November 25, 2007
FeatureFear and Recovery in Yellowstone
A gray wolf looks out from under a snow-covered pine tree.
Aspen trees are in severe decline throughout Yellowstone National Park and other areas of the Rocky Mountains. However, in a strange twist, the fate of the aspen trees in Yellowstone seems to be tied to the presence of the much-maligned gray wolf. After gray wolves were reintroduced to Yellowstone, the aspen trees started to make a comeback.
Aspen trees are symbolic of the mountainous western United States. These tall trees, easily identified by their white bark and leaves that quiver in the wind, are found in a landscape dominated by coniferous trees. Aspen groves harbor a great diversity of life: all manner of animals and plants live among the trees. But aspen groves are in trouble. The presence of aspen trees has declined by 50 to 90 percent in some areas of Yellowstone National Park and the Rocky Mountains.
A degraded aspen stand in the Lamar Valley, northern Yellowstone National Park.
Ecological Link Among Wolves, Elk, and Aspen
Enter the wolf. In 1995, gray wolves were reintroduced to Yellowstone. After the wolves integrated back into the park ecosystem, researchers noticed that the number of new aspen trees started to rise. Why would tree populations increase with the rise in wolf numbers? The answer lies in what the wolves eat: elk. There appeared to be an ecological link between wolves, elk, and aspen.
William J. Ripple and his colleagues at Oregon State University started to gather data about the interaction between the wolves, the elk, and the aspen trees. Their evidence proves that the wolves’ presence does affect the number of new aspens growing in northern Yellowstone, but in a way that goes beyond a basic predator/prey relationship.
A Cascade of Destruction
Before gray wolves were hunted to extinction in Yellowstone, aspens were booming. Aspen groves had been increasing for a period of 170 years. The loss of the wolves set off a “trophic cascade” that affected many levels of the ecosystem.
Elk now could forage in places where they had once encountered gray wolves. The elk started browsing on young aspens, so much so that new trees could not grow to maturity. The rate of new aspen growth plummeted. Other animals that relied on the trees, such as birds and beavers, also lost key resources. Stream channels eroded due to the loss of the trees that stabilized the banks.
A Desire to Eat Balanced by Fear
Once gray wolves were reintroduced, they started to prey on elk. The elk in turn avoided feeding areas where they could become an easy meal. According to the researchers, prey species are well-aware of their surroundings and will alter their foraging behavior in order to ensure that their presence is less detectable to predators. In order to stay safe, prey species spend less time foraging in high-risk locations. Sites around streams are very high-risk areas, and these are precisely the areas that have witnessed increased aspen growth.
A reintroduced male wolf explores it new territory in northern Yellowstone National Park.
Ripple calls the effect between the wolves, the elk, and the aspen trees “the ecology of fear.” The elk have changed their behavior in the face of a formidable predator and the aspens are reaping the benefits. “We think these elk need to balance the risk of being killed versus eating in their favorite places. So it’s a trade-off between food and risk in an ecology of fear,” he said.
Accumulating Evidence — and Aspens
While elk populations have decreased in Yellowstone since the reintroduction of gray wolves, the populations are not at their lowest. The number of kills that wolves make cannot by itself explain the return of the aspens. Ripple and his colleagues have documented a continued increase in the number of new aspen trees in northern Yellowstone over the past ten years, regardless of local conditions in a given year. These studies and other supporting evidence points to fear as a major factor in aspen recovery.
“The issue of aspen decline in the American West is huge, and their recovery will depend on local conditions and issues in many areas,” Ripple said. “In northern Yellowstone, we finally have some good news to report. It’s just a start, but it’s a pretty good start.”
Comments
Comment from: Susan
January 8, 2008 09:37 AM [#]
My husband and I were in Yellowstone right after the wolves were introduced back into the area which was also a few years after the big fire.
The rangers believe that the fire also played an important role in the return of the Aspen. I don't remember the whole explanation but the rangers said the high heat of the fire allowed for the release of the seeds and new Aspen trees were starting to germinate.
Comment from: Jacob
September 23, 2011 06:42 AM [#]
Wow this was amazing I love wolves it's nice to know that they also helped the environment at Yellowstone.