Can grizzlies survive global warming?

New study shows many bears still rely on dwindling whitebark pine seeds

An adult grizzly bear in the brush. Photo courtesy USFWS.

Staff Report

The long-term survival of grizzles in the Greater Yellowstone ecosystem may depend on whether they’re willing to switch from eating whitebark pine seeds to other types food.

Some of the bears have already started responding to reductions in whitebark trees by consuming more plants and berries, while others are still focused on finding stashes of the nutritious pine nuts, scientists said in a new study based on analyzing the chemical composition of what the grizzlies eat.

The research, published May 11 in PLOS ONE, focused on modeling the diets of grizzly bears in Cooke City Basin, Montana.

At issue is the fate of whitebark pines, a species listed as endangered by the International Union for Conservation of Nature. Shorter and milder winters, an introduced fungus and increasing bark beetle attacks have pushed the trees to the brink of extinction. Other potential food sources for grizzlies such as trout and ungulates have also declined in the region.

“Whitebark pine trees have declined due to an introduced fungal disease called blister rust, and, more recently, to increased infestation by the mountain pine beetle, which is exacerbated by climate change,” said study coauthor Carolyn Kurle, an assistant professor at UC San Diego’s Division of Biological Sciences. “Such declines further highlight the need to monitor diets of grizzlies as the environment continues to change.”

The scientists measured stable isotopes in bear hair and related their abundances to those found in their foods. Previous studies on bear diets have focused on bear scat, but the chemical analysis helps show what the animals are actually metabolizing and  assimilating into their tissues. The findings help show how the grizzlies have responded to changes in food availability on the landscape.”

The whitebark pine seeds are often cashed in large middens by red squirrels which are raided by grizzlies in the fall, fueling reproduction and ensuring the survival of grizzlies in the region. A main reason threatened grizzly bears have remained protected for decades is because it has not been clear how declines in whitebark pine trees, and thus the seeds they provide bears, will impact population trends over the long term.

Because their inferences are limited to a small area in the region and a small number of bears, the researchers recommend a large-scale study and urge others to use their new modeling framework to investigate the diets of other species of concern.

“Such analyses could be used to monitor grizzly bear recovery efforts and inform other wildlife conservation and management programs worldwide,” Hopkins added.

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.

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