BIRD SPECIES RICHNESS IN RELATION TO ISOLATION OF ASPEN HABITATS

We studied 14 stands of aspen (Populus tremuloides Michx.) and their associated bird and plant communities in Rocky Mountain National Park, Colorado, in June 1993, to investigate the effects of habitat isolation on bird species richness. Selected aspen stands varied in their degree of isolation from other aspens, but were similar in area, elevation, and vegetation structure. The degree of isolation of aspen stands was expressed in four ways (1) distance to nearest neighboring aspen stand, (2) area-weighted distance to nearest neighboring aspen stand, (3) area-weighted distances of all aspen stands within 1500 m, and (4) total area of aspen stands within 1500 m. Isolation had no significant effect on the bird species richness of the 14 aspen stands studied. Regressions that incorporated isolation and vegetation parameters selected one-variable models, with percent shrub cover (0.5-2.0 m height) accounting for 58% of the variation in bird species richness (r = 0.76). Abundant understory vegetation in aspen stands may provide additional cover, food, and breeding habitat to support a larger number of bird species.