Published: November 2007
Author(s): Mary J. Linders and Derek W. Stinson
The western gray squirrel is a native arboreal squirrel best known for its large size, gray pelage, and plumose, white-tipped tail. Western gray squirrels are often confused with introduced eastern gray squirrels that are increasingly common in Washington's urban areas. Historically, western gray squirrels in Washington were widely distributed in transitional forests of mast-producing Oregon white oak, ponderosa pine, and Douglas-fir. Western gray squirrels play an important role in maintaining oak woodlands by planting acorns and disseminating spores of mycorrhizal fungi that aid tree growth.
During the 20th century the Washington population of western gray squirrels experienced great reductions in both numbers and distribution. The species now occurs as separate populations in the Puget Trough, Klickitat, and Okanogan regions that are estimated to total between 468 and 1,405 individuals. These three populations are genetically isolated from one another, and have been isolated from those in Oregon and California for at least 12,000 years. None of the three current populations seem to be large enough to avoid a decline in genetic diversity and at least two may suffer from the negative effects of inbreeding.
The western gray squirrel was listed as a threatened species in Washington in 1993 by the Washington Fish and Wildlife Commission, and its native oak habitat is recognized as a Washington Department of Fish and Wildlife Priority Habitat. The U.S. Fish and Wildlife Service considers the western gray squirrel a "species of concern" in western Washington, and the U.S. Forest Service recognizes it as a "sensitive" species and a "management indicator species" for oak-pine communities. Washington populations of the western gray squirrel have not recovered from past reductions in their range and existing populations face significant threats to their survival. The western gray squirrel is vulnerable because of the small size and isolation of remnant populations. Major threats to the western gray squirrel in Washington include habitat loss and degradation, road-kill mortality, and disease. Populations of eastern gray squirrels, fox squirrels, California ground squirrels and wild turkeys are expanding and may compete with, and negatively impact western gray squirrel populations. Competition with eastern gray squirrels may be an important current issue for the Puget Trough population and in southwestern Klickitat County, while fox squirrels may affect western gray squirrels in portions of the Okanogan region. California ground squirrels, which became established in Washington in the 20th century, may compete with western gray squirrels in Klickitat and Yakima counties.
Habitat has been lost to urbanization and other development, particularly in the south Puget Sound area, and to catastrophic wild fires in Yakima County and the Okanogan. Conifer dominated stands of large diameter and mast-producing trees of pine and oak with interconnected crowns are particularly important in the life history of the western gray squirrel. Logging that removes the large mast-producing trees and results in evenly spaced trees with few or no canopy connections reduces habitat quality. Habitat also has been degraded by fire exclusion and historic over-grazing. In the south Puget Sound area oak woodland is being degraded by the invasion of Scotch broom. Road-kill is a frequent source of mortality for western gray squirrels and is known to be a major source of mortality for the Puget Trough population. Notoedric mange, a disease caused by mites, periodically becomes epidemic in western gray squirrel populations and appears to be the predominant source of mortality in some years. The incidence and severity of mange epidemics appears to be related to stresses in the local population precipitated by periodic food shortages.
Recovery actions are needed to maintain and restore western gray squirrel populations in Washington. The recovery plan identifies western gray squirrel recovery areas and interim recovery objectives for these areas. The recovery plan outlines strategies intended to restore a viable western gray squirrel population in the South Cascade Recovery Area and increase and maintain populations in the Puget Trough and North Cascades recovery areas. The western gray squirrel will be reclassified from State Threatened to State Sensitive status when management plans, agreements, regulations, and other mechanisms are in place that effectively protect the habitat values for western gray squirrel populations, and the following population levels are maintained:
- a total population of 3,300 adult western gray squirrels in the South Cascades Recovery Area;
- a total population of 1,000 adult western gray squirrels in the North Cascades Recovery Area;
- and a population of >300 adults is restored and maintained in the Puget Trough Recovery Area.
Recovery objectives may be modified as more is learned about the habitat needs and population structure of this species. Increasing and maintaining a population in the Puget Trough and the North Cascades may require augmentation with individuals from healthier populations. Western gray squirrel recovery strategies include protecting and monitoring populations, restoring depleted populations and degraded habitat, and protecting suitable oak-conifer habitat from harmful timber practices, catastrophic fires, and loss to development. Research is needed on the habitat requirements and factors limiting western gray squirrel populations, the role of disease in dynamics of populations, and to refine survey and population monitoring methods. Successful recovery of the western gray squirrel in Washington will depend on cooperative efforts of large and small private landowners, Native American tribes, counties, and multiple public agencies.
Linders, M. J., and D. W. Stinson 2007. Washington State Recovery Plan for the Western Gray Squirrel. Washington Department of Fish and Wildlife, Olympia. 128+ viii pp.
Draft documents are provided for informational purposes only. Drafts may contain factual inaccuracies and may not reflect current WDFW policy.