Dispersion of Columbian ground squirrels (Spermophilus columbianus) in meadow steppe and coniferous forest

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Columbian ground squirrels were studied from 1983 through 1985 at two locations in a mosaic of meadow steppe and xerophytic forest at the eastern border of the Columbia River Basin: Washington State University's George E. Hudson Biological Preserve and a cattle pasture near Potlatch, Idaho (30 km NE Hudson Preserve). Squirrels in the study areas began to emerge from hibernation in late February or early March and bred within a few weeks. Young emerged from natal burrows early in May. Summer precipitation was low, and squirrels began to hibernate in early summer. Females first bred as yearlings. Habitat characteristics of occupied plots differed significantly from unoccupied plots. Squirrels were found on deep, fine-textured, well-drained soils (Palouse silt loam), but did not appear to be restricted by soil depth, texture, or drainage. Burrows also occurred where soils were shallow or poorly drained. Unoccupied plots occurred in areas of silt loam, silty clay loam, gravelly silt loam, and cobbly silt loam. The author attempted to determine which age and sex classes use ungrazed coniferous forest. Twenty juvenile squirrels were fitted with radio-collars (14 at Potlatch in 1984, 6 at Hudson Preserve in 1985), and movements were monitored from June until late July, when juveniles, especially males, were dispersing. Although home ranges of some squirrels included ungrazed forest, most captures in ungrazed forest were within 10 m of the pasture edge. Permanent settlement of dispersing individuals in ungrazed forest was not observed. To study burrow densities, plots were established in areas of two populations in the Hudson Preserve in April and May 1985. There was spatial variation within the larger colony. Plots with relatively high densities of burrow entrances were located on the medium-low portions of hillsides. Density of vegetation and grass coverage were associated positively with burrow density. Soil moisture did not affect burrow density. The highest densities of burrows occurred in rose thickets juxtaposed with grassy areas. When the plots with roses were omitted, no relationship was found between density of vegetation and density of burrows in a model including density of vegetation, position of slope, and grass coverage. The relationship between density of vegetation and density of Columbia ground squirrels appears to be complex, with squirrels occupying open habitats but tending to concentrate burrows under shrubs within steppe vegetation. Variation in density of burrows in spring was not strongly correlated with plant associations or with microhabitat. The author suggests that within a large area of suitable habitat, Columbian ground squirrel settlement is influenced by social or historical factors such as the presence of squirrels or burrows. Because characteristics of favorable ground squirrel habitat change during the active season, further research is needed to determine the effects of habitat heterogeneity on spacing later in the active season.

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ELECTRONIC FILE - Zoology