Reintroduction of Wolves to Yellowstone Led to Unexpected Ecological Response

Twenty years ago, an ecological experiment started at Yellowstone National Park. Wolves, which had been hunted to extinction in the park, were reintroduced. In January 1995, eight grey wolves from Jasper National Park in Alberta were dropped off at Yellowstone. From there the population boomed—in 2014 there were 11 packs living in the park, with just over 100 individuals.

The hunting of Yellowstone wolves began shortly after the Hayden Geological Survey of Yellowstone in 1871. Over the next 50 years, populations were decimated and by the 1970s, a scientific survey showed no evidence of a wolf population in Yellowstone. With no major predators, the elk population exploded across the National Park and the vegetation was damaged through overgrazing. Experts began to realise the important role wolves played in the park and biologists began considering how to reintroduce the species.

Mark Boyce, an ecologist at the University of Alberta, has spent the last four decades studying Yellowstone and its wolves. He has now released a study showing the changes that have taken place as a result of the wolf reintroduction. The findings, published in the Journal of Mammalogy, show the park has benefited from their presence in unexpected ways, creating a more complex and thriving ecosystem.

yellowstone wolf A wolf in Yellowstone national park. iStock

As expected, his findings showed reintroducing wolves had a huge impact on the park’s ecosystems. This specifically relates to the trophic cascades—where a predator (in this case a wolf) is introduced to a food web and controls the population of its prey (ungulates, like elk), allowing the web’s next lower level (vegetation) to thrive.

“One of the unique results was how the trophic cascade created huge spatial variation in the recovery of vegetation, thereby increasing the spatial heterogeneity of the park,” Boyce told Newsweek. “This is profoundly important for biodiversity. This result was entirely unexpected.”

Boyce also found bison had overtaken elk as the dominant herbivore on Yellowstone's Northern Range, and that grizzly bears had an increasing role on elk calf mortality. In terms of vegetation, populations of willow, cottonwood and aspen trees increased across the park.

yellowstone wolves Wolves being introduced to Yellowstone in January 1995. OOL/AFP/Getty Images

He said the “unexpected” ecological consequences seen at Yellowstone following the wolf reintroduction shows how a hands-off approach by humans can lead to greater diversity than expected. “The National Park Service has adopted ecological process management, minimizing interference when possible, allowing predation, herbivory, fire and flooding to shape the ecosystem, and this policy allowed wolf recovery with the fascinating dynamics that have emerged,” the study said.

Boyce said: “I believe that national parks and other large protected areas are crucially important as ecological baseline reserves where we can document what we're doing to the rest of the planet.  We have few places large enough to capture the full complement of native species, and it has been astounding to learn what happens under an ecological-process management policy.

Yellowstone bison Bison in Yellowstone National Park. iStock

“There are abundant examples of places where species have been eliminated to foster agriculture, and there are opportunities to restore these populations. For example, The Clinton administration authorized that grizzly bears should be released in the Selway-Bitterroot Wilderness of Idaho but funding has never been allocated to implement the recovery plan.  

"Bison are gone from most of the Great Plains and there are many places with marginal agriculture where we could implement the ‘bison commons.’ Conflicts with agriculture are serious limitations to such plans, but with creative solutions we are capable of coexistence in many cases.”

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