Every organism needs food in order to live and has to get that food from somewhere. Every organism can be classified by where it fits into the food chain. Most broadly, all organisms fit into one of three camps: producers, consumers, and decomposers.

Producers are able to produce carbohydrates from the energy of the sun through photosynthesis or, in some instances, from inorganic molecules through chemosynthesis. Because they can produce their own food, producers are also called autotrophs. Producers form the foundation of every food chain because only they can transform inorganic energy into energy that all other organisms can use. On land, plants and photosynthetic bacteria are the main producers. In marine environments, green plants and algae are the main producers. In deep water environments near geothermal vents, chemosynthetic organisms are the main producers.

Consumers cannot produce the energy and organic molecules necessary for life; instead, consumers must ingest other organisms in order to get these materials. Consumers are also called heterotrophs because they must consume other organisms in order to get the energy necessary for life. There are three types of consumers; the categories of consumers are based on which organisms a particular consumer preys on. Primary consumers, such as sheep, grasshoppers, and rabbits, feed on producers. Since all producers are plants or plantlike, all primary consumers are herbivores, which is the name for a plant-eating animal. Secondary consumers eat primary consumers, making them carnivores—animals that eat other animals. Foxes and insect-eating birds are examples of secondary consumers. Tertiary consumers eat secondary consumers and are therefore carnivores. Polar bears that eat sea lions are tertiary consumers. Consumers that eat both producers and other consumers are called omnivores.

Also called saprophytes, decomposers feed on waste or dead material. Since they must ingest organic molecules in order to survive, decomposers are heterotrophs. In the process of getting the energy they need, decomposers break down complex organic molecules into their inorganic parts—carbon dioxide, nitrogen, phosphorus, etc.

All predatory interactions between producers and consumers in a community can be organized in food chains or more complex and realistic food webs. A food chain imagines a strictly linear interaction between the levels of producers and consumers we described above. An abstract food chain appears below on the left, with examples of animals that fit each category appearing on the right:

Each step in the food chain is referred to as a trophic level.

Food chains are simple and help us to understand the predation interactions between organisms, but because they are so simple, they aren’t really accurate. For instance, while sparrows do eat insects, they also eat grass. In addition, the food chain makes it seem as if there are only four populations in a community, when most communities contain far more. Most organisms in a community hunt more than one kind of prey and are hunted by more than one predator. These numerous predation interactions are best shown by a food web:

In fact, the more diverse and complicated the food relationships are in a community, the more stable that community will be. Imagine a community that was correctly described by the food chain grassinsectssparrowshawks. If some blight struck the grass population, the insect population would be decimated, which would destroy the sparrow population, and so on, until the very top of the food chain. A more complex food web is able to absorb and withstand such disasters. If something were to happen to the grass in the food web, the primary consumers would all have some other food source to tide them over until the grass recovered.

Each trophic level in a food web consumes the lower trophic level in order to obtain energy. But not all of the energy from one trophic level is transferred to the next. At each trophic level, most of the energy is used up in running body processes such as respiration. Typically, just 10 percent of the energy present in one trophic level is passed along to the next. If the energy present in the producer trophic level of a food web is kcal, you could draw an energy pyramid to show the transfer of energy from one trophic level to the next:

The energy lost between each trophic level affects the number of organisms that can occupy each trophic level. If the secondary consumer trophic level contains 10 percent of the energy present in the primary consumer level, it follows that there can only be about 10 percent as many secondary consumers as there are primary consumers. The energy pyramid is therefore also a biomass pyramid that shows the number of individuals in each trophic level.

Because biomass drops so dramatically from one trophic level to the next, any chemical present in a lower trophic level becomes heavily concentrated in higher trophic levels. Beginning in the 1940s, a pesticide called DDT was sprayed on crops to stop invading insects. The concentration of DDT in any local area was enough to kill insects, but not enough to hurt any of the larger organisms. But as each predator ate its prey, the DDT became concentrated in successive trophic levels. The small levels of DDT found in the insects became much more concentrated as it was swallowed and digested by predators. Eagles, sitting at the top of the food web, took in massive amounts of DDT in the course of eating their prey. The DDT caused the eagles to lay soft eggs that could not protect the developing embryos inside, which led to a severe population decline.