes observed in Lake Mendota after the establishment of Bythotrephes. Our findings contribute to a growing body of research which demonstrates that ecological invasions can prompt adaptive responses, altering the nature of interactions between invasive predators and native prey species.
