A food chain is the sequence in which energy and nutrients pass from producers to successive consumers in an ecosystem; each step in the chain is a trophic level, and interlinked chains form a food web.
Ecologists in the early twentieth century realised that ecosystems could be understood by following where energy goes. The British ecologist Charles Elton described food chains and the "pyramid of numbers" in 1927, noting that there are many small animals at the bottom and few large predators at the top. In 1942 the American ecologist Raymond Lindeman studied a lake in Minnesota and showed that only a small fraction of energy at one level is converted into living tissue at the next; the rest is used for respiration or lost as heat. This became the basis of the "ten per cent law".
The underlying reason is the laws of thermodynamics. Energy cannot be created, only transformed, and each transformation loses some as heat. Because so much is lost at each step, food chains rarely have more than four or five links: there simply is not enough energy left to support another level of predators.
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