Corvallis, OR — February 6, 2025 — A new study reveals the profound ecological effects of wolves and other large carnivores in Yellowstone National Park, showcasing the cascading effects predators can have on ecosystems. In Yellowstone, this involves wolves and other large carnivores, elk, and willows. The research, which utilized previously published data from 25 riparian (streamside) sites and collected over a 20 year period, from 2001 to 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 in 1995–96, and other factors. The study was led by Dr. William J. Ripple of Oregon State University and the Conservation Biology Institute in Corvallis, OR, and published today in Global Ecology and Conservation.
Trophic cascades, the effects of predators on herbivores and plants, have long been a topic of ecological interest. The study quantifies the strength of this phenomenon for the first time using willow crown volume as a proxy for aboveground biomass, demonstrating a significant three-dimensional recovery of riparian vegetation represented by the growth in both crown area and height of established willows. The strength of the Yellowstone trophic cascade observed in this study surpasses 82% of strengths presented in a synthesis of global trophic cascade studies, underscoring the strength of Yellowstone's willow recovery process. The authors note that there is considerable variability in the degree of recovery and not all sites are recovering.
Even though riparian areas in the western United States comprise a small portion of the landscape, the study has particular relevance since these areas provide important food resources and habitat for more wildlife species than any other habitat type. These areas also connect upland and aquatic ecosystems and are widely known for their high diversity in species composition, structure, and productivity.
"Our findings emphasize the power of predators as ecosystem architects," said William Ripple. "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. It's a compelling reminder of how predators, prey, and plants are interconnected in nature."
Wolves were eradicated and cougars driven to low numbers from Yellowstone National Park by the 1920s. Browsing by elk soon increased, severely damaging the park's woody vegetation, especially in riparian areas. Similar effects were seen in places like Olympic National Park in Washington, and Banff and Jasper National Parks in Canada after wolves were lost. While it's well understood that removing predators can harm ecosystems, less is known about how strongly woody plants and ecosystems recover when predators are restored. Yellowstone offers a rare opportunity to study this effect since few studies worldwide have quantified how much plant life rebounds after large carnivores are restored.
"Our analysis of a long-term data set simply confirmed that ecosystem recovery takes time. In the early years of this trophic cascade, plants were only beginning to grow taller after decades of suppression by elk. But the strength of this recovery, as shown by the dramatic increases in willow crown volume, became increasingly apparent in subsequent years," said Dr. Robert Beschta, an emeritus professor at Oregon State University. "These improving conditions have created vital habitats for birds and other species, while also enhancing other stream-side conditions."
The research points to the utility of using crown volume of stream-side shrubs as a key metric for evaluating trophic cascade strength, potentially advancing methods for riparian studies in other locations. It also contextualizes the value of predator restoration in fostering biodiversity and ecosystem resilience.
