Western gray squirrel resource selection related to fire fuel management

Abstract

One of 3 populations of the state‐threatened western gray squirrel (Sciurus griseus) in Washington occurs in the northern Cascade Range (i.e., North Cascades), where long‐term fire suppression has increased the risk of catastrophic wildfire. Land management agencies throughout this region have implemented fire fuel reduction programs that alter squirrel habitat and may affect their populations. From April 2008 to September 2011, we investigated resource selection of 38 radio‐collared western gray squirrels at 2 study sites in the North Cascades following fire fuel management activities including mechanical thinning and prescribed burning. We developed conditional logistic models to examine resource selection at 3 spatial scales: nest trees, nest sites, and core areas within home ranges. The odds of a squirrel selecting a tree for nesting increased with dwarf mistletoe (Arceuthobium spp.) presence, greater number of surrounding trees with interlocking branches, and tree size. Squirrels selected nest sites that had greater canopy cover, tree connectivity, and presence of dwarf mistletoe than available, unused sites. Core‐use areas within home ranges had greater canopy cover, a greater number of tree species, and trees with higher live crowns compared to low‐use areas. Our results indicate that fire fuel treatments may negatively affect western gray squirrel habitat across multiple spatial scales. Most variables that were positively related to habitat selection are specifically targeted for reduction in fire fuel management plans and were lower in sampled treated areas compared to untreated areas within the study sites. Key considerations in designing fuel reduction programs that benefit both squirrel habitat conservation and fire fuel management include maintaining forest patches with suitable canopy cover and connectivity, retaining large trees of a mix of species, and allowing for mistletoe infection at a reduced rate. © 2018 The Wildlife Society.

Differential Resource Use between Native and Introduced Gray Squirrels

“…In contrast, western gray squirrels used some riparian areas on the Base but were largely found in uplands dominated by conifers with little shrub cover, consistent with studies elsewhere in Washington (Linders 2000, Gregory et al 2010, Stuart et al 2018). Linders (2000) indicated that some western gray squirrels briefly moved into riparian areas during breeding season to have young in tree cavities, and we observed similar behavior on the Base.…”

“…Occupancy of high‐use areas by female gray squirrels exclusive of other conspecific females was evident on the Base (Johnston 2013) and suggests that interference competition could explain some differences in resource use. We suggest, however, that eastern and western gray squirrels also differ in their preferred habitats based on our observations, experimental removals of eastern gray squirrels (Johnston 2013), and habitat associations of these squirrels reported elsewhere (Byrne 1979, Linders 2000, Gregory et al 2010, Stuart et al 2018).…”

“…All western gray squirrels had oaks within their home ranges, and distance to oaks was the most important variable in the RUF analysis. The general distribution of western gray squirrels is mostly tied to oaks (Dalquest 1948, Carraway and Verts 1994), although there are exceptions beyond the range of oaks (Stuart et al 2018). In addition to riparian areas, eastern gray squirrels also may be restricted to the distribution of oaks in undeveloped areas of Washington.…”

“…Western gray squirrels were listed as a threatened species by the state of Washington in 1993, where habitat loss and competition with introduced squirrels are postulated reasons for population declines (Linders and Stinson 2007). Previous researchers have described habitat associations of western gray squirrels in the Cascade Mountains in some detail (Linders 2000, Gregory et al 2010, Stuart et al 2018), but only broad associations have been described for this species in western Washington (Ryan and Carey 1995, Bayrakçi et al 2001, Fimbel and Freed 2008). Most remaining habitat in western Washington for western gray squirrels occurs in areas subject to timber harvest or expanding urban development.…”

“…Western gray squirrels commonly inhabit conifer‐oak ( Quercus spp.) forests and have not been reported to have strong riparian affiliations in Washington (Linders 2000, Gregory et al 2010, Stuart et al 2018). Such habitat differences between species may be important to interspecies interactions.…”