Sink habitats can alter ecological outcomes for competing species

1. Species often compete for breeding sites in heterogeneous landscapes consisting of sources and sinks. To understand how the presence or absence of sink breeding sites influence ecological outcomes, we extend Pulliam’s source–sink model to competing species. 2. In a homogeneous landscape consisting of source sites, we prove that one species, the ‘superior’ competitor, competitively excludes the other. Dominance is determined by a simple rule: the species that at equilibrium acquires new breeding sites at a faster rate dominates. 3. We prove that the inclusion of sink sites can alter this ecological outcome by either mediating coexistence, reversing competitive dominance, or facilitating a priority effect. 4. Sink-mediated coexistence requires the species to exhibit asymmetries in acquiring sink sites, the ‘inferior’ species to have a competitive advantage on sink sites and the ratio of sink to source sites be sufficiently low. 5. For example, if the sink breeding sites are competitive refuges for the ‘inferior’ competitor and not too low in quality, coexistence occurs if the number of sink sites lies below a threshold. Alternatively, when the number of sink sites exceeds this threshold, competitive dominance is reversed and the ‘superior’ competitor is displaced. 6. Counter-intuitively, despite being unable to support species in isolation, sink habitats embedded in a geographical mosaic of sources and sinks can enhance biodiversity by mediating coexistence or alter species composition by reversing competitive interactions.