Yellowstone wolves may not have transformed the national park after all

In a formal comment published in Global Ecology and Conservation, scientists from Utah State University and Colorado State University argue that a 2025 paper by Ripple et al. overstated how much wolf recovery reshaped Yellowstone National Park’s ecosystem.

“Ripple et al. argued that carnivore recovery produced one of the world’s strongest trophic cascades,” said Dr. Daniel MacNulty, lead author and wildlife ecologist at Utah State University. “But our re-analysis shows their conclusion is invalid because it relies on circular reasoning and violations of basic modeling assumptions.”

The 1500% Willow Growth Claim

The original study reported a 1,500% surge in willow crown volume after wolves returned. That estimate was derived from plant height measurements using a regression model that both calculated and predicted crown volume from height alone.

“Because height was used both to compute and to predict volume,” MacNulty explained, “the relationship is circular — mathematically guaranteed to look strong even if no biological change occurred.”

In other words, the statistical method made the connection appear powerful by design, even if willow growth had not meaningfully changed.

Methodological Concerns and Sampling Bias

The researchers also pointed to several additional concerns:

• The height to volume model was applied to heavily browsed willows with misshapen growth forms, violating the model’s assumptions and inflating apparent increases.
• Willow plots compared between 2001 and 2020 were mostly different locations, making it difficult to separate real ecological change from sampling bias.
• Comparisons with trophic cascades around the world assumed ecological equilibrium, which does not apply to Yellowstone’s still recovering, non equilibrium system.
• Selective photographs and the exclusion of factors such as human hunting further complicated efforts to establish clear cause and effect.

According to the authors, once these issues are addressed, the evidence no longer supports claims of a dramatic ecosystem wide rebound driven by wolves.

“Once these problems are accounted for, there is no evidence that predator recovery caused a large or system-wide increase in willow growth,” said Dr. David Cooper, co author and emeritus senior research scientist at Colorado State University. “The data instead support a more modest and spatially variable response influenced by hydrology, browsing, and local site conditions.”

A More Nuanced View of Predator Effects

The researchers stress that their findings do not dismiss the ecological importance of large carnivores. Instead, they argue that complex food web dynamics require careful analysis and strong evidence.

“Our goal is to clarify the evidence, not downplay the role of predators,” MacNulty said. “Predator effects in Yellowstone are real but context-dependent — and strong claims require strong evidence.”

The new paper helps explain why scientists analyzing the same dataset reached different conclusions. Ripple et al. (2025) described wolf recovery as triggering a powerful trophic cascade. In contrast, Hobbs et al. (2024), who gathered the data during 20 years of field experiments, reported only weak cascade effects.