Ecosystems require a constant input of energy as every component of an ecosystem is regularly dissipating energy.

Two laws of thermodynamics govern this flow of energy. According to the first law of thermodynamics, energy can be transferred as well as transformed but is neither created nor destroyed. According to the second law of thermodynamics, every activity involving energy transformation is accompanied by the dissipation of energy. Except for deep hydrothermal ecosystems, the source of energy in all ecosystems is solar energy. 50% of the solar energy incident over the earth is present in PAR (photosynthetically active radiation).

Energy flow in an ecosystem is always unidirectional or one way, i.e., solar radiation → producers → herbivores → carnivores. It cannot pass in the reverse direction. There is a decrease in the content and flow of energy with the rise in trophic level. Only 10% of energy is transferred from one trophic level to the next.

Producer biomass (1000 K cal) → Herbivore biomass (100 K cal) → Carnivore I biomass (10 Kcal) Carnivore II biomass (1 Kcal)