Red squirrel (Tamiasciurus hudsonicus) midden site selection and the influence of conifer species compositions on midden occurrence in the Cooke City Basin of Montana

Abstract

Throughout the Greater Yellowstone Ecosystem (GYE), whitebark pine (WBP: Pinus albicaulis) seeds serve as an important fall food source for threatened Yellowstone grizzly bears (Ursus arctos). Grizzly bears depend on red squirrel (Tamiasciurus hudsonicus) midden sites to obtain WBP seeds. In light of recent WBP population declines, managers are concerned about the negative effects that loss of WBP may have on grizzly bears. Therefore, managing WBP for grizzly bears is facilitated by understanding red squirrel habitat requirements and identifying areas that are most likely to contain middens. Previous studies indicate that red squirrel middens are most prevalent in subalpine mixed conifer forests with interspersed WBP, but a critical gap remains in identifying a conifer species composition that is ideal for midden sites. We studied red squirrel habitat selection in the Cooke City Basin (CCB) of Montana to identify variables associated with midden sites and midden area. We also examined conifer species compositions to identify a composition where middens are most likely to occur. Habitat variables, midden counts, and midden area measurements were collected in 810, 30 meter diameter circular plots equally spaced along 27 transect lines in the CCB. General linear mixed models (GLMM) were used to assess variables associated with red squirrel midden site selection, and linear mixed models (LMM) were used to assess variables associated with midden area. Results of the GLMM indicated that red squirrel midden occurrence probability is positively associated with the amount of hillshade (light) and canopy cover in a conifer stand. Additionally, midden occurrence increased as the percent WBP in a stand increased up to 44 percent, but decreased thereafter. Results of the LMM indicated that midden area is positively associated with total canopy cover. We identified that a conifer species composition of approximately 44 percent WBP and a 56 percent mixture of subalpine fir (Abies lasiocarpa) and Engelmann spruce (Picea engelmannii) is ideal for midden sites. We concluded that managing for areas within subalpine zone mixed conifer forests containing similar compositions should be a priority to ensure availability of prime habitat for midden sites and associated WBP seeds for grizzly bears.