After a 30-year struggle, grizzlies are multiplying throughout Yellowstone National Park as another top predator — the gray wolf — has helped build the bear population in a surprising way.
The numbers tell the success of grizzly bear restoration: About 650 bears roam the Yellowstone region today — up from roughly 200 when the animal was first protected under the Endangered Species Act in 1975 — and bears have expanded their range by 40%, says Chuck Schwartz, federal scientist and head of the Interagency Grizzly Bear Study Team.
Yet as robust as the recovery has been, new threats could affect the animals in the future. So many grizzlies roam Yellowstone that young bears search for new territory outside the park. Sometimes they kill livestock on surrounding private land, prompting ranchers and their political allies to seek removal of the bear's protected status.
But others say the bears should remain protected because emergent threats to their food supply could undermine the progress of the last three decades.
Grizzlies must put on fat to fuel a winter slumber that lasts about five months. Although bears have broad tastes, the kind of fat required to pull off the big sleep comes from less than a handful of high-protein sources.
Carcasses of winterkill elk and newborn calves provide food in spring. In summer, bears switch to native cutthroat trout spawning in dozens of Yellowstone streams as well as army cutworm moths that migrate en masse from farms to the blooms of the alpine tundra. In fall, whitebark pine nuts provide high-protein, fat-rich seeds.
Yet elk populations are down from a high of 10 years ago due to drought and more predators. Infestations of blister rust and bark beetle threaten whitebark pine nut production, which is erratic in the best of times. Biologists say lake trout that anglers illegally introduced to Yellowstone Lake and whirling disease have reduced cutthroat trout populations.
Enter the gray wolf, an unexpected source of grizzly aid. Since 33 wolves were introduced to the park in 1995 — they number 170 today — bears have developed the habit of stealing their kills. John Varley, director of Yellowstone's Center for Resources, the park's science branch, says wolves provide food for at least 12 species, including bears, bald eagles and some beetles.
"The one thing we totally underestimated was how many other mouths [they] would feed," Varley says.
"In the Pelican Valley [of central Yellowstone], it's not if a bear will take a kill, but when," says Doug Smith, Yellowstone's wolf project director. "Every documented ungulate killed by a wolf pack in the last five years has been taken over by a grizzly."
Smith says that the highest number of grizzlies ever seen on wolf kills — not just in the Pelican Valley, but throughout the park — was at a time when the whitebark pine nut crop failed.
Park wildlife biologist Kerry Gunther agrees that wolf kills may be an important food source, "especially in spring, when there isn't always a lot to eat. We've actually seen bears trailing the wolf packs."
Adult male grizzlies benefit the most. Young bears can't fend off a wolf pack, and sows with cubs are reluctant to approach wolves to protect their young. Even so, wolves don't always consume an entire carcass, and females and cubs sometimes get the leftovers. In the future, Gunther will study whether sows have more young or begin having cubs at a younger age as a result.
The grizzly's uncanny ability to adapt may prove sufficient to overcome changes to diet and environmental conditions. And nothing demonstrates their power of opportunism better than the arrival of the new predator on the block.