National Geographic : 2016 May
100 national geographic • may 2016 These commercial boats have been working a long season each summer since 2011, and the cutthroat have begun showing modest signs of recovery. But the lake trout may never be extirpated, and so the suppression effort can probably never end. “If it fails,” Bigelow said, “it’ll be because we didn’t try hard enough.” The loss of whitebark pine nuts from the grizzly bear menu is a more complicated concern, lying further beyond human fixing. These fat-rich little morsels—they’re called nuts but are really pine seeds—come to ripeness within cones on the trees, which inhabit the Yellowstone ecosystem only at high elevations, above 8,500 feet. Whitebark pines grow slowly, reaching cone-bearing maturity after 50 years. Grizzly bears get the nuts mainly by raiding cone middens of red squirrels, which store them for winter. Clark’s nutcrackers, gray-and-black birds in the crow family, also harvest the nuts and hoard them buried in little caches of several seeds each, and most new whitebark pines grow from nutcracker caches that have gone unrecovered by the birds. The greatest enemy of the whitebark pine is the mountain pine beetle, a tiny bullet-shaped insect that burrows tunnels in a tree’s living tissue, which can interrupt nutrient flow and kill the tree. Whitebark forests have always suffered episodic attacks by mountain pine beetles, but in recent years the beetle kill has gotten much worse, probably because of climate change. Severely cold weather, especially deep cold snaps that occur early in winter or late in spring, can knock down the beetle population. But that sort of weather is now rare, and since 2003 a vast beetle outbreak in the Yellowstone ecosystem has resulted in unprecedented killing of whitebark pines. “The whole defensive strategy of the whitebark is escape,” said Jesse Logan, a forest entomologist, as we stood amid a grove of white- barks in the Absaroka Range. “It’s a hell of a survivor. But it’s not much of a competitor.” At high elevations, in cold and harsh condi- tions, it largely escapes competition from other conifers—ponderosa pine, lodgepole, Douglas fir. But it doesn’t escape the beetle, not anymore. Logan’s aerial surveys, done with William Macfarlane, a colleague from Utah State University, suggest that almost half the whitebark distribution in the Greater Yellowstone Ecosystem has now suffered severe mortality from the mountain pine beetle, and 82 percent has suffered at least moderate die-offs. Logan pulled some bark off a dead tree to show me the beetles’ tunnels, vertical and crisscrossing, like a subway map of New York City. “My sense is that the loss of this food resource is really important to grizzlies,” he said. He’s not a bear biol- ogist, and others disagree, but the issue is serious. Army cutworm moths are more unexpected, more fortuitous in the grizzly bear’s diet, because unlike whitebark nuts and cutthroat trout, they come from elsewhere. These little gray creatures migrate hundreds of miles in early summer from lowland farming areas—on the Great Plains and in the intermountain West, where in the lar- val stage they’re crop pests—to high elevations in the Absarokas and Bear Fare Grizzlies are omnivores, feeding on more than 260 species of plants, animals, and fungi. Depending on location and season, their diet may include berries, grasses, thistles, fireweed, dandelions, and ants as well as trash from humans. Their high- est quality meals consist of army cutworm moths, whitebark pine nuts, Yellowstone cutthroat trout, small mammals, and the carcasses of grazers such as elk and bison.