A food chain is a linear sequence of organisms in an ecosystem where one organism eats another organism, and later that organism is consumed by another larger organism. It starts with the producer organism, such as grass or algae, which produces its own food via photosynthesis, and ends at an apex predator species, detritivores, or decomposer species. The food chain also shows how organisms are related to each other by the food they eat, and each level of a food chain represents a different trophic level.

A food chain differs from a food web because the complex network of different animals feeding relations are aggregated, and the chain only follows a direct, linear pathway of one animal at a time. The length of a food chain is a continuous variable providing a measure of the passage of energy and an index of ecological structure that increases through the linkages from the lowest to the highest trophic (feeding) levels.

The food chain consists of four major parts, namely the sun, producers, consumers, and decomposers. The sun is the initial source of energy, which provides energy for everything on the planet. Producers, who make their own food using photosynthesis or chemosynthesis, make up the bottom of the trophic pyramid. Primary consumers, mostly herbivores, exist at the next level, and secondary and tertiary consumers, omnivores and carnivores, follow. At the top of the system are the apex predators: animals who have no predators other than humans.

Food chains are important because they help us understand the feeding pattern or relationship between living organisms. They also help us realize how one organism is dependent upon another organism for survival. By understanding food chains, we can better understand the impact of changes in one part of an ecosystem on other parts of the ecosystem.