According to a new study published today, the reintroduction of wolves into Yellowstone National Park is tied to the recovery of aspen trees in areas around the park.

This is the first large-scale study to show that aspen is recovering in areas around the park, as well as inside the park boundary, said Luke Painter, a wildlife ecologist at Oregon State University and lead author on the study.

The study answers the question of whether the return of wolves to Yellowstone could have a cascading effect on ecosystems outside the park where there is increased human activity such as hunting, livestock grazing, and predator control. There has also been skepticism surrounding the extent and significance of aspen recovery, he said.

Wolves didn’t cause aspen recovery all by themselves, but it is safe to say it would not have happened without them.

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