Food chain

A food chain is one straight path of who eats whom in an ecosystem. It starts with a primary producer, such as a plant or a photosynthetic alga, that captures energy from sunlight. A herbivore that eats the producer is the primary consumer, a carnivore that eats the herbivore is the secondary consumer, and so on up to an apex predator that nothing else regularly hunts. Each step is called a trophic level. Decomposers such as bacteria and fungi break down dead material from every step and return its nutrients to the soil or water.

Food chains show why energy limits ecosystems. At each transfer most of the energy is spent on staying alive or lost as heat, and often only about a tenth passes to the next level, so chains rarely run beyond four or five links. Real animals usually eat, and are eaten by, many species, so ecologists join overlapping chains into a food web, which gives a more realistic picture.

A typical exam example runs from grass to grasshopper, frog, snake and hawk. In the ocean a very short chain leads from phytoplankton to krill to the blue whale. Students also use food chains to explain biomagnification, in which long-lasting toxins become more concentrated at each step up.