The flow of energy that occurs along a food chain. Energy enters the food chain at the level of the producers (usually plants) in the form of solar energy. The plants convert solar energy into chemical energy in the process of photosynthesis. Chemical energy is passed from one trophic level to the next through feeding. Since a large proportion of energy is lost at each trophic level, mostly in the form of heat energy due to respiration, a food chain does not normally consist of more than five trophic levels: the fifth trophic level does not contain enough energy to support further levels. Energy is also lost from the food chain in excretory products and the remains of dead organisms; this is converted into heat energy by the action of decomposers. See also productivity; pyramid of energy.