A food web is made up of all the food chains in a single ecosystem. Each living thing in an ecosystem belongs to many food chains. A food chain is a path that energy takes through a certain ecosystem.

Trophic Levels

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

Producers

Producers make up the first trophic level. Producers, also known as autotrophs, make their own food and do not depend on any other organism for nutrition. Most autotrophs use photosynthesis to create food from sunlight, carbon dioxide, and water.

Plants are the most familiar type of autotroph, but there are many other kinds. Algae are autotrophic. Some types of bacteria are autotrophs. For example, bacteria living in active volcanoes use sulfur, not carbon dioxide, to produce their own food. This process is called chemosynthesis.

Consumers

The next trophic levels are made up of animals that eat producers. These organisms are called consumers. Consumers can be carnivores or omnivores.

Primary consumers are herbivores, which eat plants, algae, and other producers. In a grassland ecosystem, deer, mice, and even elephants are herbivores. They eat grasses, shrubs, and trees. In a desert ecosystem, a mouse that eats seeds and fruits is a primary consumer. In an ocean ecosystem, many types of fish and turtles are herbivores that eat algae and seagrass.

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

The next level is made up of tertiary consumers that eat secondary consumers. In the desert ecosystem, an owl or eagle may prey on a snake.

Top predators, also called apex predators, eat other consumers. Lions are apex predators in the grassland ecosystem. In the ocean, fish such as the great white shark are apex predators. 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 are organisms that eat nonliving plant and animal remains. For example, scavengers such as vultures eat dead animals.

Decomposers, like fungi and bacteria, complete the food chain by turning organic waste, such as decaying plants, into inorganic materials, such as nutrient-rich soil.

For example, grass in a forest clearing produces its own food through photosynthesis. A rabbit eats the grass and then a fox eats the rabbit. When the fox dies, decomposers such as worms and mushrooms break down its body, returning it to the soil where it provides nutrients for plants like grass.

Biomass

Food webs are defined by their biomass—the energy in living organisms. Autotrophs, the producers in a food web, convert the sun's energy into biomass. Biomass decreases with each trophic level. There is always more biomass in lower trophic levels than in higher ones.

Because biomass decreases with each trophic level, there are always more autotrophs than herbivores in a healthy food web. There are more herbivores than carnivores.

A healthy food web has an abundance of autotrophs, many herbivores, and relatively few carnivores and omnivores. This balance helps the ecosystem maintain and recycle biomass.

Every link in a food web is connected to at least two others. When one link is threatened, other links are weakened or stressed and the ecosystem's biomass declines.

The loss of plant life usually leads to a decline in the herbivore population. Plants can disappear due to drought, disease, or human activity. Forests are cut down to provide lumber for construction. Grasslands are paved over for shopping malls or parking lots.

Bioaccumulation

Toxic chemicals increase with each level in the food web.

When an herbivore eats a plant or other autotroph that is covered in pesticides, for example, those pesticides are stored in the animal's fat. When a carnivore eats several of these herbivores, it takes in the pesticide chemicals stored in its prey. This process is called bioaccumulation.

Bioaccumulation happens in marine ecosystems too. Runoff from urban areas or farms can be full of pollutants. Tiny producers such as algae, bacteria, and seagrass absorb minute amounts of these pollutants. Primary consumers, such as sea turtles and fish, eat the seagrass. Predators, such as sharks or tuna, eat the fish. By the time people eat the tuna, it may have a large amount of bioaccumulated toxins in its body.

A pesticide called DDT (dichloro-diphenyl-trichloroethane) was a major reason for the decline of the bald eagle. In the 1940s and 1950s, DDT was used to kill insects that spread diseases. DDT built up in the soil and water. Worms, grasses, algae and fish took in the DDT. As a consequence, eagles and other apex predators preyed on fish and small mammals that were poisoned with DDT.

Birds with high amounts of DDT in their bodies lay eggs with extremely thin shells. These shells would often break before the baby birds were ready to hatch. The bald eagle population declined.

Today, the use of DDT has been restricted. Food webs, which include the bald eagle, have recovered in most parts of the country.