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 potential feedback between local and regional processes in shaping metapopulation dynamics has rarely been tested experimentally. Using nectar microbes as a model system, we examine how a bacterial sink species, 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 bacterial sinks increases resident yeast densities, potentially through the provision of additional resources, and that this benefit is further enhanced when bacterial supply biases dispersal away from bacteria-inoculated patches. We supplemented our experimental results by analyzing a simple two-patch metapopulation model and demonstrated that metapopulation productivity is maximized when patch-specific dispersal rate is lower in the more productive patch (e.g., productivity enhanced by bacterial sink supply). Together, these results underscore the role of dispersal in shaping how a metapopulation experiences density-dependence across heterogeneous landscapes, highlight the relevance of material flows across ecosystem boundaries, and suggest the plausible mechanism in which transient invaders may leave lasting impacts on ecological communities.