For 70 years, the population of the wapiti deer in the Yellowstone National Park in the United States increased unchecked. Without natural predators and despite all human attempts to check the increase of the deer population they kept spreading and destroyed vast regions of the park's vegetation. They literally defoliated the National Park. However, a well-directed reintroduction of 41 wolves between 1995 and 1997 worked wonders: The animals restored the natural balance in the National Park.
