Return of Yellowstone Wolves not Enough to Restore Ecosystem: Study

A new study suggests that the reintroduction of wolves in Yellowstone National Park is not enough to restore the ecological balance of the park's landscapes.

In the beginning of the 20th century, wolves largely vanished from Yellowstone, causing complex changes in ecological processes. In a bid to increase their population and restore the ecosystem, the wolves were reintroduced to their native habitat in Yellowstone in 1995.

Earlier studies have suggested that the reintroduction of wolves initiated dramatic restoration of riparian (streamside) ecosystems. But a new study by researchers from Colorado State University (CSU) suggests the reintroduction of the wolves might alone not be enough to fully recover the ecosystem.

"The reintroduction of the wolf in Yellowstone has contributed to positive improvements in the Park's ecosystems, but it isn't a simple on and off light-switch effect," Kristin Marshall, lead author and recent CSU alumna, said in a statement.

"Our research shows that the complexity of the ecological damage caused by the eradication of a key predator species requires careful consideration of dynamic variables for restoration, and so additional caution must be emphasized to avoid predator removal in the first place."

When the wolves vanished from Yellowstone in the early 1900s, the population of elk increased, as they were no longer hunted by wolves. The elk fed on willows by streams, causing a significant decline in the population of beavers that once thrived in such small streams. This is due to the reduction in the availability of willows, as beavers need willows for food, as well as for constructing dams.

Willows also need beavers, because the beaver dams help create mud flats, where new willows emerge. In addition, the dams raise the water table, supplying more water to the willow roots. The lost wolves caused the loss of beavers and willows that are needed for a healthy riparian zone, according to a report in Science Now. 

When the wolves were reintroduced in the park, elk numbers dropped. However, it did not help in rescuing the willows. Willows need to be at least 2 meters tall to escape from browsing elk, and also to provide enough food and material for the construction of beaver dams.

For their study, Marshall and her colleagues examined willow growth at four different sites in the northern range of the park, from 2000 to 2010. At each site, the research team fenced some plots to protect the willows from browsing elk and other animals. They also constructed dams near some plots, in order to imitate the effect of beavers. While some plots were both fenced and dammed, others were neither fenced nor dammed.  

Even after 10 years, the research team found that the fenced willows without dams were shorter than 2 meters, a threshold also required for the willows to reproduce. Unfenced willows also did not reach the 2-meter mark. Only those willows that were fenced and dammed made it past the threshold point. This suggests that even if wolves were reducing the impact of elk, they still wouldn't be able to recover the willows, said the researchers. They said that the reintroduction of wolves alone is not sufficient to restore the riparian ecosystems. 

And the issue cannot be solved easily because, "the absence of beaver opposes the return of tall willows and the absence of tall willows opposes the return of beaver," the researchers write in the paper.

The findings of the study, "Stream hydrology limits recovery of riparian ecosystems after wolf reintroduction", are published in journal of the Proceedings of the Royal Society B: Biological Sciences.