Yellowstone wolves

A wolf leaps out of its crate and into the Rose Creek pen in Yellowstone in January 1996 as part of Yellowstone’s reintroduction program. New research from the University of Wyoming suggests that the popular idea of wolves creating a "trophic cascade" since reintroduction has been too simplified.

Perhaps the most-told ecological success story about the Greater Yellowstone Ecosystem is bringing back wolves, which sparked a trickle-down effect on the web of life around them that researchers are still debating.

The premise of wolf reintroduction’s effects, known as a “trophic cascade,” is that the return of a large canines cut into numbers of herbivore populations like elk, and in turn influenced vegetation like willows and the smaller species that depends on the shrubbery, like songbirds. But some scientists who have kept an eye on Yellowstone since wolves were brought back 24 years ago say the narrative has been too simplified, and charge that a new University of Wyoming research supports.

“Conventional wisdom in ecology is that trophic cascades are reversable,” University of Wyoming PhD ecology candidate Jesse Alston told the News&Guide. “So when you reintroduce a predator to an ecosystem where they existed, it’s going to reverse back to a historic state.”

Because of some “theoretical reasons,” Alston and some lab mates suspected that the relationship wasn’t so neat. Their hunch was that factors like the effects of climate change, incomplete historical records and smaller “mesopredators” filling vacant ecological niches weren’t being fully accounted for. To test the hypothesis, they mined over 1,800 academic studies, selecting 20 to analyze that assessed the effects of returning a native apex carnivore to a landscape, or removing an invasive apex predator.

The results confirmed their suspicion that some of the conventional conceptions of trophic cascades are misguided. Alston’s research was peer reviewed and published in the latest edition of the journal Biological Conservation.

“Professionals in our field,” Alston and his coauthors wrote, “might consider the conservation importance of large carnivore reintroduction (which few ecologists would question) as distinct from the cascading impacts of large carnivore reintroduction, which can be significant, muted or virtually non-existent.”

Yale University and the University of British Columbia-Okanagan researchers collaborated on the study, titled “Reciprocity in restoration ecology: When might large carnivore reintroduction restore ecosystems.”

“Removing or reintroducing apex predators from ecosystems does not appear to have predictable restorative effects,” the researchers concluded, “but solid confirmation will require additional rigorous tests of this assumption of reciprocity.”

The study suggests that the existence of a trophic cascade hinges largely on how it defined. Other smaller, carnivores considered within the 20 studies did almost invariably respond.

“When you take wolves back into an ecosystem, you get fewer coyotes,” Alston said. “The reason why is that these mesopredators perform a lot of the same functions in an ecosystem that the larger predators did.”

But the effects large carnivores’ presence had on the biomass of vegetation was unpredictable, he said. In Yellowstone, for instance, wolves reduced elk numbers, but the link to how riparian vegetation and beavers responded was muddied by drought. In other cases it was another shifting environmental condition implicated in not being able to prove a predator-prey-vegetation linkage.

Alston stressed that his conclusions are not definitive.

“We’re not claiming that predators never restore ecosystems,” Alston said. “Sometimes you get the result that you’re looking for, and sometimes you don’t.

“What we think the data shows right now,” he said, “is that we don’t have enough rigorous studies to show that it’s predictable.”

A PDF of Alston’s academic study is attached to the online version of this story at

Contact Mike Koshmrl at 732-7067 or

Mike has reported on the Greater Yellowstone Ecosystem's wildlife, wildlands and the agencies that manage them for 7 years. A native Minnesotan, he arrived in the West to study environmental journalism at the University of Colorado.

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