DENVER — The return of wolves to Yellowstone National Park long has been credited with replenishing its wetlands and spurring the beneficial rerouting of streams — an ecological boon for the crown jewel of the National Park System.
But a new study by Colorado State University researchers casts more doubt on that popular and alluringly simple restoration narrative, one cited widely by many of the groups that advocated for the recent reintroduction of wolves to Colorado.
Researchers spent 20 years studying the effects from the return of large carnivores to Yellowstone. They found that returning predators like wolves to landscapes does not immediately revert the ecosystem to how it was before they were gone.
The wolves’ return can benefit the environment generally — by increasing food for scavengers or culling sick and weak elk, for example. But exterminating them irrevocably changed their former habitat, the study says.
“It’s very important that the main conservation message is that loss of predators from ecosystems can have harmful effects,” said lead author Tom Hobbs, a professor emeritus with Colorado State University’s Department of Ecosystem Science and Sustainability and the Natural Resource Ecology Laboratory. “The best preservation strategy is not to lose them in the first place. Simply putting them back will not lead to a reversal.”
‘Enduring consequences’
The study, made public last week, is part of a body of research examining how the reintroduction of wolves impacted Yellowstone’s ecology. The study examined the idea of a trophic cascade — or how the introduction of an apex carnivore at the top of a food chain affects the rest of the system.
“It’s pretty well known that if you take apex predators out of food webs, there are enduring consequences for the ecosystems,” Hobbs said. “But what’s not known is what happens when you put predators back, when they’ve been gone for a long time. That’s a really important ecological question.”
When mountain lions and wolves were eradicated from Yellowstone National Park in the 1920s, the elk herds swelled and ate away stands of tall willows on streambanks in the park’s northern range. When the tall willows went, so did the beavers.
“Without the willows, you can’t have beavers,” Hobbs said. “And without the beavers, you can’t have willows.”
Without the beavers’ dams, streams sped up and altered the floodplain. The water table dropped, and the moist soil — needed for willows to grow — receded, speeding the decline of the willows. A landscape once dominated by small streams that were flanked by willows and aspens turned into grasslands.
In the 1990s, federal wildlife officials reintroduced wolves to the park. At the same time, the population of cougars and grizzly bears naturally swelled. The number of elk dropped precipitously in the mid-1990s.
The popular but incorrect narrative posits that the wolves caused the elk population to drop, which allowed willows to grow again and the ecosystem to revert to its prior condition.
But that’s not what the researchers found.
Hobbs and David Cooper, a research scientist emeritus in the Department of Forest and Rangeland Stewardship, created study plots in Yellowstone’s northern range that they observed over 20 years. Some plots were fenced in to keep the elk from browsing, and they included manmade beaver dams — meant to replicate the ecosystem before the eradication of carnivores.
Others were left unfenced and undammed.
If the ecosystem reverted back, willow growth should have been the same in both types of plots. But the willows in plots without fences and dams were three times shorter than those with both structures. The landscape in the study did not revert to its prior state.
Instead, the researchers concluded that the ecosystem had undergone possibly permanent change nearly 30 years after the removal of wolves. Grasslands remained grasslands. Beavers did not return.
The popular idea that the return of wolves singlehandedly reduced elk herds and restored Yellowstone’s natural systems was “wishful thinking” backed by little comprehensive science, Hobbs and Cooper said.
That narrative also ignored other factors at play. Human hunting, not wolves, was the primary cause of declining elk populations in the first 10 years after reintroduction, they said. Also, mountain lions hunt elk more effectively than wolves do, and their population boomed simultaneously.
“Claims of ecosystem restoration, resulting from a trophic cascade following the restoration of the gray wolf to Yellowstone, have been used to justify translocation of wolves to their unoccupied former range in many areas of the world,” the study states. “Careful scrutiny has revealed these claims to be exaggerated or false.”
In December, Colorado Parks and Wildlife released the first 10 of an expected 50 wolves in the state’s voter-mandated reintroduction effort.
Some beneficial effects were observed in Yellowstone, but Cooper said Colorado’s reintroduction likely would have a different impact. That’s because Colorado’s elk herds are not overgrazing the landscape and are managed through regulated hunting. The rivers and streams in Colorado also are in better health than those in Yellowstone were at the time of reintroduction nearly 30 years ago.
