Changes to zooplankton community structure following colonization of a small lake by Leptodora kindti

The predaceous cladoceran Leptodora kindti (Focke) became established in Third Sister Lake, Michigan, after individuals escaped from experimental enclosures in 1987. By 1988, the Leptodora population exhibited seasonal dynamics characteristic of natural populations. The maximum seasonal abundance of Leptodora increased to 85 individuals m23 3 yr following the introduction. After the appearance of Leptodora, small-bodied cladocerans (Ceriodaphnia and Bosmina) virtually disappeared from the lake. There were strong seasonal shifts in the dominance patterns of both cladocerans and copepods, and Daphnia species diversity increased. Results from this unplanned introduction suggest that invertebrate predators can have a rapid and lasting effect on prey populations, even in the presence of planktivorous fish. Small-scale (,20 km) geographic barriers might be as important as large-scale barriers to dispersal of planktonic animals. Both planktivorous fish and invertebrate predators have the potential to structure crustacean zooplankton communities through size selective predation. Planktivorous fish that feed visually select the largest available prey while invertebrate predators remove smaller individuals (Zaret 1980). Whereas the effects of planktivorous fish on zooplankton community structure are well established (Brooks and Dodson 1965; Hall et al. 1976), the effects of invertebrate predators are less clear. Large predatory zooplankton, such as Chaoborus and certain calanoid copepods (e.g., Hesperodiaptomus, Heterocope), are often abundant in the absence of fish and can inflict considerable mortality on populations of small-bodied prey (Von Ende and Dempsey 1981; Luecke and O’Brien 1983; Elser et al. 1987; McNaught et al. 1999). Such intense predation can lead to the exclusion of smallbodied species, thereby changing the size structure of the zooplankton community (Dodson 1974; Luecke and O’Brien 1983; Black and Hairston 1988; Hanazato and Yasuno 1989). When fish are present, large nonmigratory inverte1 To whom correspondence should be addressed. Present address: Department of Biology, Central Michigan University, Mount Pleasant, Michigan 48859.