Yellowstone's Wolf Story Is Real, but Smaller Than the Legend

Keypoints:

• Wolves were removed from Yellowstone in the 1920s and reintroduced in 1995
• A 2025 study reported a 1,500% rise in willow crown volume after wolves returned
• A 2026 peer-reviewed comment says the methods overstated the effect
• Researchers say the evidence supports a trophic cascade but not its claimed strength
• Other predators, hunters and bison also shaped the recovery

The popular version is simple. Wolves disappeared from Yellowstone National Park in the 1920s, and elk overgrazed the riverbanks. Wolves returned in 1995, elk numbers fell, and willows, aspen and beaver ponds came back. A trophic cascade is the term for that chain reaction from predator to herbivore to plants.

The first half is well established. The second half is under fire. A 2025 study by Oregon State University ecologists, including William Ripple, argued that Yellowstone showed one of the world's strongest cascades. It cited a 1,500% increase in willow crown volume and a 152-fold increase in aspen sapling density.

A peer-reviewed comment in Global Ecology and Conservation challenges those numbers. Scientists from Utah State University and Colorado State University found methodological problems. The original study used plant height both to compute and predict crown volume. Willow plots from 2001 and 2020 were often in different places. Global comparisons also assumed ecosystems were in balance, though Yellowstone is still recovering. Ripple stands by the original conclusions.

Other factors matter. Wolves eat elk, but so do bears, mountain lions and hunters. A New York Times analysis reported that for eight years after wolves returned, hunters killed more elk than wolves did. A growing bison herd also complicates recovery, because bison are harder for wolves to take.

Long-term field work found weaker effects. Thomas Hobbs of Colorado State University, who led two decades of experiments, called the popular claim greatly exaggerated. The park's lead wolf biologist has described the reality as more of a trickle than a cascade. Daniel MacNulty of Utah State University says the evidence supports a cascade, but not the strength claimed.