Yellowstone Wolves' Impact on Park Ecosystem Overstated, Study Finds

New analysis challenges claims of a dramatic 1,500% surge in willow growth after wolf reintroduction.

Published on Feb. 12, 2026

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A new peer-reviewed study is challenging the widely reported claim that the return of wolves to Yellowstone National Park triggered one of the world's strongest trophic cascades. Researchers from Utah State University and Colorado State University argue that the original study's conclusion of a 1,500% increase in willow growth was based on flawed methodology and circular reasoning, and that the evidence instead supports a more modest and spatially variable response influenced by factors like hydrology and local site conditions.

Why it matters

The debate over the ecological impact of Yellowstone's wolves has been a high-profile issue, with the original 2025 study's findings widely reported in the media. This new analysis calls into question the strength of the evidence behind those claims, highlighting the importance of rigorous scientific scrutiny and the need to avoid overstating the conclusions drawn from complex ecological data.

The details

The researchers found several issues with the original study's methodology, including the use of a regression model that both calculated and predicted willow crown volume from height alone, creating a circular relationship. They also noted that the willow plots compared were mostly different locations, making it difficult to separate real ecological change from sampling bias. Additionally, the researchers argued that the comparisons with trophic cascades around the world assumed ecological equilibrium, which does not apply to Yellowstone's still-recovering, non-equilibrium system.

  • The original study by Ripple et al. was published in 2025.
  • The new peer-reviewed comment was published in Global Ecology and Conservation in February 2026.

The players

Daniel MacNulty

Lead author of the new study and wildlife ecologist at Utah State University.

David Cooper

Co-author of the new study and emeritus senior research scientist at Colorado State University.

Ripple et al.

Authors of the 2025 study that claimed a 1,500% surge in willow growth due to wolf recovery in Yellowstone.

Hobbs et al.

Researchers who gathered the data used in the original and new studies, and reported only weak trophic cascade effects.

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What they’re saying

“Ripple et al. argued that carnivore recovery produced one of the world's strongest trophic cascades, but our re-analysis shows their conclusion is invalid because it relies on circular reasoning and violations of basic modeling assumptions.”

— Daniel MacNulty, Lead author and wildlife ecologist at Utah State University (Global Ecology and Conservation)

“Once these problems are accounted for, there is no evidence that predator recovery caused a large or system-wide increase in willow growth. The data instead support a more modest and spatially variable response influenced by hydrology, browsing, and local site conditions.”

— David Cooper, Co-author and emeritus senior research scientist at Colorado State University (Global Ecology and Conservation)

What’s next

The researchers stress that their findings do not dismiss the ecological importance of large carnivores, but rather argue that complex food web dynamics require careful analysis and strong evidence. They hope this new analysis will help clarify the evidence and lead to a more nuanced understanding of predator effects in Yellowstone.

The takeaway

This study highlights the importance of rigorous scientific scrutiny and the need to avoid overstating conclusions drawn from complex ecological data. It demonstrates how different analyses of the same dataset can lead to divergent interpretations, underscoring the value of multiple perspectives and ongoing scientific debate in advancing our understanding of environmental systems.