The return of wolves to Yellowstone National Park has contributed to an ecological recovery after decades of damage to plant and animal life. Now a new study shows that the park’s aspen populations are finally beginning to reestablish themselves – apparently thanks to the predator’s impact on the ecosystem.

During the 1930s, wolves were completely eradicated from Yellowstone National Park in the Rocky Mountains of the United States. This had major consequences for the ecosystem – as the deer population increased dramatically and began grazing intensively in the park. This led to a sharp decline in trees like aspens. Aspen is considered a keystone species in the park, and its decline affected other species that depend on the deciduous trees, including beavers, insects and birds.

In the 1990s, wolves were reintroduced to the park, which immediately changed the deer’s behavior – not only by taking some as prey, but primarily by causing the deer to stop grazing for long periods in the same places. The result was that vegetation recovered, which has had a major effect on biodiversity.

Remarkable change

Now researchers have documented the first new generation of aspens in the park. In the study, which was published in Forest Ecology and Management, researchers argue that the wolf’s return has contributed to the species’ recovery.

— The reintroduction of large carnivores has initiated a recovery process that had been shut down for decades, says the study’s lead author Luke Painter, who teaches ecology and conservation at OSU College of Agricultural Sciences, in a press release. About a third of the 87 aspen stands we examined had large numbers of tall saplings throughout, a remarkable change from the 1990s when surveys found none at all.

Another third of the examined stands had areas with tall saplings that were growing up to become new trees in the canopy layer, while the rest were still being held back by grazing. Researchers believe this may be due to bison increasing in certain parts of the park. The fact that stands with many tall saplings have low levels of grazing, while others continue to be held back, suggests that aspen recovery depends on an ecological chain reaction where predators affect plant life by reducing the number of grazing animals – rather than on factors like climate or soil fertility.

— This is a remarkable case of ecological restoration, says Painter. Wolf reintroduction is yielding long-term ecological changes contributing to increased biodiversity and habitat diversity.