The metacommunity concept as a framework for predicting patterns of diversity and stability in fragmented freshwater landscapes

Background/Question/Methods

The metacommunity concept has emerged as a powerful framework to describe and forecast effects of community connectivity on species richness across multiple spatial scales. Although metacommunity theory can predict patterns of diversity at hierarchical scales in freshwater ecosystems, its potential contribution to understanding the consequences of habitat fragmentation remains largely unaddressed. Several physical and ecological attributes of fragmented landscapes reflect metacommunity structure, including: 1) habitat patches that vary in their degree of isolation from similar patches in the landscape, and thus regulate the amount of species dispersal among communities, and 2) food web structure that often differs among habitat patches, where top predators may be absent due to landscape connectivity, movement behavior, or a small patch size. Metacommunity theory suggests that differential dispersal rates and local food web structure are likely to affect species richness and stability at multiple scales. To evaluate the combined effect of dispersal and food web structure on species richness, and population, community and ecosystem stability, we constructed mesocosm metacommunities consisting of three pond communities with heterogeneous levels of fish predation and different planktonic dispersal rates (0, 0.7%/d, 20%/d).  Two of the three communities differed continually in the occurrence of fish and supported different but constant environments reflecting spatially variable food web structure in a fragmented landscape, while the third community supported temporally variable fish occurrence reflecting a mobile top predator or one undergoing colonization-extinction dynamics.

Results/Conclusions

Dispersal significantly affected diversity such that local and regional biotas at the low dispersal rate maintained the greatest number of zooplankton species in metacommunities. The unimodal dispersal-diversity relationship was predation-dependent, however, as selective press predation excluded species regardless of dispersal. Additionally, spatially and temporally heterogeneous predation prevented spatial synchrony among prey populations despite the level of dispersal, whereas spatial synchrony of non-prey species responded positively to dispersal rate. At the local scale, predation disrupted the synchronous dynamics between species produced by dispersal. In contrast to these effects of predation on population and community stability, ecosystem stability responded only to dispersal, where the low dispersal rate stabilized temporal ecosystem productivity in metacommunities. These results suggest that dispersal rates can moderate species diversity and ecosystem stability in metacommunities, but that selective top predators can yield predation-dependent dispersal-diversity relationships and stabilize dispersal-induced population and community oscillations. Taken together, the results suggest a critical level of community connectivity may be required for the maintenance of biodiversity and ecosystem functioning in fragmented freshwater landscapes.