A food chain is a linear sequence showing how organisms are related through the transfer of energy from one organism to another as they eat and are eaten. Food chains were first introduced in the 9th century and later popularized in a 1927 book. They involve repeated eating with each group eating smaller ones and being eaten by larger ones, transferring energy unidirectionally from producers to consumers in trophic levels, with about 80-90% of energy lost at each transfer. Food chains in an ecosystem were provided as examples.