Coexistence of Specialist and Generalist Rodents Via Habitat Selection

Habitat specialization is an effective mechanism of coexistence among competing species. But many communities also include generalized species whose habitat use overlaps that of the specialists. Two hypotheses seem most likely to account for the pattern: (1) Habitat generalists may coexist in competitively structured communities by exploiting habitats unused, or underused, by habitat specialist. (2) Population densities of the specialized species may be limited (e.g., by predation) below levels where the specialists can competitively exclude habitat generalists. I evaluated these ideas in a community of small mammals occupying mesic and xeric habitats at nine different locations in the Rocky Mountains of southern Alberta, Canada. A chipmunk generalist coexists with two specialists, one with a distinct preference for xeric habitat (deer mouse), and another with a similar preference for mesic habitat (red—backed vole). Habitat selection appeared to depend primarily on the density of conspecifics. The generalist coexisted by responding to habitat at a larger scale than did the specialists. Shared predators, whose habitat selection operates on a larger scale than that of their prey, may augment coexistence of all species by reinforcing habitat selection by the specialists.