New critique prompts correction of high-profile Yellowstone aspen study, highlighting challenges in measuring ecosystem response to wolf reintroduction

LOGAN, Utah, USA — A recent critique from a team led by Utah State University ecologist Dan MacNulty and published in Forest Ecology and Management has prompted a formal correction to a high-profile study on aspen recovery while raising broader questions about how scientific conclusions are drawn and defended in complex ecological systems.

The original study, published last year by Luke Painter and colleagues, concluded that restoration of large carnivores — including the reintroduction of wolves in the mid-1990s — triggered a strong, ecologically significant trophic cascade that fostered widespread recovery of aspen trees in northern Yellowstone. Central to their evidence was a reported 152-fold increase in the average density of young aspen, a dramatic result widely cited in media coverage.

The formal correction to the reported increase in young aspen density shifted the figure down from a 152-fold increase to 17.5-fold. Painter’s team asserted that the corrected estimate still supports their original conclusion about aspen recovery.

“But the point is not just about a single number,” said MacNulty. “It’s about whether average values can reliably support claims about how strong, how widespread, and how representative aspen recovery really is.”

At the core of the disagreement is how ecological change is summarized in an uneven system: many of the measured sites have no aspen saplings, while a small minority contain large numbers.

MacNulty’s analysis indicates that a minority of sites drove the increase in average sapling density, giving an inflated impression of typical or landscape-wide recovery: 20% of sites accounted for 80% of the increase, while more than half of the sites contained no saplings at all, even as the overall average rose.

Painter’s assertion didn’t address whether averages themselves are appropriate measures of trophic cascade strength, MacNulty notes, leaving their original conclusion unchanged.

MacNulty also notes that the assertion relies on shifting interpretations of data published by his group that originally indicated a weaker trophic cascade. Although those data were previously described as methodologically limited and warranted caution, they are nonetheless used to calculate averages and cited as independent, “strikingly similar” confirmation of a strong cascade, without explaining why the same evidence is weighed differently across contexts. 

MacNulty stresses that he does not dispute that trophic cascades have occurred in Yellowstone, nor that large carnivores can have important ecological effects. Rather, his critique challenges how confidently the strength and generality of those effects are claimed — especially in a system that remains dynamic and far from equilibrium.

“Because Yellowstone is a global symbol for predator-driven ecological restoration, how we interpret the data matters,” MacNulty said. “Applying rigorous standards ensures that claims about the magnitude of plant recovery reflect what the data actually support, providing more reliable guidance for both science and environmental management.”

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