Effects of isolation on red-backed voles (Clethrionomys gapperi) and deer mice (Peromyscusmaniculatus) in a sage–steppe matrix

Abstract

Effects of habitat isolation can differ among species, thereby influencing populations both directly and indirectly. We used naturally occurring forest patches surrounded by a sage–steppe matrix in southeast Idaho to test the hypothesis that a habitat specialist, the red-backed vole (Clethrionomys gapperi), would have movements restricted and therefore population densities reduced on forest patches isolated by distances equal to or greater than their typical home-range diameter. We hypothesized that the more generalized deer mouse (Peromyscus maniculatus) would not show effects of isolation at this scale. We tested for effects of isolation on these small mammals in 1999 and 2000 in forest patches that varied in distance to a mainland forest. Densities of red-backed voles decreased with isolation and those of deer mice did not. However, strong relationships were also found between red-backed vole densities and habitat characteristics, which themselves varied with isolation. Livestock apparently had disproportionate effects on the more isolated patches, thereby altering ground cover. Isolation by distances up to 450 m appears to reduce (filter) rather than eliminate immigration of red-backed voles; however, effects of livestock on isolated habitat patches may prevent successful dispersers from populating patches.