Pining Away: Whitebark Pines, Grizzlies, and Surprising Signs of Ecosystem Resilience

WHITEBARK pine seeds weigh 10 to 60 times more than the seeds from other trees in the same forests. By weight they’re roughly half fat and 20 percent protein. They can last for a year or more when buried. Unsurprisingly, they’re a precious fall and winter food source for high mountain birds, squirrels, and foxes. Grizzly bears also eat piles of whitebark pine seeds, digging up and then devouring underground caches made by red squirrels. In years with good production, researchers found grizzly bear scat (aka poop) can be anywhere from 50 to 80 percent whitebark pine seeds.

Yellowstone grizzly bears eat so many whitebark pine seeds that tree declines were used as the primary grounds in a 2009 lawsuit against U.S. Fish and Wildlife Service to continue Endangered Species Act protections for Yellowstone grizzly bears after they were delisted in 2007. The courts ruled that more research was needed. As a result the Interagency Grizzly Bear Study Team, led by the U.S. Geological Survey (USGS), was tasked with investigating the role of disappearing whitebark pine in grizzly bear population dynamics. “Until we did this work the assumption by many, including some on our science team, was that declines in whitebark pine would lead to declines in grizzlies,” said Frank van Manen, the USGS wildlife biologist who led the report. “The biggest surprise for us was that everything pointed in the direction of flexibility.” 

Despite dramatic declines in Yellowstone whitebark pine populations since the early 2000s—with losses of around 70 percent in the age classes that produce seeds—so far there appear to be no negative impacts on the Yellowstone grizzly bear population.

As van Manen explains, grizzly bears are opportunistic generalists. (He and his team have documented grizzlies eating more than 266 different species of plants and animals.) Grizzly bears show enormous adaptability in their diets from year to year, from season to season, and from one area of the ecosystem to another. About one third of Yellowstone grizzly bears have lost all access to whitebark pine resources. Instead they now eat more animal matter—from elk carcasses to army cutworm moths, even ants—without any measurable harm to their health.

While declines in whitebark pine may affect other parts of the food web, signs of resilience in the system are good news for grizzlies and provide some welcome flexibility for Forest Service and National Park Service managers responsible for the natural resources of the Greater Yellowstone area.

The fatty, high-protein seeds of whitebark pine feed birds, squirrels, foxes, and bears. (Pinecone photo from Wikimedia; grizzly bear photo by LuRay Parker, U.S. Fish and Wildlife Service.)