Energy Flow Through Ecosystems

Energy flows in one direction through an ecosystem: from the sun to producers (autotrophs, mainly photosynthetic organisms) to primary consumers (herbivores) to secondary consumers (carnivores that eat herbivores) and sometimes tertiary or quaternary consumers. Unlike matter, energy is not recycled - it is ultimately lost as heat to the environment in accordance with the second law of thermodynamics, which is why ecosystems require a continuous energy input from the sun (or chemical energy for chemoautotroph-based systems).

Decomposers (bacteria, fungi) and detritivores act at every trophic level, breaking down dead organisms and waste products. This releases the chemical energy remaining in that matter as heat while returning nutrients (not energy) to the abiotic environment for producers to reuse - nutrient cycles are closed, but energy flow is not.

At each transfer between trophic levels, only about 10% of the energy is incorporated into the biomass of the next level; the remaining ~90% is lost as metabolic heat (respiration), used for movement and other life processes, or contained in feces and undigested/unassimilated material. This progressive loss is why food chains rarely extend beyond four or five trophic levels - insufficient energy remains to support another level.

This pattern generates an energy pyramid that is always upright (widest at the producer base). Biomass pyramids are usually but not always upright - in some aquatic systems, fast-reproducing, short-lived phytoplankton support a larger standing biomass of zooplankton, briefly inverting the pyramid at any given moment even though total energy captured over time still follows the 10% rule.

Gross primary productivity (GPP) is the total amount of energy producers capture and convert into chemical energy through photosynthesis. Net primary productivity (NPP) equals GPP minus the energy producers themselves use in cellular respiration; NPP represents the energy actually available to be consumed by heterotrophs or stored as biomass.

NPP varies dramatically by ecosystem: tropical rainforests and coral reefs/estuaries have very high productivity due to abundant light, warmth, and water/nutrients, while deserts and open ocean have low productivity due to limiting factors (water scarcity or nutrient limitation, respectively). Productivity sets the energy ceiling for how much life a given ecosystem can support at all higher trophic levels.