A small clan of researchers scrambles up a hill at nearly 10,000 feet in the Gallatin National Forest outside Yellowstone National Park. They jump tirelessly from tree to tree, running their hands over gnarled trunks and gazing up at a thick, towering canopy overhead. Uphill, a black bird glides from one branch to another, its call raucous and demanding. Squirrel chatter erupts in a nearby treetop, and someone spots a stash of seeds buried at its base. Just a few steps up, a scientist nearly steps in a fresh pile of grizzly bear scat.
At first glance, this may seem like a thriving ecosystem intact. But a closer look reveals subtle signs of whitebark pine trees battling an invasive fungus and pine beetle outbreak. It illustrates a dangerous and growing trend in the Greater Yellowstone Ecosystem—one that may pose a problem for the recently de-listed Yellowstone grizzly bears.
The relationship between bear and tree traces back to a rather ordinary-looking bird called the Clark’s nutcracker, a relative of the crow. The birds pluck large, energy rich seeds from cones that grow high up in the tree, then plant as many as 100,000 seeds in more than 20,000 locations. The nutcrackers return to about 1,000 of those caches to feed, leaving the remaining seeds to sprout into a new generation of whitebark pines.
Enter the red squirrel, which gnaws off the seed-filled cone and buries it at the base of the tree. During the fall months leading up to hibernation, grizzly bears know the chattering of a red squirrel means there are seed caches to raid.
“It’s all of these systems aligned that allow the grizzly bear to be healthy and present,” says Louisa Wilcox, a grizzly bear expert with the Natural Resources Defense Council. After all, from a bear’s perspective, a single whitebark pine can produce the equivalent of 40-50 Big Macs a day. And that makes it an important food source for females that have to provide calories for themselves and their cubs until they venture out of the den in mid-April.
But global warming has the potential to destroy that food source as lower elevation mountain pine beetles swarm into new territory. In the past, extreme winter temperatures kept the beetles at bay—up to 90 percent of the insects died off in a typical Yellowstone winter, and few could survive above 9,000-feet. But with milder winters and earlier snowmelt, the West is heating up, especially at higher elevations. New trends show that only 20 percent of the pine beetle population dies off now, allowing the insects to thrive in places where they simply should not exist.
The early stages of a mountain pine beetle attack can be difficult to detect, with only a few inconspicuous burrows on the trunk or a limb of a whitebark pine tree. But underneath the bark lies a much more destructive scene, where beetles feed, mate, and lay eggs. Each attacking adult has the potential to create 60-200 young, all the while releasing pheromones that signal nearby beetles to join ranks and attack. In time, the invaders simply overrun the tree, which first stops producing cones, then turns red, drops its needles, and dies.
One summit in particular—Mount Washburn, in Yellowstone National Park—illustrates this process to an extreme. A bird’s-eye view reveals a sprawling carpet of red, dying trees that fade into ghostly gray snags. Further along the ridgeline, it’s clear that healthy green whitebarks are an ever-decreasing minority amid an exploding beetle outbreak.
“It’s amazing to see how quickly and dramatically the changes are taking place here in Yellowstone,” says Tim Stevens, NPCA’s Yellowstone Program Manager. “The effects of global climate change are being felt in our nation’s first national park, and we need to make sure we’re planning for those changes.”
At the same time, one in four whitebark pines in the Greater Yellowstone Ecosystem are fighting a much older epidemic: blister rust, an Asian fungus that came over from Europe nearly a century ago. By the late 1980s, the disease had wiped out nearly every whitebark pine in Glacier National Park, where moist conditions favored the wind-borne spores. Some scientists fear that Yellowstone isn’t far behind. “A whitebark pine that’s weak from drought and a disease like blister rust, has barely enough energy reserves to maintain basic functions,” says Diana Six, a scientist with the College of Forestry and Conservation at the University of Montana. “Mountain pine beetles come along, and the tree’s a sitting duck.”
No one’s sure what this means for the grizzly bear. But Chuck Schwartz, a bear biologist who leads the U.S. Geological Survey’s Interagency Grizzly Bear Study Team, has faith in their adaptability. “We’re not talking about an animal that’s a specialist,” he says, referring to the lynx, which specializes on a single prey item—the hare—which in turn drives the dynamics of lynx populations. Grizzlies are generalist omnivores, meaning they keep a varied diet. In Alaska, brown bears concentrate on salmon. In the Arctic, bears forage for berries and hunt ground squirrels and the occasional caribou. It’s possible, Schwartz says, that Yellowstone’s grizzly bears may switch their diet to other sources and turn out just fine.
Besides, he adds, ecosystems are never static, and global warming is just one more factor pushing change. “There are going to be winners and there are going to be losers, and if you really want to understand the impact on grizzly bears, you can’t just focus on one dietary item that may be a loser.” Case in point: Climate change projections show that gambel oak, a tree native to the Southwest, could move as far north as Butte, Montana, as temperatures increase. Historically, when grizzly bears roamed as far south as Mexico, gambel oak was very likely a favorite food.
Some climate models also suggest that milder winters could favor grass-eaters like elk and bison, which would mean more potential prey for the grizzly bear. And insects like ants and cutworm moths—both high protein sources for grizzly bears—might also increase with rising temperatures. But Wilcox remains concerned. Other food options bears might resort to in the fall—like earthworms, pondweed root, and elk thistle—lack the concentrated caloric value of whitebark pine seeds. Although meat is always an option, taking down a bison or a hormone-crazed bull elk is riskier and more exhausting than breaking into a squirrel’s seed cache. And females not only need calories, they need those calories at the right time of year to sustain pregnancies through the winter.
For now, there’s no clear-cut solution to saving the whitebark pine. Scientists talk of pumping beetle-infested trees with pesticides, and propagating seeds from pines that resist blister rust—but that requires enormous amounts of money and manpower. Healthy stands of whitebarks still exist in Wyoming’s Wind River Range, where even higher elevations are likely to guard against a pine beetle outbreak—and if grizzly bears are forced to search for food outside of the park, they may eventually find their way into the Winds. But the bear’s safety, should they wander onto ranchland, is a complex debate—only part of the range enforces strict conservation laws for the de-listed grizzly bear. The rest is largely ranching territory, where grazing sheep could lure the bears into a free-fire zone.
“The Yellowstone Ecosystem is one of the few fully intact, fully functioning systems left in the world,” Stevens says. “We need to put our most creative, innovative thought into saving the whitebark pines—and if that doesn’t happen, then we need to do everything we can to make sure we provide alternatives for the bears. Grizzly bears are the icon of American wilderness. They represent this ecosystem. If they’re not healthy, it’s going to show.”
