The effects of an exotic fish invasion on the prey communities of two lakes

1. We examined changes in prey (zooplankton) communities resulting from the invasion of an exotic predator, the rainbow smelt (Osmerus mordax Mitchill), using long-term data sets from two lakes in Wisconsin, USA. 2. Food webs in the two lakes differed originally both in the dominant native planktivorous fish species and in relative biomasses of major zooplankton groups. 3. Zooplankton communities in each lake changed dramatically, coinciding with the exotic predator invasion. The communities in the two lakes became more similar to each other, with cyclopoid copepods becoming dominant. 4. Time-series analyses demonstrated that the long-term responses of the two lake prey communities were driven primarily by direct predation effects of planktivorous fish. Indirect effects acting through competitive interactions among zooplankton were present but were comparatively small in the long term. 5. Results suggest that zooplankton populations were subject to both predator and resource limitation but that the composition (relative species abundance) of the zooplankton community was dictated in the long term by predation.