Definition

A food chain is a line that shows who eats whom, starting with a producer such as a plant and pointing each arrow from the food to the eater, as in grass to grasshopper to frog to snake to hawk, with the arrows tracking the direction energy travels. More formally, a food chain is a linear sequence of organisms through which energy and nutrients pass as each is eaten by the next, beginning with a producer and continuing through primary, secondary, and tertiary consumers, with decomposers acting at every level; because only about $10$ percent of the energy at one level is stored in the biomass of the next, most food chains have only four or five links, and a food chain is really a simplification of the food web that better captures how real animals eat many things. At the ecosystem level, a food chain's length is constrained by energetic inefficiency, roughly $10$ percent transfer per level, by dynamic stability, since longer chains recover more slowly from disturbance, and by ecosystem size and productivity, with grazing chains beginning from living producers and detrital chains from dead organic matter, often carrying more energy in forests and streams, and biomagnification of fat-soluble pollutants such as mercury and DDT increasing with chain length.