For grizzly bears and wolves in Yellowstone, competing for food has unexpected results
Sharing a meal with a greedy tablemate makes for fraught relations, especially when there are claws and fangs involved.
For wolves and grizzly bears in Yellowstone National Park, competition over a dead elk dinner has some unexpected results. When a bear steals a meal from a wolf pack, the wolves kill less often.
"What we did was break down the wolf foraging sequence," University of Montana researcher Matthew Metz told the Missoulian. "We studied their searching time and their handling time - the amount of time they spend eating and digesting their kills."
And especially in spring, when Yellowstone's elk herds are birthing thousands of wobbly calves throughout the Lamar Valley and every predator around cruises for fresh protein, grizzly bears lean on wolves in ways that change the wolves' behavior. The reduction in wolf kills when grizzlies are around was also seen in a paired study of Scandinavian predators.
"Wolves put extra food on the landscape for bears," said Norwegian Institute for Nature Research scientist Aimee Tallian, who collaborated with Metz through UM's Yellowstone Wolf Project. "But bears are antagonistic for wolves. They take part of the supply of shared prey, and they usurp wolf kills. So wolves are kind of helping bears, but bears aren't necessarily helping wolves a lot."
The Greater Yellowstone Ecosystem surrounding Yellowstone National Park has the second-largest concentration of grizzlies in the Lower 48 states. Its estimated 1,000 grizzlies pack themselves between five and 13 per 100 square kilometers.
By comparison, Scandinavia has about 3,000 brown bears (the same species as U.S. grizzly bears). But they are dispersed at about three bears per 100 square kilometers across Norway and Sweden. And they feed on moose, which don't herd like elk but do exist in similar numbers in the Scandinavian forests.
"They're controversial there, too," Tallian said of bear relations on the far side of the Atlantic Ocean.
Scandinavia also has a sizable wolf population, which inhabits regions with and without brown bears. That added complexity to the study, because the researchers could contrast wolf behavior with and without ursine competition.
"The fact we're seeing patterns in multiple places suggests these patterns exist where wolves and bears persist together," Tallian said. "Which is a lot of their habitat."
Comparing American and Scandinavian meat-eaters has other complications. In Yellowstone, the wolves and grizzlies also have to compete with black bears, which don't exist in Europe.
While grizzly competition tends to reduce wolf kill rates, it has an opposite effect on mountain lions. Metz said lions in bear country also get driven off their kills by grizzlies, and then have to kill more to stay nourished.
The relationship can change from place to place. Along Alaska salmon streams, for example, Kodiak brown bears tend to eat only the heads of spawning fish, leaving a bounty of body meat for following wolves.
Exactly why and how these inter-species kill rates balance out remains to be discovered. But Tallian and Metz agreed the opportunity to study several top predators at work on the same landscape was enlightening.
"Relatively little had been known about how bears affected the foraging dynamics of wolves," Metz said. "Our work starts to fill in the gap by demonstrating that the dynamics do differ and provides another reminder of how changes in ecosystem complexity - in this case the presence of bears - affects the behavior of other species."
"One cool way to think about this is the historical context," Tallian added. "This is likely what these systems looked like for thousands of years, with this dynamic interplay of competitors on the landscape. There are very few places that feel that untouched these days, where ecosystems are allowed to play out on their own."