Productivity

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Productivity

Productivity is an ecological term referring to the fixation of solar energy by plants and algae, and the subsequent utilisation of that fixed energy (or biomass) by plant-eating herbivores, animal-eating carnivores, and detritivores that feed upon dead biomass. This complex of energy fixation and utilisation in an ecosystem is called a food web.

Ecologists refer to the productivity of green plants (and algae) as primary productivity, meaning it comprises the base of food webs. Gross primary productivity is the total amount of energy fixed by plants. Net primary productivity is smaller because it is adjusted for energy losses associated with plant respiration. If the net primary productivity of green plants in an ecosystem is positive, then the biomass of vegetation is increasing over time.

The productivity of herbivorous animals (which feed on plant biomass) is known as secondary productivity (both gross and net). That of carnivores (which feed on other animals) is known as tertiary productivity (again, both gross and net). The biomass of dead plants and animals in the ecosystem is consumed by decomposer organisms, in a detrital food web. (In some cases, environmental conditions may not allow decomposition to occur efficiently, usually because waterlogged soil restricts the supply of oxygen. In such ecosystems, dead biomass accumulates as peat or another kind of non-living organic matter.)

Ecological productivity within food webs always has a pyramid-shaped structure. This means that the productivity of plants is much larger than that of herbivores, which in turn is much greater than that of their predators. Plants typically account for more than 90% of the total productivity of food webs, herbivores most of the rest, and carnivores less than 1%.

Because of differences in the availability of solar radiation, water, and nutrients, the world's ecosystems differ greatly in the amount of productivity that they sustain. The least productive ecosystems are desert, tundra, and the deep ocean. These typically have a rate of energy fixation of less than 0.5 x 10³ kilocalories per square meter per year (thousands of kcal/m²/yr; a calorie is the amount of energy needed to raise the temperature of one gram of water by one degree C under standard conditions; there are 1000 calories in a kcal). Grassland, montane and boreal forest, water of the continental shelves, and rough agriculture typically have a productivity of 0.5-3.0 x 10³ kcal/m²/yr. Moist forest, moist prairie, shallow lakes, and typical agricultural systems have a productivity of 3-10 x 10³ kcal/m²/yr. The most productive ecosystems are fertile estuaries and marshes, coral reefs, terrestrial vegetation on moist alluvial deposits, and intensive agriculture, which can have a productivity of 10-25 x 10³ kcal/m²/yr.