The competitive edge of an invading specialist

Introduced species represent major threats to native and natural biodiversi ty. On the other hand, biologists may increase the understanding of ecologi cal interactions by following communities during establishment of exotic sp ecies. Accordingly, feeding ecology and habitat use were studied in native whitefish (Coregonus lavaretus) and recently invading vendace (C. albula) i n two lake localities situated 50 km apart within the subarctic Pasvik Rive r system, northern Norway and Russia. Whitefish originally dominated the na tive fish communities of both lakes. The recent invasion and successive dow nstream expansion of vendace allowed comparisons between two sites: one in which the influence of the new potential competitor on the native fish spec ies was weak, and one in which the influence was strong. In the downstream lake vendace was recorded for the first time at the time of the study, and only in small numbers, whereas in the upstream lake vendace had established a high population density and was the dominant fish species in the pelagic zone. No vertical segregation in pelagic habitat use was found between the two fish species in either lake. In the downstream lake both whitefish and vendace fed exclusively on zooplankton and had almost identical diets. In the upstream lake. in contrast, whitefish fed predominantly on zoobenthos a nd surface insects, while vendace fed mainly on zooplankton. Thus. the stro ng presence of vendace as a specialized planktivore reduced the availabilit y of zooplankton as prey for the more generalist whitefish. The food segreg ation between the two fish species in the upstream lake was apparently inte ractive and caused by a strong asymmetrical competition for zooplankton, ve ndace being the superior species. The ecological consequences (including re duced zooplankton size and species diversity, alteration of the pelagic foo d web, and eutrofication as a possible cascading effect on the primary prod uction) of the vendace invasion in the Pasvik watercourse are considerable, even after a few years. and are likely to proceed and intensify in the fut ure.