The functioning of ecosystem depends on an inflow of energy. This is provided by sunlight for most part of the ecological quanta, of which are captured by green plants and used in growth of metabolism or are perhaps stored in chemical bonds in the plant starch, carbohydrate and other components. Animal obtain their energy from sunlight indirectly in its transferred state., stored in chemical compound within the plants that herbivores eat. Carnivores in turn eat obtain their energy, third hand, from the plant eaters. The host of parasite, decay bacteria or fungi and other micro-organism that derive energy from living or dead animal may be served steps further removed from the original sunlight source.

In each energy transfer from plants to herbivores to carnivores some energy is lost. No energy transfer can be 100% efficient. Hence quantity of green plant material, with its stored energy, must be large in relation to quantity of herbivores animals that feed on it.

            Because of energy relationship, it is sometimes useful to show the distribution of living organisms within ecosystem in the form of a biotic pyramid, board at the base and tapering towards top. At the base are all green plants, in the middle the herbivores, on the top, the carnivores with their numbers or total weight (biomass), tending to decrease upward from layer to layer.