Occupancy surveys for western gray squirrels in Washington

The largest native tree squirrel in western North America, western gray squirrels (Sciurus griseus) occur along the west coast of the United States from Baja California to Washington State. The species is listed as threatened in Washington, largely due to habitat loss and a perceived reduction in population levels. Despite substantial study of the ecology of western gray squirrels in Washington, there has been limited systematic investigation of its occurrence and distribution. In 2018, Washington Department of Fish and Wildlife initiated a survey project to assess occupancy rates of western gray squirrels in the 3 primary population areas in Washington.

Three project areas were identified for occupancy surveys, each area representing current knowledge of the geographic core area for populations in the Puget Trough, the North Cascades, and the South Cascades. Geographic core here is defined as the spatial extent where past surveys and western gray squirrel observations suggest that suitable habitat occurs (or occurred in recent history) in sufficient acreage and distribution to support viable populations.

We used hair tubes to determine occupancy at survey sites and focused sampling within ecological systems that represent potential western gray squirrel habitat. The general approach was to survey along a 600m-long transect representative of the ecological system in the selected stand. Stands were selected using a geographic information system and various data layers including orthophotographs, topography, and ownership. Tubes were deployed between May and July and were checked at 2- to 4-week intervals. We used occupancy modeling on data pooled across years to derive a project-wide conditional detection probability (conditional on a squirrel being present) and to estimate occupancy for each project area.

Occupancy surveys spanning 3 years resulted in 138 sites deployed and checked across all 3 project areas. Eighteen sites were surveyed in the Puget Trough, 60 sites were surveyed in the North Cascades, and 60 sites were surveyed in the South Cascades. Conditional detection probability was high (0.91, SE = 0.03) indicating that squirrels generally were detected when present on a site. Modeled occupancy rate was 0.39 (SD = 0.12) for the Puget Trough, 0.27 (0.06) for the North Cascades, and 0.44 (0.07) for the South Cascades.

Surveys using hair tubes proved effective for documenting occupancy of western gray squirrels in Washington. An independent dataset from the South Cascades project area yielded a similar occupancy estimate based on nest surveys, providing support for our survey approach. Extant threats to habitat in all 3 population areas and recent habitat losses documented for the 2 Cascade populations make continued monitoring of these populations crucial for guiding management actions over the next few decades. Occupancy rates resulting from this survey can be considered a baseline, representing conditions extant during the survey period and providing values that can be compared with those from subsequent surveys using similar or comparable techniques.