Wildlife Conservation in Willamette Valley Grassland & Oak Habitats Species Account Western Gray Squirrel (Sciurus griseus)

Conservation Status—The western gray squirrel is classified as an ISSSSP Sensitive Species in Washington, and is an Oregon Conservation Strategy (OCS) Species as well as a game animal (ODFW 2006), however the species has no federal special status. In Washington, the western gray squirrel is designated as a state threatened species (WDFW 2009). Factors limiting western gray squirrel populations include habitat loss and fragmentation (Ryan and Cary 1995a; ODFW 2006), predation by domestic cats (Verts and Carraway 1998), competition from introduced squirrel species (Ryan and Cary 1995a), changes in woodland structure due to fire suppression (ODFW 2006), and road‐related mortality (Ryan and Cary 1995a, Verts and Carraway 1998). Distribution— The geographic range of the western gray squirrel extends from the coastal mountains near the Mexico‐California border, north along the Sierra Nevada range and along the coast to the southern Cascades (Verts and Carraway 1998). In Oregon, the western gray squirrel occurs throughout the Willamette Valley, southwest Oregon, the Columbia Gorge, foothills of the western Cascades, and lower elevations of the eastern Cascades (Verts and Carraway 1998). Ecology—In western Oregon, western gray squirrels are strongly associated with Quercus woodlands and conifer forests in which Oregon white oak co‐occurs (Verts and Carraway 1998). Dalquest (1948) stated that the occurrence of western gray squirrels in Washington is primarily regulated by the spatial distribution of Oregon white oak, the only true oak (Quercus spp) that occurs in the state. In contrast, Ryan and Carey (1995b) failed to detect western gray squirrels in pure stands of Oregon white oak at Fort Lewis, Washington, but found that the frequency of stand use was correlated with the species‐richness of trees and shrubs that produce large seeds and fruits. The geographic distribution and life history characteristics of true oaks probably prevent western gray squirrels and other vertebrates from becoming overly dependent upon acorn crops as a food source in the Willamette Valley. Oregon white oak is the only acorn‐producing species across most of the Valley (scattered populations of California black oak occur in Lane County) and its acorn production exhibits high inter‐annual variation in abundance and synchronicity across large geographic areas (Coblentz 1980, Ryan and Carey 1995a, Peter and Harrington 2009). High yields of mast may only occur once every 3 to 6 years (Ryan and Carey 1995b), thus posing a significant limit to any wildlife species that specializes in feeding upon acorns. Coblentz (1980) speculated that a poor mast crop in the Willamette Valley during 1978 affected the abundance of western gray squirrels in the region.