There was a great video that went viral a few years back about the way that wolves changed the rivers in Yellowstone National Park. As the video explains, wolves were reintroduced to Yellowstone in 1995 and through a series of fairly small, perhaps somewhat unexpected chain reactions, the ecosystem began to shift dramatically. First, the behavior and populations of the various animals in the park changed, including the way that deer and elk began avoiding certain areas of the park. With less foot traffic, the vegetation began to grow in places that it had not grown in decades and trees reached heights that they had not in decades.