A series of organisms, each successive group of which feeds on the group immediately previous in the chain, and is in turn eaten by the succeeding group. Green plants (producers) are normally the first step in a food chain, herbivores (primary consumers) the second step, with carnivores (secondary consumers) making up the remaining two or three stages. The different stages are termed trophic levels and organisms that are removed from the beginning of the chain by the same number of steps are said to occupy the same trophic level. The chain is effectively a series of energy transfers, with considerable amounts of energy being lost at each transfer (see bioenergetics
). A food chain consequently rarely has more than four or five stages since there is not enough energy in the system to maintain more. A consequence of this is that, for a given consumer level, considerably more individuals can be maintained if an earlier consumer stage is omitted. For example, a given amount of grain will support many more people if it is eaten as grain, rather than being first converted to meat by feeding to livestock.
Food chains rarely exist in isolation but are interconnected to form a food web. Thus any of the links in one food chain may, through the activity of decomposers, saprophytes, or parasites, lead into a number of different food chains. In addition, most plants and herbivores have a number of different predators.