Sink invaders can increase resident metapopulation size by modifying local conditions and regional connectivity
Populations are regulated by processes operating at both local and regional scales. As a result, species can interact not only within local communities but also through regional-scale processes. However, the feedback between local and regional processes of species interactions in shaping metapopulation dynamics have rarely been tested experimentally. Using nectar microbes as a model system, we examine how a bacterial species dispersing to a sink habitat, sustained by external supply, can interact with a resident yeast species directly by modifying local nectar chemistry and indirectly by altering metapopulation connectivity. We show that the external supply of bacteria increases resident yeast densities, potentially through increased local productivity, and that this benefit is enhanced when the dispersal rate of yeast is reduced in patches supplied with bacteria. Analysis of a metapopulation model indicates that the benefit of dispersal modification on metapopulation size occurs only when local productivity differs across patches, consistent with our interpretation that bacterial supply increases local productivity. Taken together, our results underscore the role of dispersal in shaping how metapopulations experience density-dependence across heterogeneous landscapes, highlight the relevance of material flows across ecosystem boundaries, and suggest a plausible mechanism in which transient species may leave lasting impacts on ecological communities.