The capacity to do work is known as energy. In an ecosystem energy flows through one organism to another in the form of food. The flow of energy is uni-direction and non cycle and governed by laws of thermodynamics.

1.       Energy can neither be created nor can be destroyed but may convert into different forms.

2.       Energy Transfer is always accompanied by its dispersion. The second law of thermodynamics says that when energy is changed from one form to another, some of it is lost in the form of heat.

In an ecosystem the solar energy is trapped and stored by; producers (plants) in the form of chemical energy: producers form first trophic level and pass this energy to primary consumer is unidirectional and at successive trophic level the amount of energy decreases.

Green plants capture only a small fraction of solar energy(<5% of total solar radiation) and bulk of un-utilized energy is dissipated in the form of heat. Some of this energy is used by green plants for their respiration and maintenance and rest is convened into food which is eaten by herbivores. It has been estimated that herbivores use only 10% of the gross productivity of producers.

The herbivories use a part of this energy for their growth and maintenance and rest is given out as fecal matter. When carnivores eat herbivores they get only 10% of herbivores energy. The net energy keeps on decreasing with the increase of trophic level.

Only 10% of energy of a trophic level is transferred to higher trophic level. This is known as 10% law.