The reintroduction of wolves to Yellowstone National Park, starting in 1995, has had some good effects on the food web there: The wolves keep the elk population in check, and with fewer elks to graze on the vegetation, plants including aspen and willows have rebounded. There’s further evidence that this greater availability of plants has been a boon to the beaver and bison populations in the park.
Now comes a study, just published in the Journal of Animal Ecology, that shows that grizzly bears have also gotten a boost to their diet from the wolves’ return.
Yellowstone grizzly bears are a bit different than grizzlies elsewhere — they don’t eat many berries. Studies of the bears that were conducted in the late 20th century found that Yellowstone grizzlies had some of the lowest berry consumption in the interior of North America. That was a bit weird because from July to October, female grizzlies usually gorge on the fleshy fruits. Berry calories can be easily converted to and stored as fat, perfect for bulking up for the long hibernation as well as the pregnancy and birth that usually take place during winter.
“Wild fruit is typically an important part of grizzly bear diet,” William Ripple, an ecologist at Oregon State University and coauthor on the new study, said in a statement. “At certain times of the year [berries] can be more than half their diet in many places in North America.”
Ripple and his colleagues have been studying how the reintroduction of wolves to Yellowstone has affected the Yellowstone food web, and they thought that the berry situation for the Yellowstone grizzlies might have changed with wolves’ return. The issue is more than just a note of academic interest. After the wolf population reached sufficient numbers that the government wasn’t worried they’d die out, the wolves were removed from the endangered species list in 2011, letting hunting of them start up again. If it turns out that the wolves have an impact on other endangered critters, such as grizzly bears, conservationists might have a better case for stopping the hunt.
Since asking grizzly bears to complete a survey of their diet isn’t exactly practical, the researchers analyzed bear poop, comparing scat collected from 2007 to 2009 with data from 1968 to 1987. That latter time was one when elk were increasing in numbers (they went from 3,000 in 1968 to 19,000 in 1994) and bears weren’t doing so well. The bears lost their easy meals in 1971 when the park closed all of its garbage dumps. That led to a lot more bear-human conflicts, and many bears were removed or killed. By 1975 the grizzlies were listed as threatened.
The team found that the percentage of fruit in bear scat declined from 1968 to 1987, as the elk were increasing. But after the wolves had come back into the picture, the bears began eating a lot more berries. In August, for example, the male diet was as much as 29 percent fruit and a female’s up to 39 percent.
What’s happening is that with fewer elk to munch on the park’s vegetation, the berry plants can thrive and bears can eat their fill. It’s likely that other critters, including birds and rodents, are benefiting from the recovering fruit harvest as well, the researchers say.
“Studies like this also point to the need for an ecologically effective number of wolves,” study coauthor Robert Beschta, of Oregon State University, said in a statement. “As we learn more about the cascading effects they have on ecosystems, the issue may be more than having just enough individual wolves so they can survive as a species. In some situations, we may wish to consider the numbers necessary to help control overbrowsing, allow tree and shrub recovery, and restore ecosystem health.”
Photo courtesy of Yellowstone National Park
