I think it's because one animal can change one thing in the ecosystem (how all living things interact with each other) and that triggers a chain reaction which leads to the other animal being helped e.g. in Yellowstone national park, the deer had overpopulated and there was barely any vegetation. This also led to other animals not thriving and the river changed course. But when they introduced wolves, they brought down the numbers of deer, which led to the vegetation growing back, which gave other animals a chance to thrive and the river went back to normal. Hope this solves your problem!
