In this topic you will learn about how all living things depend on other living things to meet their needs for survival.

Organisms depend on one another to meet their needs. An ecosystem with fish and plants shows how organisms depend on each other. The plant gives off oxygen for the fish to use. The fish gives off carbon dioxide for the plant to use. The fish and the plant take part in the carbon dioxide and oxygen cycle. The carbon dioxide and oxygen cycle is the process of trading or exchanging the gases oxygen and carbon dioxide between producers and consumers. Fish are the consumers, and plants are the producers. If gases were used up instead of exchanged in this cycle, living things would die.

Gases are recycled in the carbon dioxide and oxygen cycle. Animals breathe in oxygen and breathe out carbon dioxide. Plants take in carbon dioxide and give off oxygen. Plants need some oxygen, too. During the day, plants make oxygen. At night, plants don't make food or oxygen. They get oxygen from the air, just as animals do.

Organisms depend on one another in other ways, too. Cattle egrets are birds that spend their time following cattle. Wherever the cattle go, the egrets follow. The cattle stir up insects and other small animals wherever they feed. The egrets follow behind the cattle to get food. The cattle aren't always helped by the egrets. They aren't always harmed, either.

Sometimes one population does harm another population. A tapeworm cannot live on its own. It lives inside the body of another organism. Tapeworms are parasites. A parasite is an organism that lives on or in another organism called a host. A host is an organism that a parasite lives on or in.

Some animals help plants reproduce. Squirrels bury oak tree seeds called acorns in the ground. Most of the acorns get eaten by the squirrels, but some stay buried and grow into trees.