Climate change implications for grizzly bears (Ursus arctos) in the North Cascades Ecosystem

Ransom, Jason; Krosby, Meade; Lyons, Andrea

The North Cascades Ecosystem of north-central Washington State and southern British Columbia, Canada, is one of six designated recovery zones for grizzly bears (Ursus arctos) in the conterminous United States. The National Park Service and US Fish and Wildlife Service evaluated a range of alternatives for restoring grizzly bears in the Cascades, and carrying capacity models estimated the ecosystem could support approximately 250–300 bears under current habitat conditions. As climate change shapes the North Cascade Ecosystem, there is considerable uncertainty as to how grizzly bear habitat may change through time. Projected increases in growing season length, winter and spring water surplus, summer water deficit, wildfire, and decreases in snow pack will likely to lead to substantial vegetation changes by the end of the century. These changes are in turn likely to affect grizzly bear foraging and denning behaviors, and thus could influence their population dynamics through time. The North Cascades Ecosystem supports many of the primary food sources used by grizzly bears in other populations, including graminoids, starchy tubers like Hedysarum spp., montane forbs like glacier lilies, forest plants like horsetails and cow parsnip, clovers, and a wide variety of berry-producing plants (Vaccinium spp. and others), as well as ants, ungulates, and carrion. Because grizzly bears are habitat generalists, they are projected to be relatively insensitive to climate change effects. More grizzly bear food resources in the North Cascades are expected to increase in abundance over time than those projected to become more scarce. For example, some important bear food sources, like Vaccinium spp., are projected to significantly increase in abundance as meadows become shrubbier and fire opens forests over the coming decades. Many grizzly bear food sources are projected to migrate up in elevation, potentially creating higher quality habitat farther from low elevation roads and human settlements where human-bear conflict is more likely. Disease and pest outbreaks such as blister rust disease in whitebark pine, beetle infestations of conifer forests, and salmon poisoning disease introduce a suite of uncertainties for the future of specific potential grizzly bear foods. Changes in winter snowpack may delay denning, exposing grizzly bears to negative human interactions for a longer period of time each year, underscoring the importance of human sanitation practices and education. The complex relationship between changes in climate, natural processes, and natural and anthropogenic features will expose grizzly bears to a range of changing resource conditions, but the species low sensitivity to changing climate and high adaptive capacity portends positive long term outcomes if a successful founding population can be re-established. This report aims to synthesize the scientific literature and develop the conceptual basis for understanding potential climate impacts on grizzly bear habitat quality in the North Cascades Ecosystem. It is expected that the outcome of this effort will be used to inform modeling efforts for estimating grizzly bear carrying capacity under future climate change scenarios.

Climate change implications for grizzly bears (Ursus arctos) in the North Cascades Ecosystem

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