Energy Flow

Energy flow is the process by which energy enters, moves through, and is ultimately dissipated from an ecosystem, and it is one of the two defining currencies -- energy and nutrients -- that every ecosystem continuously handles, though the two behave very differently once inside the system.

For almost every ecosystem on Earth, the sun is the ultimate and near-exclusive source of energy (the only notable exceptions being certain deep-sea hydrothermal-vent communities, which rely instead on chemosynthetic bacteria). Of the total solar radiation that reaches the Earth's surface, however, only a portion falls within the wavelength range that plants can actually use for photosynthesis; this usable portion is called photosynthetically active radiation, or PAR, and it makes up less than half -- roughly 40 to 50 per cent -- of the total incident solar radiation. Of this PAR itself, green plants are able to capture and convert into stored chemical energy only a small fraction, generally cited as somewhere between about 2 and 10 per cent, with the remainder reflected, transmitted through leaves, or simply not absorbed by chlorophyll and other pigments. Despite this seemingly tiny efficiency, the absolute quantity of energy involved is enormous, and this small captured fraction is what sustains essentially all life in the ecosystem.

Once energy has been captured by producers, it moves through the ecosystem strictly in one direction -- from producers to primary consumers, from primary consumers to secondary consumers, and so on up the food chain -- and it can never flow in reverse, from a consumer back down to a producer, and it can never be reused once it has been dissipated as heat. This unidirectional character sets energy flow sharply apart from the flow of mineral nutrients through an ecosystem, which move in repeating cycles (biogeochemical cycles) and can be used again and again by successive generations of organisms.

The amount of energy actually transferred from one trophic level to the next is governed by what is widely known as the ten per cent law, first articulated by the ecologist Raymond Lindeman in 1942: at each transfer from one trophic level to the next, only about ten per cent of the energy present at the lower level is incorporated into the biomass of, and thus made available to, the level above it, while the remaining roughly ninety per cent is lost, chiefly as metabolic heat released during respiration, and to a smaller extent through matter that is simply never consumed or is excreted as waste. Because this loss compounds at every single step, the total energy available shrinks very rapidly indeed as one moves up a food chain -- for instance, starting from 10,000 kilocalories per square metre per year fixed by producers, only about 1,000 kilocalories would typically become available to primary consumers, about 100 kilocalories to secondary consumers, and a mere 10 kilocalories to tertiary consumers. …