A new peer-reviewed study challenges one of the most well-known claims about the impact of wolves on the Yellowstone National Park ecosystem.

Reevaluating Previous Conclusions on Wolf Impact

In an official commentary published in the journal Global Ecology and Conservation, researchers from Utah State University and Colorado State University argue that the article by Ripple and colleagues (2025) exaggerated the effects of wolf reintroduction on Yellowstone’s ecosystem. According to Dr. Daniel MacNulty, lead author and ecologist at Utah State University, the earlier study claimed that the return of predators triggered one of the world’s most powerful trophic cascades. However, a reanalysis revealed that these conclusions are invalid, as they are based on circular reasoning and violations of basic modeling assumptions.

The Problem with the 1500% Willow Growth Claim

The original study reported a 1500% increase in willow canopy volume after wolves returned, basing this on plant height measurements and a regression model that predicted canopy volume solely from height. MacNulty explains that using height both for calculation and for predicting volume creates a circular dependency, which mathematically guarantees a strong correlation—even if no real biological change occurred. As a result, the chosen statistical method artificially inflated the perceived relationship, even if actual willow growth did not occur.

Methodological Issues and Sampling Bias

The researchers also highlighted several additional concerns:

• The “height-volume” model was applied to heavily browsed willows with deformed growth forms, violating model assumptions and inflating apparent growth.
• Willow plots compared between 2001 and 2020 were mostly in different locations, making it difficult to separate real ecological changes from sampling bias.
• Comparisons with trophic cascades in other regions assumed ecological equilibrium, which is not characteristic of Yellowstone’s still-recovering, non-equilibrium system.
• The use of selective photographs and the omission of factors such as human hunting further complicated the establishment of causal relationships.

According to Dr. David Cooper, co-author and senior researcher at Colorado State University, after accounting for these issues, the evidence no longer supports claims of dramatic ecosystem recovery driven by wolves. The data instead point to a more modest and spatially variable response, influenced by hydrology, browsing, and local conditions.

A More Nuanced View of Predator Roles

The authors emphasize that their findings do not diminish the ecological importance of large predators. They note that the complex dynamics of food webs require careful analysis and robust evidence. As MacNulty points out, their goal is to clarify the evidence, not to downplay the role of predators. The impact of predators in Yellowstone is real, but it depends on context, and strong claims require equally strong evidence.

Why Scientists Reach Different Conclusions

The new article helps explain why scientists analyzing the same data can arrive at different conclusions. Ripple and colleagues (2025) described wolf reintroduction as triggering a powerful trophic cascade, while Hobbs and colleagues (2024), who collected 20 years of field experiment data, reported only weak cascade effects.