Over the last three decades, Yellowstone National Park has undergone an ecological cascade. As elk numbers fell, aspen and willow trees thrived. This, in turn, allowed beaver numbers to increase, creating new habitats for fish and birds.
The shift has largely been attributed to the reintroduction of wolves to the park — as predators, they helped control the elk numbers. But their return may not have reshaped the entire ecosystem in the way that scientists thought, and has sparked a fierce debate among scientists over exactly why and how Yellowstone has rebounded.
According to a study published in January, the reintroduction of gray wolves (Canis lupus) in the 1990s created a trophic cascade — a chain reaction in the food web — that benefitted the entire ecosystem. The study linked wolves in the area to a reduction in the elk population, which in turn reduced browsing and allowed willow trees to grow. Between 2001 and 2020, this led to a 1,500% increase in crown volume, the total space filled by upper branches of the willows.
But now, scientists have written a response letter to the editor, published in Oct. 13 in the journal Global Ecology and Conservation, in which they argue that the original study's methodology was flawed, and that Yellowstone wolves' effect on willow shrubs is not so clear.
Large predators were targeted in Yellowstone from the end of the 1800s. By the 1920s, wolves were largely extinct from the park. Their disappearance created an ecological imbalance — the elk population exploded, which decimated plant populations and in turn threatened beavers, among other impacts. This is known as a trophic cascade, where the removal of one species causes ripples throughout the food web.
While the reintroduction of wolves to Yellowstone has led to changes within the park, the authors of the response letter claim the original study reinterpreted existing data to fit an oversimplified story.
The study converted willow height measurements collected and published by another research group into a metric called crown volume, response author Daniel MacNulty, a wildlife ecologist at Utah State University, told Live Science in an email. Crown volume was used as a proxy for willow size, meant to capture the shrub’s entire three-dimensional growth more than simply measuring its height.
"Because crown volume was built directly from height, [the study] only showed that height predicts height," MacNulty said. "They did not reveal anything new about how willow growth changed after wolf reintroduction."
The response letter suggests other inconsistencies in data analysis, like comparing willow measurements from different locations across years. This is problematic because it shows a misleading time series of willow growth, and MacNulty's research group has previously published research noting sampling biases in other studies supporting this same trophic cascade theory.
"There is substantial scientific evidence of a definitive effect of wolf recovery on the rest of the Yellowstone ecosystem," MacNulty said, like wolves increasing the supply of carrion to bears, coyotes, eagles and other meat-eating species. But the effect of wolves on vegetation is less clear because it operates through the decline of elk populations, which wolves were likely not solely responsible for. As MacNulty points out, humans, grizzly bears and cougars also hunt elk. "A major problem with the simple trophic cascade story is that it ignores the role of these other predators."
William Ripple, an Oregon State University wildlife ecologist and author of the original paper, stands by the original conclusions of the paper, maintaining that a large carnivore, elk, and willow trophic cascade occurred in Yellowstone. "Our methods are sound, the modeling approach is standard," Ripple told Live Science in an email. "So we reject the idea that there are fatal flaws."
The debate about Yellowstone wolves and the impact of their reintroduction goes beyond this study and the latest response. While scientists widely agree that there is a trophic cascade in Yellowstone, its strength — and which predators are most responsible for it — form the center of the disagreement, MacNulty said.
Some scientists argue the story is more complex. "There are reasons other than trophic cascades by which carnivores and plants can be positively associated," Jake Goheen, a wildlife ecologist at Iowa State University told Live Science in an email. Goheen, who was not involved in the research or response, said he doesn't believe that the authors of the original study provided enough evidence to support their conclusion that reintroducing wolves in Yellowstone caused a strong trophic cascade that affected willows.
"There is a growing body of literature at this point that has scrutinized the hypothesized cascade in Yellowstone," Goheen said. He adds that this does not mean there's no wolf-to-elk-to-willow trophic cascade in Yellowstone, only that the evidence presented so far is not clear enough.
To establish a clear trophic cascade from Yellowstone wolf reintroduction to willows, researchers would need to account for other predators and herbivores, said MacNulty. The ideal study would then analyze how much more total willow biomass there is now compared with before wolf introduction, to identify the strength of the effect; then calculate how much of that increase can be attributed solely to wolves, to identify its cause.
Ripple and his research team are now preparing a detailed reply, which explains that criticisms of the original study come from misunderstandings of what they did, Ripple said. "The basic scientific logic of the paper is solid," Ripple said.
Conservation priorities might be fueling the controversy over large carnivores' beneficial effects on ecosystems, said Goheen, adding that even if wolves are not definitively causing a trophic cascade to willows, they are still important to conserve.
