Productivity of an Ecosystem

Productivity is the rate at which an ecosystem produces new biomass per unit area in a unit of time, usually expressed in grams per square metre per year or kilocalories per square metre per year. The chapter recognises three classes. Primary productivity is the organic matter or chemical energy that autotrophs generate through photosynthesis and chemosynthesis, and it underwrites the energy needs of every other organism in the ecosystem, right up to humans. Within primary productivity, Gross Primary Productivity (GPP) is the total amount of organic matter an ecosystem's producers manufacture through photosynthesis, while Net Primary Productivity (NPP) — also called apparent photosynthesis — is what remains once the plant's own respiratory losses are subtracted, so that NPP = GPP − Respiration; globally, NPP across the whole biosphere is estimated at roughly 170 billion dry-weight tonnes a year, of which the world's oceanic producers alone contribute about 55 billion tonnes. Secondary productivity is the energy stored in the tissues of heterotrophs (consumers), split into gross secondary productivity (total plant material ingested by herbivores, minus what is lost as faeces) and net secondary productivity (what consumers actually retain as stored biomass after their own respiratory losses). Community productivity is the net rate at which an entire group of plants together synthesises organic matter per unit area per unit time. Primary productivity is shaped by the plant species present, their photosynthetic capacity, nutrient availability, solar radiation, rainfall, soil type, and topographic factors such as altitude, latitude and slope direction — which is why it is size-independent: a sm …