The return of wolves to Yellowstone National Park is having an interesting — though not surprising — effect on the larger ecosystem, affecting everything from grizzly bears to elk to berry bushes, according to new research from Oregon State University and Washington State University.
The new research has found that grizzlies are benefiting greatly from the return of the wolf — with the return of the wolf, a historically important part of the grizzly bear diet has returned as well, berries. Berries were previously abundant within Yellowstone but largely disappeared during the last century with the removal of wolves, mostly as a result of elk over-browsing. With the return of wolves, elk numbers have fallen to levels which allow for a far greater abundance of shrubs — benefitting everything from grizzly bears to bees and butterflies.
This new research is some of the first to identify some of the — often very complex — interactions between these large, ecologically important predators. And some of the first to provide clear examples of how the removal or extinction of an important — or keystone — species can have vast reverberations that travel throughout the ecosystem affecting nearly every other species living within it.
In this case — the researchers discovered that the overall “level of berries consumed by Yellowstone grizzlies is significantly higher now that shrubs are starting to recover following the re-introduction of wolves, which have reduced over-browsing by elk herds. The berry bushes also produce flowers of value to pollinators like butterflies, insects and hummingbirds; food for other small and large mammals; and special benefits to birds.”
The researchers noted in the paper detailing their findings that “berries may be sufficiently important to grizzly bear diet and health that they could be considered in legal disputes — as is white pine nut availability now — about whether or not to change the ‘threatened’ status of grizzly bears under the Endangered Species Act.”
“Wild fruit is typically an important part of grizzly bear diet, especially in late summer when they are trying to gain weight as rapidly as possible before winter hibernation,” stated William Ripple, a professor in the OSU Department of Forest Ecosystems and Society, and primary author of the new research paper. “Berries are one part of a diverse food source that aids bear survival and reproduction, and at certain times of the year can be more than half their diet in many places in North America.”
Back when wolves were first removed from Yellowstone back early in the early 1900s, “increased browsing by elk herds caused the demise of young aspen and willow trees — a favorite food — along with many berry-producing shrubs and tall, herbaceous plants. The recovery of those trees and other food sources since the re-introduction of wolves in the 1990s has had a profound impact on the Yellowstone ecosystem, even though it’s still in the very early stages.”
“Studies like this also point to the need for an ecologically effective number of wolves,” stated co-author Robert Beschta, an OSU professor emeritus. “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 over-browsing, allow tree and shrub recovery, and restore ecosystem health.”
Oregon State University provides more:
As wolves help reduce elk numbers in Yellowstone and allow tree and shrub recovery, researchers said, this improves the diet and health of grizzly bears. In turn, a healthy grizzly bear population provides a second avenue of control on wild ungulates, especially on newborns in the spring time.
Yellowstone has a wide variety of nutritious berries — serviceberry, chokecherry, buffaloberry, twinberry, huckleberry and others — that are highly palatable to bears. These shrubs are also eaten by elk and thus likely declined as elk populations grew over time. With the return of wolves, the new study found the percentage of fruit in grizzly bear scat in recent years almost doubled during August.
Because the abundant elk have been an important food for Yellowstone grizzly bears for the past half-century, the increased supply of berries may help offset the reduced availability of elk in the bears’ diet in recent years. More research is needed regarding the effects of wolves on plants and animals consumed by grizzly bears.
There is precedent for high levels of ungulate herbivory causing problems for grizzly bears, who are omnivores that eat both plants and animals. Before going extinct in the American Southwest by the early 1900s, grizzly bear diets shifted toward livestock depredation, the report noted, because of lack of plant-based food caused by livestock overgrazing. And, in the absence of wolves, black bears went extinct on Anticosti Island in Canada after over-browsing of berry shrubs by introduced while-tailed deer.
It’s also likely that an increased abundance of berries will help to provide a buffer against other changes in the ecosystem, providing an alternate means of nutrition for the bears if other food sources diminish, As an example, whitebark pine nut production — an important source of nutrition for bears — is predicted to diminish notably in the near-future as a result of climate change. It’s been noted previously that grizzly bear survival rates declined considerably during years of low nut production.
“Livestock grazing in grizzly bear habitat adjacent to the national park, and bison herbivory in the park, likely also contribute to high foraging pressure on shrubs and forbs, according to the new report. In addition to eliminating wolf-livestock conflicts, retiring livestock allotments in the grizzly bear recovery zone adjacent to Yellowstone could benefit bears through increases in plant foods.”
The new research was just published in the Journal of Animal Ecology.