“A keystone species is a plant or animal that plays a unique and crucial role in the way an ecosystem functions.”
In 1995, wolves were reintroduced into Yellowstone National Park for the first time in 70 years.
Deer had overpopulated due to lack of predators, causing them to depopulate the vegetation along the rivers. After the reintroduction of the wolves, deer and elk couldn’t remain in one place for too long. As a result, lands were no longer over grazed and were able to fully recover between migrations.
This caused the vegetation along the rivers in the valleys to regenerate, resulting in new stands and forests of aspen, willow, and cotton trees.
With the return of the trees and vegetation, beavers, who are also ecosystem engineers, returned to the waters and began to build dams. These dams changed the ecology, providing new homes for otters, muskrats, ducks, reptiles, and amphibians.
The wolves killed coyotes which brought the coyote population back into balance; as a result the number of rabbit and mice began to rise.
Which in turn lead to an increase in the numbers of other predators such as hawks, eagles, weasels, badgers, and foxes.
Grizzly bears of Yellowstone also benefited because they often successfully steal wolf kills, so they had more food to feed their cubs.
But the most striking change was to the rivers. With the return of the trees and vegetation, erosion slowed drastically and the rivers became more healthier and more stable.
All because a few wolves were brought back to Yellowstone.