As climate change continues to alter the environment in Yellowstone National Park and its surrounding region, Unity College’s Dr. Jack Hopkins is studying the impacts of such changes on the diets of the region’s iconic, threatened grizzly bears.
In a study published May 11 in PLOS ONE, lead author Dr. Hopkins and his team found that whitebark pine seeds and other plant foods contribute more to grizzly bear diets than meat for bears sampled in Cooke City Basin, Mont., an area within the Greater Yellowstone Ecosystem (GYE).
Once pervasive across western North America, the slow-growing whitebark pine trees have steadily declined in recent decades and are now listed as endangered by the International Union for Conservation of Nature. Past research has found that roughly 70% of marked whitebark pine trees alive in Greater Yellowstone in 2002 were dead by 2009. Shorter, milder winters brought about by climate change have exacerbated beetle infestations, and fungal disease was also rampant in past decades. It is also important to note that in recent decades other important food sources for grizzlies, namely cutthroat trout and elk, have also declined in the region.
The team measured stable isotopes from bear hair collected from barbed wire hair snares to determine what bears had been eating that year. Scat analysis doesn’t provide reliable dietary estimates for digestible foods, Dr. Hopkins explained, saying, “It is difficult to know how much meat or seeds a bear really consumed during the year when all you have to work with is some bone, hair or pine cone material found in their scat.”
“Faculty research is essential in adding experiential value to our students’ education,” President Dr. Melik Peter Khoury said about the recent study. “People often say that those who can’t do, teach. The faculty at Unity College disprove this premise with every published research paper and experiment. Our teachers put theory into practice every day, both inside and outside the classroom.”
The main reason the threatened status of the grizzly was reinstated in March 2010 was because it was unclear how declining whitebark pine would impact long-term population trends. While prior research suggests whitebark pine seeds are important to grizzly populations because grizzly bear survival increases during years of good whitebark pine productivity and females produce more cubs the year following good cone production, no studies previous to the one released by Hopkins and his team have put forward a framework for estimating the contribution of whitebark pine seeds and other important food sources to the diets of grizzlies through time using stable isotopes.
“This project was really interesting to me because I’d like to understand how bears and other wildlife respond to changes in their environment. I think there’s been a lot of top-notch bear research done in Yellowstone, but it seems we still don’t have a comprehensive understanding of how bears diets have changed as their major food sources, such as whitebark, have declined,” Hopkins said. ”It takes time to understand the effects of such change and it also requires a look into the past. Stable isotope analysis is really nice for this kind of work because it provides researchers both a current and historical perspective of animal foraging behavior. Such an understanding is not only useful to grizzly bear managers in Yellowstone but is central to many ecological questions.”
Hopkins hopes other researchers will use the team’s new statistical methods to estimate the dietary responses of grizzlies to declines in whitebark pine seeds and other important food sources through time. Such information could be useful in predicting how Yellowstone’s grizzly population will adapt to future environmental change — more important now than ever, as Yellowstone grizzlies are currently on the table to be delisted under the Endangered Species Act, resulting in regional states taking control of population management through permitted hunts.
The team found that in spite of the tree’s decline, grizzly bears continue to forage for whitebark pine seeds as a diet staple in the northeast region of the GYE, a stronghold of relatively healthy whitebark pine. They also learned that some bears could be responding to reductions in whitebark trees by consuming more plants and berries.
Because results of the study were derived from a small area in the GYE and a small number of bears, researchers recommend a larger scale study that uses the modeling tools described in their paper “Selecting the best stable isotope mixing model to estimate grizzly bear diets in the Greater Yellowstone Ecosystem”’. Dr. Hopkins and his team also urge researchers to investigate the diets of other species of concern.
Coauthors of the study also include Carolyn Kurle, an assistant professor at UC San Diego’s Division of Biological Sciences, Jake Ferguson of the University of Idaho, and Daniel Tyers of the U.S. Forest Service. The research was supported by the U.S. Forest Service, Federal Highway Administration-Western Federal Lands, UC San Diego, the National Institute for Mathematical and Biological Synthesis at the University of Tennessee Knoxville, the Center for Modeling Complex Interactions, D. Ohman and G. Bennett.