Response of cladocerans to native and invasive invertebrate predators in Lake Simcoe, Canada
How populations respond to changing environmental conditions is critical to their survival. Organisms can respond adaptively to new environmental conditions, for example an invasive predator, by the expression of phenotypic plasticity, or evolution through selection for certain phenotypes. Lake Simcoe, one of the largest inland lakes in Ontario, has been invaded by several non-native species, including the zooplanktivore Bythotrephes cederstroemii, which is native to north-central Europe and Asia. To investigate whether the arrival of Bythotrephes and/or increase of Epischura abundance contributed to the zooplankton shifts observed in Lake Simcoe in the mid-1990s, we conducted two laboratory experiments. Firstly i) a live predation experiment to assess prey vulnerability and consumptive effects of each predator and ii) a phototactic response experiment to assess differences in phototactic behaviour when prey encounter the presence or absence of each predator cue. We found that bosminids were more vulnerable than daphniids to Bythotrephes and Epischura predation and that all prey responded to specific predator cue by altering their phototactic behaviour. Our predation results aligned with the observed abundance changes in Lake Simcoe for some species, however they were opposite to the abundance changes for others. Our phototactic results demonstrated that all species are capable of changing their behaviour in response to predators, but in Lake Simcoe other factors could be contributing to the observed changes. Although these results only explain some of the observed changes in Lake Simcoe, they provide predictive insight into how future invasion sites may respond to an invasive predator.