Biology teachers worldwide often use Yellowstone National Park in the United States to explain ‘trophic cascades‘ to their students. This example begins with the elimination of wolves and culminates in the downfall of a forest.
The reintroduction of wolves to the park has been celebrated as a significant achievement in forest restoration. However, some biologists and geographers argue that these claims might be exaggerated.
Let’s trace the history. In the 1920s, gray wolves (Canis lupus) were eradicated from Yellowstone National Park due to government hunting programs.
Elk (Cervus canadensis), one of the wolves’ primary prey, are herbivores that feed on aspen and cottonwood saplings and disturb exposed soils with their hooves.
With natural predators like wolves keeping their numbers in check, elk-caused damage was minimal. But without these predators, the elk population ballooned, leading to significant overgrazing of saplings.
frameborder=”0″ allow=”accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share” referrerpolicy=”strict-origin-when-cross-origin” allowfullscreen>The once prominent skyline of quaking aspen trees (Populus tremuloides) in Yellowstone began to decline as mature trees naturally died off. Without new mature trees to replace them, the landscape altered dramatically.
Species dependent on mature aspen, such as beavers and cavity-nesting birds, were left without habitat. The absence of wolves led to an ecosystem unraveling.
After years of advocacy, gray wolves from Jasper National Park in Canada were reintroduced to Yellowstone in 1995, with hopes of restoring the forest.
In July 2025, a team led by Oregon State University ecologist Luke Painter revealed that introducing gray wolves had initiated a notably strong trophic cascade compared to similar situations in other ecosystems.
Their research focused on measuring the height of aspen trees, positing that trees taller than a specific height are relatively safe from browsing herbivores.
However, another group of scientists, led by wildlife ecologist Daniel MacNulty from Utah State University, has challenged these findings. They published their critique in the same journal as Painter’s team: Forest Ecology and Management.
This team initially raised their concerns in a letter to the editor of another journal in November 2025. Now, they have presented a comprehensive paper on the issue.
“Painter et al. assert that large-carnivore recovery in Yellowstone National Park has produced a strong trophic cascade compared to other systems, citing a 152-fold increase in aspen sapling density and widespread recruitment of new trees,” MacNulty and his team write.
“We demonstrate that these conclusions significantly exaggerate the cascade’s strength due to key methodological and interpretive errors.”
Such criticism, while challenging, is part of scientific progress—a constructive dialogue that refines our understanding as new evidence emerges.
MacNulty’s team points out errors in the baseline density calculations of trees in Painter et al.’s data. Instead of a 152-fold increase in aspen sapling density observed from 1998 to 2021, the actual increase is closer to 17.5-fold.
Additional issues with data analysis were identified. Treating measurements from the same tree stands as independent samples inflated the effect size, according to MacNulty’s team.
Furthermore, they argue that Painter et al.’s reliance on mean-based metrics allowed a minority of plots to disproportionately affect results. The presence of a few tall stems within a plot does not indicate widespread tree recruitment, contrary to what was reported.
“Finally, their assumptions that stems of two meters or more have escaped browsing, and that reduced browsing alone drives height growth, are contradicted by long-term data” indicating significant browsing above these heights, MacNulty and team write.
MacNulty’s team emphasizes that their critique does not diminish the role of large predators in ecosystems, but underscores the need for scientific precision in studying complex systems.
“Predator effects in Yellowstone are real but context-dependent – and strong claims require strong evidence,” MacNulty stated in an earlier press release.
While the reintroduction of wolves has influenced Yellowstone’s forest recovery, its impact may not be as pronounced as originally reported by Painter’s team.
“The evidence supports the occurrence of a trophic cascade in Yellowstone, but not the magnitude of strength claimed,” MacNulty and team conclude in their latest correspondence.
“Accurate assessment of trophic cascade strength in Yellowstone is vital to ensure that this iconic system reliably informs ecological understanding and restoration practice.”
Related: There’s One Super Predator in Africa That Instills More Fear Than Lions
MacNulty and team’s 2026 response to the 2025 study has been published in Forest Ecology and Management. Their 2025 letter to the editor was published in Global Ecology and Conservation.
