“The reintroduction of wolves to Yellowstone National Park triggered a trophic cascade that fundamentally transformed the park's ecosystem.”

Our results are consistent with the hypothesis of a wolf-ungulate-alder trophic cascade in northern Yellowstone, supported by comparisons of alder age structure across time, before and after wolf reintroduction at our study sites. This evidence demonstrates that alder stems recruited only after wolf reintroduction at our study sites. Thus, alder recruitment was likely linked to changes in top-down forces (reduced ungulate browsing).

Stream hydrology limits recovery of riparian ecosystems after wolf reintroduction. Recovering aspen follow changing elk dynamics in Yellowstone. The National Park Service acknowledges ongoing scientific debate about the extent and mechanisms of trophic cascades following wolf reintroduction, with studies showing varied influences including hydrology and elk behavior.

We show how a tradition of non‐random sampling has confounded this understanding in a textbook system (Yellowstone National Park) where carnivore (wolf) recovery is associated with a trophic cascade involving changes in herbivore (elk) behaviour and density that promote plant regeneration. To the extent that annual decreases in browsing and annual increases in height of woody deciduous plants in northern YNP reflect the cascading effects of reintroduced wolves, our results indicate the existence of a trophic cascade that was weaker than is often claimed.

Ripple et al. (2025) recently argued that large carnivore recovery in Yellowstone National Park triggered one of the world's strongest trophic cascades, citing a ∼1500 % increase in willow crown volume derived from plant height data. In this comment, we show that their conclusion is invalid due to fundamental methodological flaws, including a tautological volume model, violations of key modeling assumptions, comparisons across unmatched plots, and the misapplication of equilibrium-based metrics in a non-equilibrium system. These shortcomings explain the apparent conflict with Hobbs et al. (2024), who found evidence for a relatively weak trophic cascade based on the same height data and a long-term factorial field experiment.

Data from a 20-year study (2001–2020) revealed a relatively strong trophic cascade, with a ∼1500% increase in average willow crown volume following the 1995–96 reintroduction of gray wolves. Reduced herbivory pressure from Rocky Mountain elk followed wolf reintroduction, leading to increased growth in willows. This ratio surpassed 82% of those reported in a global meta-analysis of trophic cascades.

A new scientific review challenges the headline-grabbing claim that Yellowstone’s returning wolves triggered one of the strongest trophic cascades on Earth. Researchers found that the reported 1,500% surge in willow growth was based on circular calculations and questionable comparisons. After correcting for modeling and sampling flaws, the supposed ecosystem-wide boom largely disappears.

With the advent of better technology, satellite imagery of Yellowstone's Northern Range show that aspen are still in major decline since wolf reintroduction. In conclusion, recovery of aspen and willow communities in Yellowstone since wolf reintroduction has not been as strong or widespread as claimed. Still, there has been some recovery, suggesting a trophic cascade has unfolded.

Next month will mark the 30th anniversary of a landmark wildlife experiment: the reintroduction of wolves into Yellowstone National Park. Diane Boyd, a wildlife biologist... supported the U.S. Fish and Wildlife Service’s broader reintroduction effort... “The return of wolves has been wildly successful beyond all expectations.” The article notes the reintroduction's success in wolf population recovery but does not address trophic cascade claims directly.

A Colorado State University experiment spanning more than two decades has found that removal of apex predators from an ecosystem can create lasting changes that are not reversed after they return, challenging the commonly held belief that the reintroduction of wolves to Yellowstone National Park restored an ecosystem degraded by their absence. The study, published in Ecological Monographs, suggests that Yellowstone's northern range shifted into an alternative ecological state that resisted returning to previous conditions once carnivores were restored.

“Our findings emphasize the power of predators as ecosystem architects,” said William Ripple from Oregon State University, who led the research. “The restoration of wolves and other large predators has transformed parts of Yellowstone, benefiting not only willows but other woody species such as aspen, alder, and berry-producing shrubs.” The research, utilizing data from 25 riparian sites over a 20-year period (2001-2020), revealed a remarkable 1,500% increase in willow crown volume along riparian zones in northern Yellowstone National Park, driven by the effects on elk due to a restored large carnivore guild following the reintroduction of wolves.

Despite studies claiming to show early evidence of a tantalizing relationship between wolves and regenerating riparian ecosystems since the canines returned to Yellowstone, scientists are still debating how large carnivores impact vegetation and other animals, a dynamic scientists call a “trophic cascade.” Chris Wilmers, a professor of wildlife ecology at the University of California Santa Cruz, noted that while there is evidence consistent with a trophic cascade, “the effects are a lot more complicated and weaker than what was initially thought.”

Wolves are causing a trophic cascade of ecological change, including helping to increase beaver populations and bring back aspen, and vegetation. In Yellowstone, biologists have the rare, almost unique, opportunity to document what happens when an ecosystem becomes whole again, what happens when a key species is added back into the ecosystem equation.

“It is the source of so much confusion,” says Dan MacNulty of Utah State University. Tom Hobbs, an ecologist at Colorado State University, agrees with MacNulty, describing the idea of a widespread trophic cascade stemming from the wolves as a great idea that doesn’t hold up. “Firstly, has vegetation dramatically improved everywhere? Absolutely not. There has been a very patchy, spatially variable change in woody deciduous plants.” “During the decade after wolves were reintroduced, the decline in the elk population was not caused by wolves – there weren’t enough of them – but by human harvest outside the park. People were the apex predator.

Over the last three decades, Yellowstone National Park has undergone an ecological cascade, with aspen and willow trees thriving as elk numbers fell, which in turn allowed beaver numbers to increase. This shift has largely been attributed to the reintroduction of wolves, but a fierce debate among scientists questions whether their return reshaped the entire ecosystem in the way that was initially thought, with critics arguing that a 2025 study oversimplified the story and reinterpreted existing data.

When wolves were reintroduced, they not only killed elk but changed their prey’s behavior patterns; herbivores avoided valleys and gorges, allowing areas to regenerate. Plant life thrived along riverbanks, erosion decreased, and riverbanks stabilized, causing rivers and streams to change course. With just a small population of wolves, the landscape of the whole park transformed.

Multiple peer-reviewed studies, including Ripple and Beschta (2003, 2012), document increased recruitment of aspen, cottonwood, and willow following wolf reintroduction in 1995, attributed to reduced elk browsing due to predation risk and density effects. However, critiques like those from Winnie (2012) and recent analyses (e.g., MacNulty et al., 2025) highlight sampling biases, non-random site selection, and confounding factors like drought recovery and human hunting, suggesting effects are real but not ecosystem-wide or transformative.

Ripple et al. (2025) recently argued that large carnivore recovery in Yellowstone National Park triggered one of the world's strongest trophic cascades, citing a ~1500% increase in willow crown volume derived from plant height data. This preprint comment shows that their conclusion is invalid due to fundamental methodological flaws in the analysis.

Thirty years ago, wolves were reintroduced to Yellowstone National Park and triggered one of the most important ecological recoveries ever recorded. Through a powerful trophic cascade, wolves changed elk behaviour, restored riverbanks, brought back trees and beavers, and ultimately healed the rivers themselves.

When wolves were reintroduced to Yellowstone National Park after being absent nearly 70 years, the most remarkable "trophic cascade" occurred. George Monbiot explains how wolves changed rivers through ecosystem recovery.