Here is the explanation:

Wolves were reintroduced to Yellowstone National Park in 1995 after being absent for over 70 years. Since their reintroduction, the wolf population has grown and has had a significant impact on the park's ecosystem. One of the most noticeable changes has been the recovery of aspen trees. Aspen trees are a keystone species in Yellowstone and provide food and shelter for a variety of animals.

Before the reintroduction of wolves, the elk population in Yellowstone was growing rapidly and aspen trees were being overgrazed. This was preventing aspen trees from regenerating and was causing them to decline. However, since the reintroduction of wolves, elk numbers have been reduced and aspen trees are now able to regenerate. This has helped to restore the balance of the ecosystem and has benefited a variety of other species that rely on aspen trees.