Habitat Use by Insular Small Mammals: Relative Effects of Competition and Habitat Structure

This study tests two predictions about habitat use by the small mammal species of Assateague Island, the largest of the Virginia barrier islands: (1) habitat segregation between species should be conspicuous, with centers of species abundance located in different plant associations, and (2) competition should exert a strong influence on habitat use by each species, with the effects of the other species at least as great at the effects of habitat structure. Prediction 1 was supported by analyses of mammal species abundances (based on live—trapping) in five vegetation types. Habitat segregation was pronounced and species' distribution reflected coarse—grained habitat patchiness. However, prediction 2 was not supported by analysis of the independent effects of competition and habitat structure on the local abundance of each species. Despite the apparent ubiquity of competition, all of the interaction coefficients were °1.00 in absolute value, suggesting that the competition was relatively “weak”. Furthermore, for most species the effects of other species were found to be relatively less important than those of habitat structure. Only Mus musculus, and to a much lesser extent Zapus hudsonius, appear to be affected more strongly by competition. These findings impugn an earlier suggestion that competition plays a significant role in promoting the strong positive relationship between rodent diversity and habitat complexity on the Virginia barrier islands. Although no data on turnover rates are available, there is no compelling reason to invoke a dynamic equilibrium to explain the biogeography of small mammals on the Virginia barrier islands. The number and combination of species found on an island may simple represent a sample drawn from the pool of available species, weighted primarily by island area and habitat complexity and by the colonizing abilities of the species.