Hedysarum (Hedysarum spp.) roots are a major food for grizzly bears (Ursus arctos) over much of their Canadian and Alaskan range. In Banff National Park, grizzly bears typically dig roots of pink hedysarum (H. alpinum) in willow (Salix glauca, S. farriae) – dwarf birch (Betula glandulosa) shrubland. This shrubland type often burned in the past, but the effects of shrubland fire on grizzly bear feeding ecology have not been studied. We applied shrubland fire to pink hedysarum digging habitat in Banff National Park and measured grizzly bear response by counting their excavations for pink hedysarum roots over the subsequent 4–6 years. In 4 of 6 study sites, a positive fire effect was recorded: the digging density ratio (digging density in burned habitat divided by digging density in unburned control) increased 4.5x to 14.3x following fire compared to the ratio we recorded in the same treatment and control areas before fire was applied. In the remaining 2 study sites, grizzly bears dug 1 site essentially the same following fire (postfire ratio = 1.1 x prefire ratio), and 1 site showed a weak negative fire effect (postfire ratio = 0.8 x prefire ratio). Overall, fire resulted in >6x increase in the digging density ratio compared to the prefire ratio (n = 6 sites). Ease of digging did not appear to be an important factor in our study: the pull required to release a steel bar driven 11 cm into the substrate immediately beside diggings was approximately equal in burned and unburned habitat. The strong preference grizzly bears showed for excavating pink hedysarum roots in burned habitat suggests that shrubland fire may benefit grizzly bears in Banff National Park and perhaps elsewhere in North America where comparable digging habitat occurs.
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Vol. 17 • No. 2