LOGAN, Utah, USA — A new peer-reviewed analysis challenges one of the most publicized claims about Yellowstone's wolves.
In a detailed comment published in Global Ecology and Conservation, researchers from Utah State University and Colorado State University demonstrate that the 2025 study by Ripple et al. overstated the ecological effects of wolf recovery in Yellowstone National Park.
"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."
Ripple et al. based their conclusion on a 1,500 percent increase in willow crown volume, calculated from plant height data using a regression model that defines and predicts volume from the same variable. "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."
The authors identified several additional issues:
- The height-to-volume model was applied to heavily browsed willows with distorted shapes, violating model assumptions and exaggerating apparent growth.
- Willow plots compared between 2001 and 2020 were largely unmatched, conflating ecological change with sampling bias.
- Global comparisons of trophic cascade strength ignored equilibrium assumptions that do not apply to Yellowstone's still-recovering, non-equilibrium system.
- Selective photographic evidence and omission of key factors such as human hunting further distorted causal interpretation.
"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."
The authors emphasize that their critique does not diminish the ecological significance of large carnivores but underscores the need for rigorous methods when evaluating complex food-web interactions.
"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 study reconciles conflicting interpretations of the same dataset. Ripple et al. (2025) concluded that carnivore recovery produced a strong trophic cascade, whereas Hobbs et al. (2024), who collected the data through two decades of field experimentation, found only weak cascade effects.
