Read what the study says Reintroduced Wolves 

New research has demonstrated the powerful impacts the reintroduction of predators can have on an ecosystem. The presence of wolves in Yellowstone National Park has driven a cascading effect that has literally changed the landscape.  The new study used data from previous studies on 25 riparian (streamside) sites collected over a 20-year period – from 2001 to 2020. The results show a 1,500 percent increase in willow crown volume along these streams in northern Yellowstone National Park. The effects were produced by changing behavior among elk adapting to the presence of a large carnivore guild (wolves) reintroduced into the Park in 1995-96, and other factors.