Shared sinks alter competitive outcomes via edge effects

Most work on source-sink dynamics in metacommunities assumes that species have minimal or no niche overlap and thus different sources and sinks. We explore the alternative possibility: competing species have an overlapping set of sources and sinks. Using both implicit-space two-patch (ordinary differential equations) and explicit-space reaction–diffusion (partial differential equations) models, we find that the presence of shared sinks (where neither species can persist indefinitely) allows for a species that would otherwise be driven extinct to exclude its superior competitor, assuming that the species that benefits the most in the source incurs a greater cost than its competitor in the sink. Competitive outcomes are altered when there is an abrupt transition between the source and the sink (i.e., due to an edge effect) because the species that is more tolerant of the sink has a lower net emigration rate at the edge. We discuss how shared sources and sinks relate to previously described trade-offs and potential applications for conservation and restoration.