Organisms are complex biochemical machines that require a constant consumption of energy to grow, reproduce, and maintain their biological integrity. The use of energy must obey physical principles: the laws of thermodynamics. Constraints imposed by these principles have profound influence in the flow and conservation of energy and therefore the structure of an ecological community.

Energy from the sun powers the world's ecological communities. Solar energy is channeled into an ecological community by way of photosynthesis in green plants and many other photosynthetic microorganisms. Energy harvested by photosynthesis is used to produce plant tissue where light energy is saved as chemical energy. This chemical energy is transferred when plants are eaten by herbivores (plant-eating animals). Energy stored in herbivores can further be transferred to carnivores (animal-eating animals). This sequence of energy transfer from plants to herbivores and then carnivores is called a food chain...