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Study: Yellowstone grizzlies still fattening up despite climate and habitat stresses

NPS Photo
Ken Conger

News brief: 

Grizzly bears in the Greater Yellowstone Ecosystem are adapting to climate change and other ecological pressures, a new U.S. Geological Survey study suggests. It found that the bears have been able to maintain healthy body fat levels over the past couple of decades despite shifting availability of popular foods and increasing population density.

Body fat is critical for grizzlies, especially females raising cubs. Bears need it to property hibernate through long winters and build sufficient energy reserves. So the Interagency Grizzly Bear Study Team – a group of scientists representing federal agencies, states and tribes that's housed at the USGS – used more than 20 years of data to see if body fat levels have changed over time.

“They're basically shifting their diets in response to the changes in the availability of food resources,” said Frank van Manen, a USGS ecologist who leads the study team. “Bears are very resilient. They've demonstrated based on our data that they can prioritize body fat storage.”

Van Manen said many foods that Yellowstone-area grizzlies have historically relied on have declined, including the whitebark pine, cutthroat trout and some elk herds. Yet bears are omnivorous and have been able to find other resources, like a variety of plants, bugs and winter carcasses. Researchers said the bears have eaten more than 260 different foods in recent decades.

“All those changes were pretty dramatic, but yet bears were still able to find the calories in alternative resources, switching among the high-calorie resources as needed,” van Manen said.

Another stressor on grizzlies is an increase in density. The bear population has grown since it was listed under the Endangered Species Act in 1975, yet the habitat of the animals has stopped expanding as their range bumps up against human development. Van Manen said some of the leanest grizzlies tend to live in areas with high bear concentrations, and thus more competition for food. He said this is something for game managers to keep in mind as they make decisions on future species management.

This year, several politicians from the Mountain West have renewed efforts to delist the Greater Yellowstone grizzly bear.

This story was produced by the Mountain West News Bureau, a collaboration between Wyoming Public Media, Nevada Public Radio, Boise State Public Radio in Idaho, KUNR in Nevada, the O'Connor Center for the Rocky Mountain West in Montana, KUNC in Colorado, KUNM in New Mexico, with support from affiliate stations across the region. Funding for the Mountain West News Bureau is provided in part by the Corporation for Public Broadcasting.

Will Walkey is a contributing journalist and former reporter for Wyoming Public Radio. Through 2023, Will was WPR's regional reporter with the Mountain West News Bureau. He first arrived in Wyoming in 2020, where he covered Teton County for KHOL 89.1 FM in Jackson. His work has aired on NPR and numerous member stations throughout the Rockies, and his story on elk feedgrounds in Western Wyoming won a regional Murrow award in 2021.

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