Metacommunity ecology provides a framework for understanding the relative importance of local and regional processes in structuring species diversity across hierarchical spatial scales. Although metacommunity theory can predict empirical patterns of multi-scale diversity, the potential effects of strongly interacting non-native species on diversity and ecosystem function in metacommunities remain unaddressed. In particular, non-native omnivores may exert large impacts on local diversity, beta diversity, and ecosystem structure. Omnivory is hypothesized to reduce local species richness and biomass of prey and primary producers, and increase dissimilarity between invaded and uninvaded communities and ecosystems. However, high levels of community connectivity via native species dispersal rates may override these predicted responses to non-native omnivory. In this study, we evaluated the effects of an invasive omnivore, the zebra mussel (Dreissena polymorpha), on species diversity and local ecosystem attributes within partially invaded reservoir metacommunities. Zebra mussels, zooplankton, phytoplankton, transparency, and nutrients were systematically sampled in invaded and uninvaded reservoirs. Multi-scale species diversity and ecosystem properties were contrasted as a function of reservoir invasion status. Additionally, the relative importance of local and regional processes in structuring invaded reservoir metacommunities was compared between two adjacent riverine networks that differ in community (hydrologic) connectivity.
Results/Conclusions
Zebra mussel veligers were present in all invaded reservoirs, indicating established reproducing populations of the non-native species and potential for local ecological impacts. While local and beta diversity of zooplankton did not differ with reservoir invasion status, zooplankton biomass was significantly lower in invaded reservoirs. Phytoplankton biomass and phosphorus were also lower in invaded reservoirs. Likely due to observed reductions in phytoplankton, water column transparency was significantly greater in invaded reservoirs. These results are consistent with predictions that omnivory and effective filtration by zebra mussels will reduce grazer and primary producer biomass and alter ecosystem function. Zooplankton communities in the less connected river basin, where reservoirs were located along tributaries and hydrologic dispersal was reduced, were most strongly structured by local environmental processes. In contrast, zooplankton communities in the highly connected reservoir metacommunity located along a main river channel were more structured by regional dispersal processes. Taken together, the results indicate that the large effects of omnivory from a single invasive species can influence metacommunity and metaecosystem properties in landscapes under both low and high levels of connectivity. Thus, a strongly interacting non-native species has the potential to structure multiple trophic levels and ecosystem attributes within differentially connected invaded landscapes.