A new study calculates the long-term effects of reintroducing wolves to Yellowstone National Park in the 1990s, which ultimately helped willow shrubs that feed wildlife in stream habitats.
A new study reveals that the reintroduction of wolves to Yellowstone National Park in the 1990s triggered a cascading effect that positively impacted the entire ecosystem. The research highlights how the presence or absence of apex predators can influence every level of the food web.
By the 1920s, gray wolves (Canis lupus) had been extirpated from Yellowstone, and cougar (Puma concolor) populations were significantly reduced due to government efforts to control large predators. Without these predators, Rocky Mountain elk (Cervus canadensis) populations flourished, leading to overgrazing and the destruction of various plant species. This damage to plant life then threatened beaver populations, showcasing a classic trophic cascade — where the actions of top predators indirectly affect species lower on the food chain, ultimately reshaping the ecosystem.
Similar cascading effects have been observed in other parks, such as Olympic National Park in Washington after wolves vanished.
Restoring the balance in Yellowstone, cougar populations began to recover in the 1980s, and wolves were reintroduced in 1995 and 1996. However, scientists were uncertain whether the return of these predators would restore ecological balance.
A recent study, published on January 14 in Global Ecology and Conservation, used two decades of data collected from 2001 to 2020 to examine willow shrubs (Salix) along streams in Yellowstone. The researchers focused on measuring willow crown volume — the total area covered by the plant’s branches, stems, and leaves. This metric allowed them to estimate the shrubs’ biomass, which represents the organic material available at the plant level of the food web and the energy passed through the food web when herbivores feed on these plants.
According to study lead author William Ripple, an ecologist at Oregon State University, Yellowstone’s northern range offers an ideal setting to study such changes. “It is one of the few places in the world where we can observe the effects of restoring an apex predator guild, including wolves and cougars, after a long absence,” Ripple said. “The lessons we learn here can apply to other ecosystems globally.”
The study found a dramatic 1,500% increase in willow crown volume along streams during the study period, signaling a significant recovery of these shrubs. This surge in willow growth is linked to reduced elk browsing, likely influenced by the reintroduction of predators, allowing the willows to regenerate in certain areas.
Ripple noted, “One of the most striking results was how strong the trophic cascade has been. A 1,500% increase in willow crown volume is remarkable. It is one of the most significant trophic cascade effects reported in the scientific literature.” These streamside areas, though small, play a crucial role in the Yellowstone ecosystem by providing food and shelter to a wide variety of wildlife.
The study also emphasizes the importance of long-term monitoring, as such ecosystem changes take time to unfold. “Understanding these changes requires patience and persistence,” Ripple explained.
Ripple and his team are now looking into how other factors, such as climate change and a growing bison population, may impact the trophic cascade and how beaver populations further influence the landscape. “The Yellowstone story is still unfolding,” Ripple said. “It will be fascinating to see where it goes next.”