top of page

Climate Change

Yellowstone Grizzly Bears, Whitebark Pine and Climate Change

Yellowstone grizzly bears are already suffering from the impacts of climate change, which has precipitated the rapid decline of whitebark pine, a key fall food source for the bears. Other foods may be next. 


The importance of whitebark pine seeds to Yellowstone grizzly bears is well documented, including the effects of this food on human-caused mortality rates and female reproductive success.  In the fall, just before hibernation, grizzlies typically raid caches of whitebark pine cones stored by squirrels, which is an efficient way to get large, nutritious, whitebark pine seeds at a critical time of year. Females consume roughly twice as many whitebark pine seeds as males do, and are more likely to reproduce—and produce larger litters—following good pine seed crops. [1] 

Whitebark pine also reduces human-caused mortality by attracting grizzly bears to remote high-elevation areas during years when seed crops are large, away from people (especially hunters) and thus out of harm’s way.[2]  When pine seed crops are poor, grizzly bear mortality doubles compared to years when seed crops are good, which results in an average 5% rate of population increase following good seed crops, versus an average 7% rate of decrease when crops are poor.[3]  Furthermore, during poor whitebark pine seed crop years, grizzly bears turn to other foods such as elk during fall and late summer, but with predictably negative consequences for females and their cubs. 1, [4]  Because male bears are better able to dominate elk carcasses, females place their cubs at risk if they rely on this food source.1 Elk also bring grizzlies into conflict with big game hunters.


In addition to being a critical food source for grizzly bears and other wildlife, whitebark pine also stabilizes soils, and provides habitat for both plants and animals. Moreover, whitebark pine forests stabilize and shade the snowpack, reducing avalanches and extending precious snowmelt flows into the summer months. This slow melting process not only keeps rivers cool for trout and other aquatic wildlife but also helps maintain sufficient water resources for the people living in the arid American West.

But an unprecedented climate-driven mountain pine beetle epidemic is now decimating whitebark pine forests across the West, threatening mountain ecosystems and devastating this key food resource for bears. Tiny mountain pine beetles bore into mature pine trees, killing them by eating critical tissue under the bark. When the beetles hatch in the summer, huge swarms attack a forest all at once. Cool year-round temperatures and sustained cold snaps, especially during spring and fall once kept this beetle confined to low-elevation forests, but global warming has allowed the mountain pine beetle to expand its range into high-elevation forests, where the whitebark pine is virtually defenseless against this newcomer and its explosive attacks. Swaths of dead whitebark pines now stretch across the landscape, their telltale red needles bearing witness to the unprecedented impacts of climate change in this iconic ecosystem.


A 2009 survey conducted by the U.S. Forest Service, Ecoflight, and Natural Resources Defense Council showed that 51 percent of the whitebark pine forests in Greater Yellowstone have already suffered high mortality from mountain pine beetles, with another 31 percent experiencing significant mortality.[5] While beetles kill the mature trees, an introduced pathogen known as blister rust is killing young trees, creating a perfect storm for whitebark. Moreover, climate warming is shrinking the suitable habitat for whitebark pine, with projections of greater than 95% habitat loss by the end of the century.[6] Every year is now a bad year for whitebark pine nuts – and for the bears that depend on them.


The potential consequences of a dramatic rapid loss of whitebark pine to grizzly bears have been recognized and anticipated by federal government researchers.  In 2006, Schwartz et al. stated, “should whitebark pine decline rapidly, we speculate that we would witness a scenario similar to what occurred when dumps were closed in Yellowstone National Park: more management problems, particularly outside the Recovery Zone, with a substantial increase in measurable bear mortality.” [7] This is exactly what we are witnessing now: grizzly bears are turning more to meat, which precipitates more conflict with livestock operators and big game hunters.


