Abundance and habitat associations of Washington ground squirrels in north-central Oregon

Graduation date: 2000Much of the Columbia River Basin has been cultivated for agriculture,\ud which has reduced and fragmented the native shrub-steppe habitats. As a result, the\ud range of the Washington ground squirrel (Spermophilus washingtoni) has been\ud significantly reduced, prompting the states of Washington and Oregon to review\ud their status. Consequently, I investigated abundance and habitat selection of\ud Washington ground squirrels on the Boardman Bombing Range, Morrow County,\ud Oregon during February through July of 1996 - 1997. Transect surveys were used\ud to determine relative abundance among seven habitats, and live-trapping was used\ud to compare density of squirrels in habitats in which they were present. Habitat\ud associations were determined by comparing vegetative and soil characteristics at\ud occupied sites and unoccupied sites. I made comparisons at two levels of\ud resolution: a macro-habitat and a micro-habitat comparison at the colony level.\ud Micro-habitat analysis used matched-pair logistic regression to compare habitat\ud characteristics of colonies to a paired site where squirrels were not present.\ud I located 44 colonies to investigate the habitat associations of this species.\ud Transect surveys indicated highest densities in sagebrush, followed by grassland\ud habitat. No squirrels were detected in bitterbrush or low-shrub habitats during the\ud transect surveys. Results from capture-recapture methods also suggested higher\ud densities in sagebrush habitat. Recruitment was highest in sagebrush followed by\ud bunchgrass and low-shrub habitats. Mean weight for adult and juvenile squirrels\ud were highest in bunchgrass habitat, followed by sagebrush and low-shrub. Mean\ud maximum distance moved was greater for males than females (P = 0.0006) for\ud adults and juveniles alike, while sex-ratios favored females in both age classes.\ud Habitat associations at the macro-habitat level revealed selection for sites with a\ud higher silt content (Warden soils) of the soil (P = 0.0006) and higher vegetative\ud cover (P = 0.032). Micro-habitat associations indicated a selection for sites with a\ud lower clay content of the soil (P = 0.004).\ud The continued existence of Washington ground squirrels depends upon\ud maintenance of the remaining suitable habitat, particularly large contiguous tracts.\ud Moderate grazing does not appear to be incompatible with squirrels, though grazing\ud intensity should be investigated in further studies