Use of Arctic ground squirrels (Urocitellus parryii) by brown bears (Ursus arctos)

Abstract

Brown bears (Ursus arctos) in the Mackenzie Delta in the Northwest Territories of Canada are at the northern edge of their range and persist in a nutrient poor environment with limited available protein. Arctic ground squirrels (Urocitellus parryii) are widespread across northern Canada and are often cited as an important protein source for brown bears. We hypothesized that Arctic ground squirrels form an important part of brown bear diet in the Mackenzie Delta region, and that their importance varies seasonally, with peak use occurring during bears' pre-dormancy hyperphagic period, as well as annually with fluctuating availability of other food. To investigate the seasonal use of Arctic ground squirrels by brown bears, we used resource selection function analysis to describe habitat use, stable isotope analysis to determine individual trophic level and examine its relationship with habitat use, and GPS satellite telemetry to facilitate site investigations. Arctic ground squirrel habitat was not a significant covariate in resource selection functions for pooled sexes, pooled females, and pooled males. However, at the individual level, bears displayed a range of behaviors from selection against, no selection, to selection for Arctic ground squirrel habitat. We found no significant interannual variation in use of Arctic ground squirrel habitat. Site investigations and stable isotopic values for individual bears also suggest a range of foraging behaviors for Arctic ground squirrels. Individual resource selection models combined with trophic level and site investigations provide support for limited use of Arctic ground squirrels that varied between individuals and provided additional evidence for the diversity of niches exploited by individuals.

Time-dependent memory and individual variation in Arctic brown bears (Ursus arctos)

“…Our landscape data provide information on the spatial heterogeneity in vegetation and topography. We used three 30 x 30 m raster layers to describe the study area: a digital elevation model (DEM) measuring elevation (ranging from 0 m to 1676 m), a vegetation class raster describing dominant vegetation in each portion of the landscape, and a raster approximating the density of Arctic ground squirrels ( Urocitellus parryii ), which are a common brown bear prey species [ 39 , 40 ]. The vegetation class raster classified each 30 x 30 m grid cell into one of 46 vegetation classes, describing the age, size, and/or dominant plant species present in each area ([ 41 ]; but also see the Appendix).…”

“…The food available in riparian habitats (including whitefish, which generally migrate in early October; [ 47 ]) is most prominent from May 10 to October 16 [ 55 ] when the ice has melted from the Mackenzie River. Arctic ground squirrels are always present, but they are easier for brown bears to hunt when they are hibernating [ 39 ], so we used an interval from September 11 to November 30 to approximate when most squirrels would be dormant [ 56 ]. Sweetvetch is also available year-round, but provides the highest nutritional return in the early spring, so we used an interval from April 1 (the beginning of the active season) to June 15 [ 40 ].…”

Time-dependent memory and individual variation in Arctic brown bears (Ursus arctos)

“…Our landscape data provide information on the spatial heterogeneity in vegetation and topography. We used three 30 x 30 m raster layers to describe the study area: a digital elevation model (DEM) measuring elevation (ranging from 0 m to 1676 m), a vegetation class raster describing dominant vegetation in each portion of the landscape, and a raster approximating the density of Arctic ground squirrels (Urocitellus parryii ), which are a common brown bear prey species (MacHutchon and Wellwood, 2003;Barker et al, 2015). The vegetation class raster classified each 30 x 30 m grid cell into one of 46 vegetation classes, describing the age, size, and/or dominant plant species present in each area (Ducks Unlimited, 2002; but also see the Appendix).…”

“…The food available in riparian habitats (including whitefish, which generally migrate in early October; Barker and Derocher, 2009) is most prominent from May 10 to October 16 (Macdonald et al, 1995) when the ice has melted from the Mackenzie River. Arctic ground squirrels are always present, but they are easier for brown bears to hunt when they are hibernating (Barker et al, 2015), so we used an interval from September 11 to November 30 to approximate when most squirrels would be dormant (Buck and Barnes, 1999). Sweetvetch is also available year-round, but provides the highest nutritional return in the early spring, so we used an interval from April 1 (the beginning of the active season) to June 15 (MacHutchon and Wellwood, 2003).…”