The reintroduction of wolves to Yellowstone National Park in the 1990s had a cascading effect that benefited the entire ecosystem, a new study finds.
The finding shows how the return or loss of apex predators can affect every part of the food web.
By the 1920s, gray wolves (Canis lupus) were no longer present in Yellowstone National Park and cougar (Puma concolor) populations were very low, as a result of government initiatives to control large predator populations. Rocky Mountain elk (Cervus canadensis) thrived without these predators, which in turn decimated some plant populations. The loss of some trees and shrubs then threatened beaver populations. This sequence of events is known as a trophic cascade — when the actions of top predators indirectly affect other species further down the food web, ultimately affecting the entire ecosystem.
Similar trophic cascades have shaped the ecosystems of other parks, such as Olympic National Park in Washington after wolves disappeared.
Restoring balance
In the 1980s, cougar populations in Yellowstone rebounded, and wolves were reintroduced in 1995 and 1996. But scientists were uncertain whether the return of these carnivores could restore balance to the park's ecosystem.
Related: Do wolves wag their tails?
The new study, published Jan. 14 in the journal Global Ecology and Conservation, used 20 years' worth of data, collected from 2001 to 2020, regarding willow shrubs (Salix) along streams in Yellowstone. The researchers looked at willow crown volume — the total space occupied by a shrubs' branches, stems and leaves. Measuring crown volume enabled the researchers to calculate the shrubs' overall biomass: the amount of organic material available at the plant level of the food web, and the energy that will be passed on through the food web when animals eat these plants.
"Yellowstone's northern range is the perfect natural laboratory for studying these changes. It is one of the few places in the world where we can observe what happens when an apex predator guild, including wolves and cougars, is restored after a long absence," study first author William Ripple, an ecologist at Oregon State University, told Live Science in an email. "The lessons we learn here can apply to other ecosystems globally."
The analysis found a 1,500% increase in willow crown volume along streams over the study period, demonstrating a major recovery of these shrubs. The study links this significant willow shrub recovery to a reduction in elk browsing, probably influenced by the return of predators to the region, which enabled willows to grow back in some areas.
"One of the most striking results was just how strong the trophic cascade has been," Ripple said. "A 1,500% increase in willow crown volume is a big number. It is one of the strongest trophic cascade effects reported in the scientific literature."
These streamside regions are a small but crucial part of the Yellowstone landscape, providing food resources and habitats for a huge, diverse group of wildlife.
"Our study also highlights the importance of long-term monitoring," Ripple said. "These ecosystem changes do not happen overnight, and understanding them requires patience and persistence."
Ripple and his team now want to explore how other factors like climate change and a growing bison population will affect this trophic cascade, and how beaver populations shape the landscape and ecosystem. "The Yellowstone story is still unfolding," Ripple said, "and it is going to be fascinating to see where it goes next."
Editor's note: This story was updated at 11:55 a.m. EST to note that the willows researched in this study are shrubs, not trees.
