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 is a network of links in a food web starting from producer organisms (such as grass or algae which produce their own food via photosynthesis) and ending at an apex predator species, detritivores, or decomposer species. The food chain also explains the feeding pattern or relationship between living organisms. Each level of a food chain represents a different trophic level, starting with producers at the bottom, followed by primary, secondary, and tertiary consumers. The food chain consists of four major parts, namely the sun, producers, consumers, and decomposers.

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. A food web, on the other hand, shows different paths where plants and animals are connected.

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. Because energy, in the form of heat, is lost at each step, or trophic level, chains do not normally encompass more than four or five trophic levels.

In summary, a food chain is a sequence of organisms in an ecosystem where one organism eats another organism, and later that organism is consumed by another larger organism. It explains the feeding pattern or relationship between living organisms and consists of producers, consumers, and decomposers. 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 levels.