Productivity is the rate of biomass production. It is expressed in terms of g–2yr–1 or (kcal m–2) yr–1 to compare the productivity of different ecosystems. Biomass refers to the total weight of living matter and organic material per unit area. Productivity is the rate at whichchemical energy is produced in an ecosystem – expressed as gram of organic matter per square meter per year. In other words, ecosystem productivity represents the total amount of energy (organic matter) fixed or stored by the autotrophs per unit time in theecosystem. Productivity is divided in to two, primary productivity and secondary productivity.
Primary production is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight or energy. Or Primary productivity means the rate of food produced by producers(autotrophs) or the rate of solar energy trapped at first trophic level. Primary productivity is again classified in to two, Gross primary productivity and Net primary productivity. Gross Primary Productivity (GPP) means the total amount of energy produced by producers(autotrophs) at trophic level one. Or in another words it is the rate at which an ecosystem’s producers convert solar energy into chemical energy as biomass. This gross productivity depends up on photosynthetic capacity of producers and environmental factors including climatic conditions such as temperature, rainfall and total solar radiation and other nutrient materials of the abiotic environment such as nitrogen, phosphorus and sulphur.
Net Primary Production (NPP) represents the amount of energy or organic matter fixed or stored at trophic level one (Producers) after respiration and which is available to higher trophic levels.
Net Primary Productivity = Gross Primary Productivity – Respiration Rate (Rate at which producers use biomass)
The primary productivity of a natural ecosystem largely depends on the amount of solar radiation; there is positive correlation betweenprimary productivity and solar radiation. Since there is marked decrease in solar radiation received at the earth’s surface from equator towards the poles, primary productivity also, on an average (besides a few intermediate zones of exception), decreases markedly towards the poles. This results in spatial variations in primary productivity at regional and local scales. At a very large scale the primary productivity of the terrestrial ecosystems is far more than the marine ecosystems. E.P Odum (1959) has identified three levels of productivity at world scale as:
- The regions of high ecological productivity represented by shallow water areas, moist forest (tropical and temperate), alluvial plains and regions of intensive farming.
- The regions of low ecological productivity represented by arctic snow-covered wastelands deserts and deep ocean areas.
- Regions of intermediate ecological productivity e.g., grasslands, shallow lakes and farmlands except intensively cultivated areas.
Tropical rainforest have highest primary productivity and is least in desert. In aquatic ecosystem primary productivity is limited by light.
The potential energy resulting from primary production furnishes the energy required by other trophic levels in an ecosystem. Some energy in the form of food is consumed by herbivores or omnivores which may be eaten by carnivores which in turn may be eaten by other carnivores. However, much of this ingested food is not assimilated; herbivores may assimilate only 10 per cent of the ingested food, whereas the unassimilated materials leave the animal’s body as waste materials to serve as energy source for other organisms. Assimilated energy is used by the consumers for various metabolic processes like respiration, excretion, reproduction etc. The resultant amount of energy storedin the tissues of heterotrophs is termed as net Secondary Production.
In terrestrial ecosystems the order of productivity in decreasing order is 1. Swamps, marshes, tropical rain forests (most productive), 2.Temperate forest, 3.Northern coniferous forest (taiga), 4. Savanna, 5.Agricultural land, 6.Woodland and shrub land, 7.Temperate grassland, 8.Tundra (arctic and alpine), 9.Desert scrub, 10. Extreme desert (least productive). In Aquatic ecosystems, productivity in decreasing manner is 1.Estuaries (most productive), 2.Lakes and streams, 3.Continental shelf, 4.Open ocean (least productive).