Explain how ecosystem can change even if people are not involved

The animals could change it. For example, a bunny could dig a hole in the ground, making a home for other animals inside of it, too, changing the animals' living environment.

Or natural disasters could change their environment, too. For example, if a landslide occurred and covered a pond, frogs and snakes could lose their living areas and food, which would make them have to move to a different area, changing another ecosystem.