Resources and predators can affect the vertical distributions of zooplankton

Two short-term field experiments in large enclosures indicate that Daphnia pulicaria can facultatively alter habitat selection in response to changes in food distributions and Chaoborus and fish predators. Responses to food and to fishes appear to be adaptive, although their magnitude does not seem to be optimal. Responses to Chaoborus were not necessesarily adaptive, but more data are needed to evaluate this issue. Although indirect evidence indicated the presence of additional genetic factors, the experiments demonstrate the existence of facultative components in the vertical distributions of Daphnia.