Moreover, the native Yellowstone cutthroat trout population – historically another one of the four most important grizzly bear foods in Greater Yellowstone – has collapsed due to the introduction of predatory Lake trout, drought and other factors.[8] The loss of cutthroat trout could exacerbate the adverse impacts of whitebark pine loss.


So what can we do? The most critical step is to protect grizzly bears and their habitat so they can adapt to these staggering changes. In 2001, Charles Schwartz, then Leader of the Interagency Grizzly Bear Study Team, recommended that additional habitat be protected outside the Grizzly Bear Recovery Zone because of the threats to whitebark pine. In a February 28, 2001, email [9], Schwartz stated:


“Loss of whitebark pine will amplify the interface between humans and grizzly bears, and could increase the mortality rate in the population…We know the major foods in the GYE are likely to decline.  The net result of this decline is a lower carrying capacity for grizzly bears in the GYE.  To ensure that there is an adequate number of healthy individuals, and hence a healthy population, likely will require a larger area than currently defined by the Recovery Zone.  Protecting these lands before they are drastically altered by human impacts is important to ensure the long-term conservation of the bears.”


Ultimately only turning the thermostat down – addressing global climate change – will secure a future for whitebark pine forests and the wildlife and human communities that depend on them. But for grizzlies, maintaining stringent federal protections and allowing bears to expand their occupied habitat, can give these animals a chance to adapt to the rapidly changing world that we are creating for them.

[1] D.J. Mattson, Causes and Consequences of Dietary Difference Among Yellowstone Grizzly Bears (Ursus arctos), Ph.D. Dissertation, University of Idaho, Moscow, Id, USA, 2000.


[2] D.J. Mattson, B.M. Blanchard, and R.R. Knight, “Yellowstone grizzly bear mortality, human habituation, and whitebark pine seed crops,” Journal of Wildlife Management, vol. 56, pp. 432-442, 1992.


[3] C.M. Pease and D.J. Mattson, “Demography of the Yellowstone grizzly bears,” Ecology  vol. 80, pp. 957-975, 1999.


[4] M.A. Haroldson, C.C. Schwartz, and D.S. Moody, “Possible effects of elk harvest on fall distribution of grizzly bears in the Greater Yellowstone Ecosystem,” Journal of Wildlife Management, vol. 68, pp. 129-137, 2002.


[5] Macfarlane W.W, J. A. Logan and W.R Kern, “Using the Landscape Assessment System (LAS) to Assess Mountain Pine Beetle-Caused Mortality of Whitebark Pine, Greater Yellowstone Ecosystem, 2009: Project Report.” Prepared for the Greater Yellowstone Coordinating Committee, Whitebark Pine Subcommittee, Jackson, Wyoming, 69 pages, 2010. Available at


[6] M. V.Warwell, G. E. Rehfeldt and N. L. Crookston. 2007. Modeling contemporary climate profiles of whitebark pine (Pinus albicaulis) and predicting responses to global warming. Proceedings of the conference Whitebark pine: a pacific coast perspective. USDA Forest Service R6-NR-FHP-2007-01. Cited in: U.S. Fish and Wildlife Service, Endangered and Threatened Wildlife and Plants; 12-Month Finding on a Petition To List Pinus albicaulis as Endangered or Threatened With Critical Habitat, 50 CFR part 17, pp. 42631-42654, 2011.


[7] C. C. Schwartz, C.C., M.A. Haroldson, G.C. White, R.B. Harris, S. Cherry, K.A. Keating, D. Moody, and C. Servheen, “Temporal, spatial, and environmental influences on the demographics of grizzly bears in the Greater Yellowstone Ecosystem,” Wildlife Monograph, no. 161, 2006.


[8] Interagency Grizzly Bear Study Team, Annual Report of the Interagency Grizzly Bear Study Team, USGS Northern Rocky Mountain Science Center, Bozeman, Mont., 2008.


[9] C. Schwartz, E-mail to Dr. Christopher Servheen, 28 Feb 2001.

bottom of page