Over the past 30 years, Yellowstone National Park has experienced an ecological cascade. As the elk population declined, aspen and willow trees flourished. This has increased beaver numbers and created new habitat for fish and birds.
This change was largely due to the reintroduction of wolves into the park, which as predators helped keep elk numbers in check. But their return may not have reshaped the entire ecosystem as scientists thought, sparking intense debate among scientists about why and how Yellowstone returned.
The reintroduction of gray wolves (Canis lupus) in the 1990s created a trophic cascade that benefited the entire ecosystem, according to a study published in January. The study linked wolves in the area to a decline in the elk population, which in turn is believed to have reduced the animal’s population and allowed willow trees to grow. Between 2001 and 2020, this led to a 1,500% increase in canopy volume, or the total space filled by willow upper branches.
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But now the scientists have written a reply to the editor, published Oct. 13 in the journal Global Ecology and Conservation, in which they argue that the original study’s methodology was flawed and that the impact of Yellowstone’s wolves on willow shrubs is less clear.
Large predators have been targeted in Yellowstone since the late 1800s. By the 1920s, wolves were nearly extinct from the park. Their disappearance caused an ecological imbalance. Moose populations exploded, plant populations decimated, and beavers threatened, among other impacts. This is known as a trophic cascade, where the removal of one species causes ripples throughout the food web.
Although the reintroduction of wolves to Yellowstone brought about changes in the park, the response authors argue that 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 canopy volume, responding author Daniel McNulty, a wildlife ecologist at Utah State University, told Live Science in an email. Canopy volume was used as a proxy for willow size and was intended to capture the entire three-dimensional growth of the shrub, rather than simply measuring height.
“The crown volume is built directly from the height, so [the study] “The study did not reveal anything new about how willow growth changed after wolf reintroduction, only that height predicted height,” McNulty said.
The response letter also suggests other inconsistencies in the data analysis, such as comparing willow measurements from different locations across years. This is problematic because it presents a misleading time series of willow growth, and McNulty’s research group has previously published research pointing to sampling bias in other studies supporting this same trophic cascade theory.
Just as wolves increase the supply of carrion to bears, coyotes, eagles and other carnivorous species, “there is substantial scientific evidence that wolf recovery will have a critical impact on other ecosystems in Yellowstone,” McNulty said. However, the impact of wolves on vegetation is less clear, as it is caused by declining moose populations, but wolves are probably not the only culprit. As McNulty points out, humans, grizzly bears and cougars also hunt elk. “A big problem with the simple trophic cascade story is that it ignores the role of other predators.”
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William Ripple, a wildlife ecologist at Oregon State University and author of the original paper, supports the paper’s original conclusions, arguing that a large-scale carnivore-elk-willow trophic cascade occurred in Yellowstone. “Our methodology is sound and our modeling approach is standard,” Ripple told Live Science via email. “We therefore reject the idea that it is fatally flawed.”
The debate about Yellowstone wolves and the impacts of their reintroduction extends beyond this study and the most recent response. Scientists broadly agree that there is a trophic cascade in Yellowstone, but disagreements center on its strength and which predators are most responsible, McNulty said.
Some scientists argue that the story is more complicated. “There are reasons other than trophic cascades for the positive association between carnivores and plants,” Jake Goheen, a wildlife ecologist at Iowa State University, told Live Science via email. Goheen, who was not involved in the study or response, said he did not believe the original study’s authors provided enough evidence to support the conclusion that wolf reintroduction to Yellowstone triggered a strong trophic cascade affecting willows.
“At this point, there is a growing body of literature examining a hypothetical cascade in Yellowstone,” Goheen said. He added that this doesn’t mean a wolf-to-elk-to-willow trophic cascade doesn’t exist in Yellowstone, just that the evidence presented so far isn’t clear enough.
To establish a clear trophic cascade from the reintroduction of Yellowstone wolves into willows, researchers will need to account for other predators and herbivores, McNulty said. An ideal study would analyze how much total willow biomass has increased today compared to before wolf introduction and determine the strength of the impact. We then calculate how much of that increase can be attributed to wolves alone and identify the cause.
Ripple and his team are currently preparing a detailed response, which Ripple said will explain that criticism of the original study stems from a misunderstanding of what they did. “The basic scientific logic of the paper is sound,” Ripple said.
Goheen said conservation priorities may be fueling the debate over the beneficial ecological effects of large carnivores, adding that even if wolves don’t conclusively trigger a trophic cascade in willows, it’s still important to conserve wolves.
