Ecological productivity refers to the primary fixation of solar energy by plants and the subsequent use of that fixed energy by plant-eating herbivores, animal-eating carnivores, and the detritivores that feed upon dead biomass. This complex of energy fixation and utilization is called a food web.
Ecologists refer to the productivity of green plants as primary productivity. Gross primary productivity is the total amount of energy that is fixed by plants, while net primary productivity is smaller because it is adjusted for energy losses required to support plant respiration. If the net primary productivity of green plants in an ecosystem is positive, then the biomass of vegetation is increasing over time.
Gross and net secondary productivities refer to herbivorous animals, while tertiary productivities refer to carnivores. Within food webs, a pyramid-shaped structure characterizes ecological productivity. Plants typically account for more than 90% of the total productivity of the food web, herbivores most of the rest, and carnivores less than 1%. Any dead plant or animal biomass is eventually consumed by decomposer organisms, unless ecological conditions do not allow this process to occur efficiently, in which case dead biomass will accumulate as peat or other types of nonliving organic matter.
Because of differences in the availabilities of solar radiation, water, and nutrients, the world's ecosystems differ greatly in the amount of productivity that they sustain. Deserts, tundra, and the deep ocean are the least productive ecosystems, typically having an energy fixation of less than 0.5 × 103 kilocalories per square meter per year (thousands of kcal/m2/yr; it takes one calorie to raise the temperature of one gram of water by 34°F [1°C] under standard conditions, and there are 1,000 calories in a kcal). Grasslands, montane and boreal forests, waters of the continental shelf, and rough agriculture typically have productivities of 0.5-3.0 × 103 kcal/m2/yr. Moist forests, moist prairies, shallow lakes, and typical agricultural systems have productivities of 3-10 × 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 productivities of 10-25 × 103 kcal/m2/yr.
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