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.
All these animals rely on each other. Just like people in a town or community. The pond ecosystem is a community too. If you can, go to a pond and see all the different varieties of plants, insects, birds and other living things. Other examples of ecosystems include forests, rivers, oceans, the desert, the cold arctic tundra, the high mountains and jungles.