In this topic you will learn about how living things depend on each other for energy.

Every population needs energy in order to survive. The energy in an ecosystem comes from the Sun. The energy of the Sun is stored in food. The energy in food is passed from one organism to another in a food chain. A food chain is the path of energy in food from one organism to another.

The first organisms in a food chain are plants. Plants capture the Sun's energy during photosynthesis. This energy is stored in the foods the plant makes. When a plant eater eats the food it takes in the energy-rich sugars. The energy is used by the plant eater or is stored in its tissues. When these plants and animals die they become food for organisms like worms and ants.

Every community includes organisms that compete for the same resources. This means that, in an ecosystem, many small food chains may overlap each other. A food web is the overlapping food chains in a community. A food web shows how one population can be part of more than one food chain.

Food chains and food webs exist in all ecosystems. They all have producers. The producers on land include grasses and trees. Producers use the Sun's energy to produce their own food. Organisms that cannot make their own food are consumers. Consumers get energy from food made by other organisms. Consumers can be grouped according to the type of food they eat.

Herbivores are animals that eat plants. Herbivores may be as small as grasshoppers or as large as elephants. Herbivores, in turn, are eaten by carnivores—animals that eat other animals. All cats, big and small, are carnivores. So are dogs, foxes, and other sharp-toothed animals.

Living things that hunt other living things for food are called predators. The hunted are called prey. Not all meat eaters are predators. Some animals eat meat but don't hunt it. Such meat eaters are called scavengers—they feed on the remains of dead animals. Crows and vultures are scavengers. An animal that eats both animals and plants is an omnivore. You are an omnivore. Bears are omnivores, too—they eat foods ranging from berries to salmon. Every food chain ends with decomposers such as worms, bacteria, and fungi. Decomposers break down dead organisms and wastes into simpler molecules. Some of the molecules are returned to the soil.

Energy is lost as it passes from one organism to another in a food chain.