Red Squirrel (Tamiasciurus hudsonicus) Midden Site Selection and Conifer Species Composition

Abstract

Reductions in whitebark pine (Pinus albicaulis) due to blister rust (Cronartium ribicola) and mountain pine beetle (Dendroctonus ponderosae) have prompted some forest managers to consider selective thinning and prescribed fire to reduce competition of whitebark pine with other conifer species. Whitebark pine is an important food source for grizzly bears (Ursus arctos horribilis) in the Greater Yellowstone Ecosystem, but most of the seeds are obtained by raiding red squirrel (Tamiasciurus hudsonicus) middens. Therefore, it is important to understand which attributes maximize red squirrel midden site selection. The objectives of this study were to estimate active midden site selection criteria and quantify the ideal conifer composition associated with red squirrel middens in the Cooke City Basin, MT. Active midden counts were collected in 810, 30-meter diameter circular plots, equally spaced along 27 transect lines in the basin. We used generalized linear mixed models to assess variables associated with middens. Midden occurrence probability was positively associated in a curvilinear relationship with conifer canopy cover and the amount of topographic shading. Midden occurrence increased as the percent of whitebark pine increased up to 44 percent. A conifer composition of 44 percent whitebark and 56 percent mixture of subalpine fir (Abies lasiocarpa) and Engelmann spruce (Picea engelmannii) was ideal for midden sites. Whitebark pine treatments to reduce fir and spruce competition should acknowledge the tradeoff to foraging grizzly bears.