Communities in a Landscape: The Influence of Habitat Heterogeneity on the Interactions between Species

A model similar to the single-species source-sink models developed by Pulliam and by Pulliam and Danielson is described for species that have "distinct" habitat preferences and preemptive occupation of sites within habitats. The model shows that the interactions between two species do not result solely from the intrinsic properties of the species involved. They are also functions of the landscape in which the species occur. One species may cause an increase (facilitation) in the equilibrium population size of another species if the high-quality source habitat for the second species is rare relative to the abundance of low-quality sink habitat. A species may inhibit another if the second species' source habitat is abundant. Combinations of facilitation and inhibition effects describe the interactions between two species. There are two sets of possible interactions for any landscape of two habitat types. These are (-, +), (-, -), (+, -) and (-, +), (+, +), (+, -). Species for which individuals can sample a large number of sites are likely to exhibit one of the interactions in the first set. Species for which individuals sample only a few sites are likely to exhibit interactions from the second. Within each set of interactions, the relative abundances of each habitat will determine which interaction will occur in the landscape.