Think of an ecosystem as any place where a lot of living things exist in balance. That's a little too simple, but it's a good place to start. Imagine a small body of water. Insects, worms, birds, fish, mice, muskrats, ducks, deer, wolves.
Insects eat plants, fish eat worms and insects, birds eat fish, worms and insects.
Mice eat grain. Muskrats eat baby ducks and duck eggs. Ducks eat insects and worms. Deer eat grasses. Wolves eat muskrats and deer.