Wolves were reintroduced to Yellowstone National Park in 1995 and 1996. (Photo credit: Mark Miller Photos/Getty Images)
A new study has found that the reintroduction of wolves to Yellowstone National Park in the 1990s caused a chain reaction that positively impacted the entire ecosystem.
The study's findings show how the return or disappearance of large predators can impact all levels of the food chain.
By the 1920s, gray wolves (Canis lupus) had disappeared from Yellowstone National Park, and the cougar (Puma concolor) population was extremely low due to government large-carnivore control programs. Rocky Mountain elk (Cervus canadensis) thrived without these predators, which in turn led to the decline of some plant populations. The loss of certain trees and shrubs then threatened beaver populations. This sequence of events is known as a trophic cascade—where the actions of top predators indirectly affect other species lower on the food chain, ultimately affecting the entire ecosystem.
Similar trophic cascades shaped ecosystems in other parks, such as Olympic National Park in Washington, after the disappearance of wolves.
Restoring balance
In the 1980s, Yellowstone's cougar population recovered, and wolves were reintroduced in 1995 and 1996. Still, scientists remained unsure whether the return of the predators would restore balance to the park's ecosystem.
A new study published Jan. 14 in the journal Global Ecology and Conservation analyzed 20 years of data collected between 2001 and 2020 on willow (Salix) shrubs along streams in Yellowstone. The researchers looked at the willows’ canopy volume — the total space taken up by the shrub’s branches, stems, and leaves. Measuring canopy volume allowed the researchers to calculate the shrubs’ total biomass: the amount of organic material available at the vegetation level of the food chain and the energy that will be transferred through the food chain when animals feed on the plants.
“The North Range of Yellowstone provides an ideal natural laboratory to study these changes. It’s one of the few places on the planet where we can observe what happens when a group of top predators, including wolves and cougars, rebounds from a long absence,” lead study author William Ripple, an ecologist at Oregon State University, told Live Science via email. “The lessons we learn here can be applied to other ecosystems around the world.”
The analysis showed a 1,500 percent increase in willow canopy volume along streams over the study period, indicating significant recovery of these shrubs. The study attributes this remarkable recovery of willow shrubs to a decrease in elk predation, likely due to the return of predators to the region, allowing willows to regrow in some places.
“One of the most striking results was how powerful the trophic cascade was,” Ripple said. “A 1,500 percent increase in willow crown volume is an impressive number. It’s one of the largest trophic cascade effects reported in the scientific literature.”
These riparian areas are a small but important part of the Yellowstone landscape, providing food resources and habitat for a large and diverse group of wildlife.
“Our study also highlights the importance of long-term monitoring,” Ripple added. “These changes in the ecosystem do not happen overnight, and understanding them requires patience and persistence.”
Ripple and his team now aim to study how other factors, such as climate change and growing bison populations, will affect this trophic cascade and how bison populations
Sourse: www.livescience.com
