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. All these animals relay on the pond for the water they need. The deer keep the grasses back, wolves take the sick and weak deer out of the herd. Muskrats keep the duck population down.