Communities of coexisting organisms are formed by both dispersal among habitat patches in a region and local interactions within patches. The supply of colonists from the regional pool has the potential to alter the outcome of local interactions by providing species that fill different ecological roles. I examined the consequences of invasion by fish (juvenile Lepomis macrochirus) and insect (Notonecta undulata) predators on experimental plankton communities that were either connected to or isolated from the regional pool of zooplankton species. The effects of predators on zooplankton composition and diversity depended on dispersal by members of the regional zooplankton species pool. Fish and notonectids reduced the abundance of large zooplankton and thereby facilitated invasion by new species from the surrounding region. Both predators reduced zooplankton species richness in the absence of dispersal, while fish had positive effects and notonectids had no effects with rapid dispersal. Fish facilitated invasion by more species than they excluded, while similar numbers of species invaded with notonectids as were driven extinct. The effects of predators on planktonic trophic structure (zooplankton size structure and biomass, phytoplankton biomass) were independent of the level of dispersal. Both predators reduced zooplankton mean body size, but only fish enhanced phytoplankton density. Invasion by members of the regional species pool slightly increased zooplankton mean body size and reduced phytoplankton biomass; however, the effects were small and independent of the predator treatment. Predator-facilitated invasion of new species altered the effects of predators on zooplankton species composition and diversity, but not on plankton trophic structure. The results suggest that spatial heterogeneity in predator abundance promotes regional coexistence among zooplankton. Connection to a diverse regional species pool via dispersal may therefore be important for determining the impact of local interactions such as predation on communities.
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