The wolves in Yellowstone have helped the park’s ecology. If wolves kill an elk, they will leave a lot of meat for other animals, because an elk is food to animals and birds like eagles. The wolves also helped the trees. If there are no wolves, there will be too many elk. The elk won’t have enough food, so they eat small willow trees and kill some kinds of forests. After the wolves came back to the park, they killed and ate the elk, and the willow trees started to grow again.