Energy flow in an ecosystem is unidirectional. It means that energy which is absorbed by the producers from sun and is converted into chemical energy, always flow from a lower trophic level to a higher trophic level as in a food chain when herbivore consumes producers the energy is transferred from producers to consumers and so on. Some of this energy is assimilated some is used by the organism and some is dessipated in the environment but the flow of energy is never reverted, that is obviously a producer can not consume a consumer.
