Published: August 2023
Author(s): Gary J. Wiles, Derek W. Stinson and Mary J. Linders
The Western Gray Squirrel is one of three native tree squirrel species in Washington. It was historically distributed at low elevations from Pierce County southward to Clark County, through the Columbia River gorge, and in low to mid-elevations along the eastern Cascade Mountains from Klickitat to Okanogan counties. Current distribution in the state is now primarily limited to three areas: the Klickitat region (Klickitat, southern Yakima, and southeastern Skamania counties); the North Cascades (Okanogan and Chelan counties); and the southern Puget Trough (Joint Base Lewis-McChord and small areas off-base in Pierce and Thurston counties).
Although not well documented, Western Gray Squirrels were probably once uncommon to locally common across much of their range in Washington. The species was in decline by the late 1800s and was considered rare by 1970. In 2007, the statewide population was estimated to be between 468 and 1,405 squirrels (937 ± 50%) based on data from 1994-2005. Populations have not been estimated since, but occupancy surveys were conducted 2018-2020 to establish a baseline for monitoring trends in coming years.
In the past decade, the southern Puget Trough population may have increased somewhat due mainly to habitat work and augmentation of the squirrel population on Joint Base Lewis-McChord (JBLM) in 2007- 2012. Research and local surveys in the North Cascades since that time suggested the population was higher than the 2007 estimate, but likely declined following wildfires in 2014, 2015, and 2021. An assessment of change in availability of Western Gray Squirrel primary habitat between 1993 (listing) and 2017 found gains from successional processes (e.g., tree recruitment and tree growth) did not compensate for habitat loss. Estimated net loss of habitat totaled 20.8% for the North Cascades and 21.2% for the South Cascades (Vander Haegen and others 2022). Wildfire was the dominant disturbance in plots examined in the North Cascades while timber harvest predominated in the South Cascades.
Known threats important to Western Gray Squirrel populations in Washington are habitat loss, degradation, and fragmentation; small population size and isolation; disease; and road mortality. The factors most linked to habitat loss for Western Gray Squirrels include timber harvest, wildfire, land conversion, and fire exclusion. Climate change is both a current and potential future threat to habitat through increased size and/or frequency of stand-replacement fires, changes in resulting stand composition and ongoing effects on food supply such as production of fungi and seeds.
Although the southern Puget Trough population may have increased slightly since the recovery plan was completed in 2007, it is very limited in size and constrained by the area and fragmentation of its habitat. Because of the species’ relatively small total population size throughout the State, isolation of the three populations, continuing threats of wildfires and timber harvest, and a likely decline in primary habitat of >20% in both the North Cascades and Klickitat regions it is recommended that the Western Gray Squirrel be uplisted to endangered in Washington.
Wiles, G. J, D. W. Stinson, and AM. J. Linders. 2023. Periodic status review for the Western Gray Squirrel. Washington Department of Fish and Wildlife, Olympia, Washington. 35 + ii pp.
Draft documents are provided for informational purposes only. Drafts may contain factual inaccuracies and may not reflect current WDFW policy.