Assessment of habitat change for western gray squirrels in the eastside Cascades of Washington, 1993-2017

Western gray squirrels (Sciurus griseus) occur in mixed oak‐conifer habitats along the west coast from Baja California north to Washington State. In Washington, they occur in 3 disjunct populations: 2 on the eastern slope of the Cascade mountains, and a third, smaller population in the south Puget Trough. The western gray squirrel was listed as threatened in Washington in 1993, largely due to habitat loss and fragmentation and perceived declines in the population. Over the decades since listing there has been no quantitative assessment of how availability of habitat has changed for this species, while factors causing loss of habitat appear to be increasing.

Wildfire is a constant and significant agent of change in the dry forests of the east‐slope Cascades. Structural and compositional changes to eastside forests have resulted in larger and more severe fires in recent decades; in 2014 and 2015 alone, >310,000 ha of forest burned in the North Cascades of Washington. Timber harvest also has changed the forests in the east‐slope Cascades and has been especially prevalent in the South Cascades where ≥70% of the forested area is privately owned, with over half of that managed by industrial timber companies. Wildfires that result in high levels of mortality of overstory trees, and commercial harvest of trees that removes a high proportion of the forest overstory, reduce western gray squirrel habitat by creating forest stands with insufficient canopy cover to allow arboreal travel, fewer mature trees for nest sites, and reduced availability of tree seeds as food.

In 2017, Washington Department of Fish and Wildlife initiated a project to examine changes in availability of primary habitat for western gray squirrels in the years since the species was listed. Our objectives in this study were to estimate the change in primary habitat availability for western gray squirrels between 1993 and 2017 within key areas of importance to the species in eastern Washington, and to estimate the relative contribution of different drivers of habitat change. We focused our analysis on the 2 populations in the Cascade Range, areas that currently support the vast majority of squirrels in the state and that have the greatest potential for sustaining populations into the future.