Bears butting in on Yellowstone wolf kills

Battle of carnivores ultimately has little effect on population.

By Cory Hatch, Jackson Hole, Wyo.

April 27, 2011

Wolves and bears in Yellowstone National Park squabble over elk carcasses, but the two species have little impact on each other’s overall population, a park biologist said last week. Park wolf biologist Doug Smith outlined research and observation regarding the interaction of the two species in front of a group of bear managers who met at Spring Creek Ranch last week. The Interagency Grizzly Bear Committee and the Yellowstone Ecosystem Subcommittee of federal and state land and wildlife officials provide oversight for grizzly bear management in the ecosystem.

“When you see wolves and bears next to each other, 95 percent of the time there’s something dead that they’re both feeding on,” Smith said. “Typically what happens is wolves kill it, and bears take it.”
“Bears generally will find and take a carcass,” Smith said. “It’s not a matter of if, but when.”

During confrontations between wolves and bears, especially over food, bears in Yellowstone win roughly 80 percent of the time, Smith said. In other places such as Banff National Park in Canada, bears win a carcass about 50 percent of the time. The reason for the discrepancy is unclear, Smith said.
That doesn’t mean wolves give up on what’s often their own hard-earned kill.

Opportunistic grizzlies

“Wolves will harass bears because they’re much quicker,” Smith said. “Bears are more powerful.”
It’s usually male grizzly bears that will claim a carcass from wolves. Researchers have documented up to 12, and perhaps as many as 20, grizzly bears on a single kill, with wolves typically hanging around the periphery as “bystanders,” Smith said.

Grizzlies tend to take advantage of wolf-killed carcasses and other carcasses during poor whitebark pine seed crop years, according to data. Bears are found on wolf kills during August, September and October more often on bad whitebark years than during good years.

More work is needed to discern whether those data are significant, Smith said. Whitebark pine nuts are an important fall grizzly food, and the high-elevation tree is under threat from global warming, beetles and blister rust.

Smith’s report on wolf-grizzly interactions comes as the wolf population has taken a 60 percent plunge in the park’s northern range, echoing a similar decline in the region’s elk herd, Smith said. “We peaked ... and now we’re going down,” he said. “Wolves are adjusting to their food base.”

When wolves were first brought back to Yellowstone starting in 1995, pack sizes were generally large and wolves had plenty to eat. More recently, wolf packs have been documented fighting and even killing each other for the best territories.

Behavior varies with habitat

Smith showed one photo of a wolf that appeared to have starved to death. Back in the early days of the wolf recovery, “it was unheard of to have a wolf that died of starvation,” Smith said. Researchers have found some wolf carcasses have had low fat content in their bone marrow, which can be a sign malnutrition, Smith said. Diseases such as mange and distemper also have impacted the park’s wolf population.

On the other hand, grizzly bears appear to have little impact on the wolf population, and vice versa, Smith said. “There’s no relationship at all,” he said. “These species have coexisted for a long time.” Researchers have documented four grizzly cubs that were killed by wolves, Smith said. Unlike in the Yukon, where grizzlies have been know to dig out wolf dens, in Yellowstone that behavior is thus far unrecorded.
“We’ve never seen a bear dig a den out,” Smith said.

“Wolves have very different behavior around a kill compared to a den,” he said. “They kind of act like a mosquito on the bear. I’ve seen wolves biting bears on the butt ... harassing them away from the den.”
As for the decline in Yellowstone’s northern range elk herd, Smith said the answer is complicated. Drought has likely caused some of the decline, and predators certainly play a role. “When you have this many carnivores, you probably can’t expect to have as many prey as you did in a carnivore-free system,” he said.

Still, this regulation of the elk population by grizzly bears and wolves could be a good thing. Instead of a boom-and-bust cycle, where the elk population increases then declines dramatically in the absence of predators, wolves and grizzlies might cause a smoothing effect on elk fluctuations. “Wolves could be a buffer against climate change because of that smoothing effect,” Smith said.

Source