Productivity

Productivity describes the rate at which biomass, or organic matter, is generated within an ecosystem, and is one of the key functional processes -- alongside decomposition, nutrient cycling and energy flow -- that keep an ecosystem running. Because it is defined as a RATE, productivity is always measured over a stated interval of time, not as a single fixed quantity, and this is what distinguishes it from simply describing how much biomass is present at one instant.

Primary productivity is the rate at which producers (autotrophs) generate new biomass through photosynthesis, and it is conventionally split into two related measures. Gross primary productivity (GPP) is the TOTAL rate at which producers fix or capture energy through photosynthesis, before any of that captured energy is spent on the producers' own life processes. Net primary productivity (NPP) is what remains of that captured energy after subtracting the producers' own respiratory losses (R), and is calculated as GPP minus R. NPP is the ecologically more important figure of the two in practice, because it represents the biomass that is genuinely available to be consumed by herbivores and, further up the food chain, by carnivores -- GPP alone overstates how much organic matter is actually accumulating and available for the rest of the ecosystem, since a portion of it is immediately used up by the producers themselves simply to stay alive.

Because productivity is a rate measured over time, ecologists distinguish it carefully from two related but different snapshot concepts. The standing crop is the quantity of biomass present in a community at one particular point in time, usually expressed as the mass of living organisms (biomass) or as the number of individuals present per unit area or volume; a forest's standing crop of trees, for instance, is enormous even though the forest's yearly rate of NEW biomass production (its productivity) may be comparatively modest once the forest has matured. The standing state, by contrast, refers to the amount or concentration of a particular nutrient present in the soil, water or sediment of an ecosystem at a given time -- it describes the pool of an available chemical resource rather than the pool of living biomass, and the two terms should not be used interchangeably even though both are 'snapshot in time' rather than 'rate over time' measures.

Primary productivity is typically expressed in one of two comparable kinds of units: as a mass of dry organic matter produced per unit area per unit time, most often written as grams per square metre per year (g/m^2/yr), or as an energy equivalent expressed in kilocalories per square metre per year (kcal/m^2/yr), since a known mass of organic matter corresponds to a known amount of stored chemical energy. …