In every ecosystem, there's a balance. Of plants, of animals, and of the rivers and lakes. The amount of every species keeps the others to a reasonable level. When something is taken away, it messes up that balance. When grey wolves were killed off in Yellowstone National Park in 1926, it messed up the balance in the park. The amount of deer, elk, coyotes, and other animals the wolves hunted multiplied, eating tons of grass, trees, and bushes. The amount of beavers, rabbits, bears, songbirds, and fish lowered. But when 14 grey wolves were reintroduced to Yellowstone in 1995, they brought back the balance, making the park a greener place with more animals than before. If we can reintroduce grey wolves to other places too, we'll have a better, more balanced world.