Every living being is part of a food chain. Food and the animals that eat the food make up a food chain. For example, plants and grasses are food for zebras. Zebras are food for lions. Plants, zebras, and lions make up a food chain.

There are many different food chains in an ecosystem. All together, the food chains in the ecosystem make a food web.

Trophic Levels

Organisms in food webs are grouped into categories. These categories are called trophic levels.

Producers

Organisms in the first trophic level are called producers. Plants are producers. Algae and some bacteria are also producers. Each producer makes its own food. Most producers use photosynthesis. This is a series of chemical reactions. Plants use these reactions to make energy from sunlight, carbon dioxide, and water.

Consumers

The next trophic levels are animals that eat producers. These animals are called consumers.

Consumers can be carnivores or omnivores. Carnivores only eat meat. Omnivores eat both meat and plants.

The first level of consumers is made of herbivores. These animals are also called primary consumers. They eat plants, algae, and other producers. Deer, mice, and elephants are herbivores. They eat grasses, 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 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.

Animals in the next level are called tertiary consumers. They eat secondary consumers. In the desert, an owl or eagle may hunt snakes.

Top predators are also called apex predators. They eat other consumers. No other consumers eat them. Lions are apex predators on the grasslands of Africa. In the ocean, the great white shark is an apex predator. In the desert, bobcats and mountain lions are top predators.

Detritivores and Decomposers

Detritivores and decomposers make up the last part of food chains. Detritivores eat plants and animals that are not alive. For instance, vultures eat dead animals.

Some organisms, like fungi and bacteria, are decomposers. They turn decaying plants into soil. Decomposers allow food chains to start over. For example, grass makes its own energy 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 body returns to the soil. There, it leaves nutrients for plants to grow.

Biomass

Biomass is the energy in living organisms. Producers use the sun's energy to create biomass. The higher the trophic level, the lower the biomass. There is more energy in lower trophic levels than in higher ones.

There are always more producers than herbivores in a healthy food web. A healthy food web has many producers and many herbivores. It only has a few carnivores and omnivores.

Every part of a food chain is connected to other food chains. When one part is in danger, others are also at risk. If plants are destroyed, herbivores don't have enough to eat. Their numbers go down. The number of plants can decrease because of drought or disease.

Humans can also destroy food chains by destroying habitats. People cut down forests. We use the lumber for buildings. We also pave over grasslands to build shopping malls or parking lots.

Bioaccumulation

Sometimes, pesticides can affect food chains. Pesticides get into the soil and water. Animals eats plants that are covered in pesticides. The pesticides stay in the animals' fat. When a carnivore eats that animal, it also eats the pesticides. This is called bioaccumulation.

Bioaccumulation happens in water ecosystems, too. Runoff from cities or farms can be polluted. Algae, bacteria, and seagrass absorb the pollutants. Sea turtles and fish eat the seagrass. Then, sharks or tuna eat those fish. When people finally eat the tuna, that meal is full of pesticides.

In the 1940s and 1950s, bald eagles began disappearing. One major cause was a pesticide called DDT. The name DDT stands for dichloro-diphenyl-trichloroethane. It was used to kill insects that spread diseases. DDT builds up in soil and water. Worms, grasses, algae, and fish ate organisms with DDT. Bald eagles ate the fish. They had high amounts of DDT in their bodies. They got it from their prey. These eagles started laying eggs with thin shells. These shells often broke before the baby birds hatched.

The U.S. government decided to ban DDT. Food webs have come back in most parts of the country. Bald eagle chicks are now able to hatch.