Important Food Source for Yellowstone Bears In Trouble

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A grizzly bear wandering among the pine trees near Swan Lake in Yellowstone

A grizzly bear wandering among the pine trees near Swan Lake in Yellowstone

A history of fire suppression, rampant insect infestation, an invasive fungal plague and global warning adds up to likely extinction for the whitebark pine, and serious trouble for the species that depend on it – in particular, the grizzly and black bears of the Northern Rockies. 

A black bear in a Whitebark Pine on Dunraven Pass in Yellowstone.

A black bear in a Whitebark Pine on Dunraven Pass in Yellowstone.

“I am very worried, but I can’t get depressed and give up,” says Professor Diana Tomback of University of Colorado – Denver, one of the nation’s leading experts on whitebark pine. Biologists like Tomback have known for only a few decades that the high-fat content of whitebark pine nuts helps bears fatten up for winter’s hibernation. The bears raid squirrel middens or caches of stored whitebark pine nuts in the fall, adding on the layers of fat that will get the bears through long winters, and improve the odds that grizzly mothers will have successful pregnancies.

Grizzly bears are attracted into the high country – away from people – where the whitebark pine dominates the timberline with its spreading canopies and wind-battered trunks and limbs. Yet when whitebark pine nut crops fail or do poorly, grizzlies abandon the high country in search of food. That means trouble for bears and people alike. Good years for whitebark pines mean good years for grizzly bears – abundant food far from livestock, birdfeeders, orchards, garbage and other temptations in valley bottoms crowded with roads, development and people. Trouble is, said Tomback, whitebark pines aren’t having many good years these days.


The first problem facing whitebark pines is an altered fire regime in the last century or so, said Tomback. In pre-settlement times, Native Americans made frequent use of fire as a tool to improve forage for game, and later horses. These frequent fires, over several centuries, created diverse patterns and generations of forest species. In and around timberline, the whitebark pine would eventually be replaced by shade-tolerant conifers – if fires didn’t periodically remove the conifers and give the whitebark pines an opening. Fire suppression in the post-settlement era has meant fewer fires and thus fewer opportunities for regeneration of whitebark pines. That means fewer young whitebark pines and a majority of older, mature whitebark pines.

Pine Bark Beetles

Mountain Pin Beetle

Mountain Pin Beetle

That sets the stage for problem number two: mountain pine beetles, which attack mature whitebark pines and kill healthy trees with inner bark thick enough to support the larvae of the beetles. Pine bark beetles have long infested the lower-altitude lodgepole pine forests and historically have invaded the higher-altitude stands of whitebark pine at the peak of their cyclical epidemics – particularly in time of drought when the trees are weakened. Problem three, and arguably the worst, is white pine blister rust, an exotic species native to Eurasia and inadvertently introduced to western North America in 1910 near Vancouver, British Columbia. The blister rust attacks the family of five-needled white pines, said Tomback, entering through the needle stomata. It grows into branches and stems, then erupts as spore-producing cankers that kill the branches and end cone production. While the pine bark beetle can kill a tree within one or two growing seasons, blister rust can take up to a decade before it kills a tree. As the fungal disease spreads south and east, it leaves behind “ghost” forests, said Tomback – stands of dead whitebark pine and mortality rates of 90 percent or higher. Blister rust has already spread to southern California, east through Idaho and Montana, south to Colorado and Nevada. It is already present in the Greater Yellowstone Ecosystem, said Tomback.

While whitebark pines are often too high in altitude to be of interest to the timber industry, blister rust has hammered other five-needled pine species, such as western white pine, sugar pine, limber pine and southwestern white pine. Lastly, said Tomback, there’s the growing impact of global climate change. “I call it the great unknown,” said Tomback. What’s already known, she said, is that warmer temperatures and milder winters have allowed the mountain pine beetle to speed up its life cycle from two years into one, thereby allowing more rapid infestations. “There’s also growing evidence that the mountain pine beetle is beginning to prefer the whitebark pine,” said Tomback. Other scientists are predicting that climate change will push subalpine species like the whitebark pine, up into higher altitude slopes. Yet if there is no “up,” no higher slopes, said Tomback, the whitebark pine could in theory be pushed up and off the mountains of the Northern Rockies.


Whitebark pine trees on Mt. Washburn in Yellowstone

Whitebark pine trees on Mt. Washburn in Yellowstone

From the Bob Marshall Wilderness south to the mountains of western Wyoming, said Tomback, a major food source for grizzly bears is being lost as the whitebark pine disappears. “Given the stresses imposed on the grizzly bear population in this region – including reduction of historic range, development and urbanization, and human intrusion – the loss of whitebark pine could be the final blow to an already precarious situation,” wrote Tomback, with co-authors Stephen Arno and Robert Keane in Whitebark Pine Communities, published by Island Press. “One outcome is certain: the absence of whitebark pine seeds in the subalpine zone will send bears wandering far and wide for food in late summer and fall, thereby increasing the incidence of encounters with humans. Inevitably, this will lead to many bears being destroyed,” they added.

While grizzly bears are the most prominent of the impacted species, the disappearance of whitebark pines will also affect black bears, 12 bird species and eight small mammal species. Other impacts of losing the whitebark pine include:

Altered watershed hydrology

Growing where other conifers cannot, the loss of the whitebark pine will alter high country patterns of snow accumulation and snowmelt, with watershed effects on the timing, levels and quality of stream flow.

Accelerated erosion

As whitebark pines disappear, their roots won’t hold the rocky soils of the high country, and woody plant succession will be slowed in disturbance sites.

Homogenization of timberline forests

As whitebark pine disappears, forest areas will shrink at high elevations, and will see whitebark pine replaced by subalpine fir, Englemann spruce or mountain hemlock. These new forests will be more vulnerable to large, severe fires.

Reduced nutcracker populations

As whitebark pine declines, so do populations of Clark’s nutcracker, the prime disperser of whitebark pine seeds. As seed numbers decline, fewer are left over after nutcrackers eat them, and fewer seedlings sprout. The mutual decline of whitebark pines and nutcrackers continue until there is no hope of restoring the tree to its historic range.


Tomback is founder and board member of the Whitebark Pine Ecosystem Foundation ( – a group of scientists who’ve focused on restoration of the beleaguered whitebark pine forests. The Forest Service and timber companies have been worried as well about blister rust, particularly when it hit high value timber species, such as the western white pine. For 40 years, foresters focused on the eradication of currant or gooseberry bushes, where the blister rust spores play out half of their life cycle. Millions of dollars have also been spent on fungicide spraying. 

Neither approach was very successful. The most encouraging news, says Tomback, is that not all whitebark pine trees die when hit by blister rust – some seem to be resistant to the disease. By harvesting these seeds, scientists have conducted a breeding program with rigorously controlled pollination and seed germination for over 50 years. Tomback and her fellow scientists recommend an aggressive program of planting rust-resistant whitebark pine seedlings, thinning underbrush, removing trees selectively, and applying a range of fire treatments from low- to high-intensity.

Restoration of whitebark pine forests won’t be easy, cheap or quick. The whitebark pines are in the high country, which is difficult to access; it will take a lot of money to restore them; and a new seedling takes 75 years before it bears seeds, says Tomback. It has been said that the definition of an optimist is an old man planting a tree for his grandchildren to enjoy. Tomback and her associates aren’t old, but the restoration of the whitebark pine forests will take several generations. They’re all working as if they’ll see that day come.


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