All living things are in a food chain. For example, zebras eat plants and grasses. Lions eat the zebras. Lions, zebras, and plants all belong to one food chain.

Other animals eat zebras, too. Other animals eat the same plants. They all form different food chains. Put together, these food chains are called a food web.

Trophic Levels

Food chains have different levels. These are called trophic levels.

Producers

Organisms in the first trophic level are called producers. Most producers are plants. Algae and some bacteria are also producers.

Each producer makes its own food. Most producers use photosynthesis. That means they use sunlight, carbon dioxide, and water to create energy.

Consumers

The next trophic levels are animals that eat producers. They are called consumers. Some consumers are herbivores while others are carnivores. They can also be omnivores, which means they eat both meat and plants.

Primary consumers are herbivores. They eat plants and other producers. Deer, mice, and elephants are herbivores. They eat grass, shrubs, and trees. In the desert, a mouse is a primary consumer. It eats seeds and fruit. In the ocean, many fish and turtles are herbivores. They eat algae and seagrass.

Secondary consumers are animals that eat herbivores. In a desert, a secondary consumer may be a snake. It eats mice. In underwater kelp forests, sea otters are secondary consumers. They hunt sea urchins. Secondary consumers are at the third trophic level.

Animals at the next level eat secondary consumers. In the desert, an owl or eagle may hunt snakes. Top predators are also called apex predators. No other animal eats an apex predator. Lions are apex predators. Great white sharks are apex predators in the ocean.

Detritivores and Decomposers

Detritivores and decomposers are at the end of food chains. Detritivores eat things that are not alive. Vultures eat dead animals, for example.

Some organisms, like fungi and bacteria, are decomposers. They turn dead plants and other organisms into soil. They let food chains start over.

For example, grass makes food through photosynthesis. A rabbit eats the grass. Then, a fox eats the rabbit. When the fox dies, worms and fungi break down its body. The animal becomes part of the soil. There, it leaves nutrients for plants to grow. This way, the cycle keeps going.

Biomass

Biomass is the energy in living creatures. It starts with the sun's energy. Plants use sunlight to create energy, or biomass, for themselves. When animals eat these plants, they also create energy, or biomass. At each step, energy gets lost, though. Animals are only able to take a part of it.

As you go up the food chain, there is less and less biomass. There is more energy in the lower trophic levels. That is why food webs need more producers than herbivores. They can only have a few carnivores and omnivores.

Sometimes, food chains can fail. If plants are destroyed, herbivores do not have enough to eat. Their numbers go down. Humans can also destroy a food chain. People cut down forests for wood.

Bioaccumulation

Pesticides get into the soil and water. Sometimes, an animal eats a polluted plant. The pesticides stay in the animal's fat. Then, a carnivore eats that animal. It takes in more and more pesticide. This is called bioaccumulation.

In the 1940s and 1950s, bald eagles began disappearing. DDT was a major reason. DDT stands for dichloro-diphenyl-trichloroethane. It is a pesticide. DDT was building up in soil and water. Fish ate organisms with DDT. Bald eagles ate the fish. Because of the DDT, they started laying eggs with thin shells. These shells often broke and the baby birds died.

The U.S. government decided that using DDT was against the law. Many food webs have come back. Baby eagles are now hatching again.