DDT is found in water, soil, and organisms all over the Earth. DDT was originally used on plants and soil, but now it is found in most organisms. This is because of the feeding relationships among the organisms in an ecosystem. The feeding relationships within an ecosystem can be grouped into trophic levels, based on the organism's main source of nutrition.

For the most part, the organisms in the first trophic level are photosynthetic organisms. They use light energy to synthesize organic compounds. These autotrophs are known as primary producers. On land, the main primary producers are plants. In lakes and oceans, aquatic plants, algae, and cyanobacteria are the main primary producers. Primary producers support all other organisms in an ecosystem.

The organisms in the trophic levels above the primary producers are heterotrophs. Organisms that survive by consuming primary producers are called primary consumers. Since they eat mostly plants, the primary consumers are usually herbivores. Organisms that eat herbivores are carnivores, which make up the secondary consumers. Carnivores that eat carnivores are tertiary consumers. There can be even higher level consumers: carnivores that eat carnivores that eat carnivores. Omnivores feed on primary producers and at least one level of consumers.

A group of consumers called the detritivores link all of the trophic levels. Detritivores obtain energy from detritus. Detritus is nonliving organic material, including the remains of dead organisms, leaves, and feces. Because of how they get energy, detritivores are sometimes called decomposers. Detritivores are generally prokaryotes and fungi. But some animals, like earthworms, also feed on detritus.

To follow the path between trophic levels, we can build a food chain. A food chain shows how food is transferred between levels. Usually, feeding relationships of the organisms in an ecosystem aren't as simple as the food chain shown here. Several primary consumers may feed on the same primary producer. Some higher level consumers, may feed on several levels of lower consumers.

It is easier to interlink the feeding relationships in an ecosystem into a food web. A food web shows how some organisms feed at several different trophic levels. All levels of the web are connected to the detritivores. This food web shows the particular organisms involved. The diagram can be simplified to show what trophic level each organism occupies. Some food webs don't explicitly state what each branch point represents, but we can figure it out.

Since all organisms are food for H, H must represent the detritivores. Organism A doesn't have a food source on the diagram, so it must represent a primary producer. B, C, and D feed on the primary producer, so they must be primary consumers. D also feeds on C, so it’s a secondary consumer. In this way, we can figure out the trophic level of each symbol in the food web.

To see how well you understand food webs, drag the labels to the appropriate letter. Click Submit to see if you're correct.

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All organisms are food for C, so it's a detritivore. B has no food source, so it's a primary producer. D has only B as a food source, so it's a primary consumer. F has both B and D as food sources, so it’s a primary/secondary consumer. A has D and F as food sources, making it a secondary/tertiary consumer. E feeds on A, making it a tertiary/quaternary consumer.

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