Source-sink effects occur where environmental heterogeneity at the regional scale allows a species to persist in unfavorable habitats because of immigration from favorable habitats. Theoretical evidence suggests that community structure and species coexistence in source-sink metacommunities is strongly determined by the proportion of dispersal between localities. However, in many experimental tests it is difficult to parse the effects of source-sink dynamics from trade-offs between competitive and dispersal abilities. Although these two mechanisms likely work in concert in natural communities, competitive-colonization dynamics arise from a particular distribution of species traits within an interacting community whereas source-sink effects can result from purely random dispersal in a heterogeneous environment. We used a model microbial system to examine how the source sink dynamics affect specific aspects of community structure: coexistence at the local and metacommunity level over time, relative abundance of species, and spatial and temporal variation in population sizes. We directly manipulated the proportion of dispersal in a multi-trophic metacommunity consisting of bacteria, bacterivores, and omnivore/predators. Regional heterogeneity was introduced via temperature differences between local communities.
Results/Conclusions

We found that local (alpha) species richness reached a maximum when 50% of the community dispersed and that between-community (beta) richness decreased with increasing dispersal due to increased homogenization. Relative abundance distributions varied with the level of dispersal. As predicted by source-sink metacommunity models, these patterns appear to be driven by a shift in dominance from a strong local competitor to a strong regional competitor as dispersal increases. Overall, dispersal had a stabilizing effect on species richness; temporal variation in local communities and spatial variation within metacommunities were greatest in the absence of dispersal. These results suggest that even relatively weak dispersal (1% of total community) can change the outcome of local interactions and that these effects are not dependent on species’ variation in dispersal ability.