In 1926 the last wolf was killed in Yellowstone National Park. For 70 years the top predator was missing and elk, its main prey, increased dramatically to over 20.000, turning vast areas of Yellowstone into an ecological disaster zone, browsing down the woody vegetation to almost nothing. In 1995 wolves were reintroduced. Wolves not only preyed on elk. They had an impact on almost all animals down the food chain, also on the fauna of the park, turning the eco-system of Yellowstone back into balance.