For seventy years, Yellowstone National Park had been wolf-free. During that time, since wolves are major hunters of elk and deer and other herbivores, the ecosystem at the park underwent a slow but steady change. Masses of grass-grazers wandered Yellowstone, eating all vegetation in sight.

However, once their natural predators the wolves returned to Yellowstone, the entire ecosystem radically changed — for the better.

Find out how reintroducing the wolf has had a major impact on Yellowstone’s ecosystem, and how it changed the flow of its river — watch the video.

You can help more populations of wolves restore habitats in the American west! Find out more about the project.