Huckleberry Habitat and Its Influence on Two Small Populations of Grizzly Bears (Ursus arctos)

Huckleberries (Vaccinium spp.) are a nutritionally important food to grizzly bears (Ursus arctos) in interior, western North America. They provide sugar-rich calories in late summer and fall prior to denning. We developed a resource selection function of high-quality huckleberry habitat important to Cabinet-Yaak and Selkirk female grizzly bears using field-verified huckleberry foraging radiolocations acquired during prime months of huckleberry fruiting (July 15-September 15, 2010-2019, n = 22). Logistic regression analysis identified a model with 12 significant variables in predicting huckleberry habitat important to female grizzly bears (Somers' D = 0.729; K-S statistic = 0.570, p < 0.00001). Most influential variables (p < 0.00001; positive [+] or negative [-] relationship) include canopy closure (-), moisture deficit (-), time since last wildfire (-), solar radiation (+), snow water equivalent (-), and growing degree days above 5°C (-). On average, 28% of an adult female annual home range includes predicted huckleberry habitat ( $\overline{x}$ = 61 ± 6.4 [SE] square kilometers). Seasonal ranges of females overlap extensively within predicted huckleberry habitat, and degree of overlap trends with quality of habitat. Mothers and daughters display similar selection patterns for predicted huckleberry habitat, suggesting huckleberries are an important component of dispersal patterns, range expansion, and connectivity (via linkage areas) to other populations. Model values were significantly associated with long-term berry production, allowing estimates of amount of this food resource available on the landscape. Using energetic predictions of huckleberry foraging in these areas, we find that a smaller average body size of Cabinet-Yaak and Selkirk adult females ( $\overline{x}$ = 94 kg lean body mass) lessens the energetic constraints of a huckleberry-dominant diet and may be a direct outcome of huckleberries being a primary food resource for these populations.