In the 1920s, the wolf population in Yellowstone National Park was eradicated. They were reintroduced in 1995 in order to manage a rising and overgrazing elk population. This impact cascaded beyond expectations, resulting in a dramatic regeneration of the park’s willow stands, a nine-fold increase in the number of beaver colonies, and most dramatically: a change in the course of the park’s rivers.
(https://bit.ly/3eJFgKu, https://bbc.in/3hlI0zk)