Energy flow

Apart from deep-sea hydrothermal ecosystems, every ecosystem on Earth runs on energy from the sun.

Less than 50 per cent of the incident solar radiation is photosynthetically active radiation (PAR).

Of this PAR, plants trap just 2-10 per cent, yet that small share keeps the whole living world going.

All organisms depend on producers for food, directly or indirectly, so energy flows one way: from the sun to producers and then to consumers.

Ecosystems obey the second law of thermodynamics. They need a constant energy supply to build the molecules they need and to counter the universal trend towards disorder.

Producers in terrestrial ecosystems are mainly herbaceous and woody plants; in aquatic ecosystems they are phytoplankton, algae and higher plants.

Energy trapped in an organism does not stay there for ever. It passes to a consumer, or the organism dies, and death starts the detritus food chain.