Except for the deep sea hydro-thermal ecosystem, the sun is the only source of energy for all ecosystems on Earth. Of the incident solar radiation, less than 50 per cent is photosynthetically active radiation (PAR).

Plants and photosynthetic bacteria (autotrophs) capture only 2-10 per cent of the PAR and fix the sun's radiant energy to make food from simple inorganic materials. This small amount of energy sustains the entire living world. In a terrestrial ecosystem, major producers are herbaceous and woody plants, while in an aquatic ecosystem, producers include phytoplankton, algae, and higher plants.

All organisms are dependent for their food on producers, either directly or indirectly. Organisms of different trophic levels in nature are connected to each other for food or energy relationships, forming food chains and webs. The energy trapped by the producer is either passed on to a consumer or the organism dies, which marks the beginning of the detritus food chain.

The flow of energy is unidirectional – from the sun to producers and then to consumers. Food is transferred from the producers to decomposers. No energy that is trapped into an organism remains in it forever.

Ecosystems are not exempt from the Second Law of thermodynamics. They need a constant supply of energy to synthesise the molecules they require, to counteract the universal tendency toward increasing disorderliness.