The ecosystem is a web of plants and animals. If the web loses something it can break. If the web breaks then the whole forest can change. It can hold a whole forest together. An ecosystem works because of who eats what, and what eats that, etc.
Animals eat other animals or animals eat plants.
Some people think of it as a food chain. A food chain has a top, a middle, and a bottom. The fox is at the top and the grass is at the bottom. If the plants are healthy the animals will be healthy too. If one link in the chain is unhooked the chain will totally fall apart.