Field of Science


Why Yellowstone's Grizzlies Should Be Grateful for Wolves

There's only one time a giant domino chain isn't fun: when you're a domino. Humans are great knockers-down of ecosystem domino chains, and sometimes we don't even know which species we've felled until we start propping things back up. When we knocked every last wolf out of Yellowstone National Park, for example, we didn't know how we were hitting bears at the other end of the chain.

When Yellowstone was first created, visitors were free to kill the animals. Then in the early 20th century, government "predator control" efforts gradually finished the job of wiping out the park's wolves. The last ones were killed in 1926. Wolves remained absent from Yellowstone until the mid-1990s, when officials lugged pens of them into the park and set them free.

William Ripple, an ecologist at Oregon State University, and his colleagues investigated how the return of wolves affected grizzly bears in Yellowstone. They compared data from old studies of bear scat to data collected after the wolves came back. Specifically, they wanted to know how many berries bears had been eating.

What do wolves have to do with berries? Nothing. But wolves kill elk—and elk eat all kinds of plants, including berry shrubs that bears would otherwise dine on.

For a few decades after wolves were wiped out of Yellowstone, the booming elk population was culled to keep it under control. But the elk culling ended in the 1960s. Ripple found that in the late 1960s, while the elk population was still low, bears had a relatively high percentage of fruit in their dung. Over the next 20 years, as the elk population more than tripled, the grizzlies' berry diet dropped to almost nothing. And in the late 2000s, after wolves had reestablished themselves in the park and elk numbers had dropped, a hearty helping of berries returned to bear diets.

The researchers also looked at berries themselves, choosing a representative plant called serviceberry. They saw that plants in fenced-off areas (where elk and other grazers can't reach them) had been growing for decades. Outside the fences, though, the berry plants were young—they had all sprung up after wolf reintroduction. The evidence all pointed the same way: more wolves means fewer elk, which means more berries, which means better-fed bears.

Berries are good for a grizzly's diet, the authors explain, because bears use them to fatten up before entering hibernation. Female bears will give birth during their winter doze, so having enough energy stored is crucial to the population's survival.

Ripple says there's not yet any evidence that having more berries in their diets is actually helping bears—his study didn't ask that question. But he is optimistic about the future of grizzlies in Yellowstone. "It is good to see that the bears can at times get significant calories from...the berries," he says. Berries made up as much as 39% of female grizzlies' diets in recent years.

Other studies have shown that beaver and bison numbers both increased after wolves came back to the park. This might be, the authors write, because herbivores have less competition now from the once-ubiquitous elk. All the species that were knocked down while the wolves were gone can now stand back up and give them a thank-you.

Images: Grizzly bear and wolf at Yellowstone from YellowstoneNPS (via Flickr); diagram from Ripple et al.; bear hug card from OldEnglishCo on Etsy (available for $4.71!).

William J. Ripple, Robert L. Beschta, Jennifer K. Fortin, & Charles T. Robbins (2013). Trophic cascades from wolves to grizzly bears in Yellowstone Journal of Animal Ecology DOI: 10.1111/1365-2656.12123

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