Productivity in an ecosystem is the quantity of biomass being generated over a specified time period within the given geographic area. Productivity exists at all levels but they differ from each level. Based on the level and nature of dependency of th eindividuals for productivity, it can be classified as primary productivity and the secondary productivity. In order for the productivity to be transferred from one trophic level to another, the direction of energy flow must be ascertained, however, it is usual that the energy transfers from autotrophs towards the tertiary organisms. Let us discuss the various concepts of productivity in an ecosystem.
Types of productivity
Primary productivity is the synthesis of new organic material by utilizing the inorganic molecules such as H2O and CO2( i,e the process of photosynthesis). Secondary productivity is the production of biomass by heterotrophic or consumer organisms in a system by consuming the primary (plant) products. Various animals, fungi and bacteria will assist in secondary production. Productivity can be mathematically expressed by using certain units.
Formula for productivity
Productivity is expressed either in terms of dry matter produced or energy captured within a unit area of land, in a given unit of time. So, mathematically it is the energy in calories/cm2/year or dry organic matter in grams/m2/year (g/m2 x 8.92 = lb/acre). Productivity can also be divided into gross primary productivity (GPP) and net primary productivity (NPP). Gross primary productivity is simply the production of biomass by photosynthesis where plants consume a lot of carbon dioxide (respiration) to synthesize energy. Whereas the net primary productivity (NPP) is written as GPP – R = NPP. “R” stands for respiration. In simple terms, net primary productivity is the amount of biomass available for the consumption of herbivores or heterotrophs. The NPP per year across the whole biosphere is roughly about 170 billion tons, out of which 70 % is occupied by earth’s surface productivity and the rest by oceans.
Factors affecting productivity
A. Standing crop
B. Materials removed, and
C. Production rate
Standing crop is the abundance of organisms at a given point of time in a given ecosystem. It can be expressed in terms of a number of individuals, or the amount of energy. The standing crop reveals the concentration of different populations within an ecosystem.
The materials removed is calculated from the particular area per unit time. It is the cumulative yield of mean organisms removed from the ecosystem through migration or death (which is existing as an organic deposit).
It is the rate at which the growth processes are going forward within the specified area. The amount of material formed by each link in the food chain per unit of time in a unit area or volume is the production rate.
Environmental Factors Affecting Productivity in the Ecosystem
Various factors influence the productivity of an ecosystem. Some of them are solar radiation, amount of temperature, moisture, and fertility level of the soil. Mineral nutrition, rhizosphere effects, fire effects, salinity, heavy metals, and nitrogen metabolism are also important factors in the formation of biomass. Some of the biotic activities such as grazing above ground herbivores, below ground herbivores, predators, parasites and diseases of primary producers are also essential activities that have a direct impact on the populations of species. In an aquatic system, productivity is generally limited by light, which decreases while the water depth is increasing. In tropical areas, primary productivity may remain continuous throughout the year, provided adequate soil moisture available. In the temperate regions, primary productivity is delimited by a harsh climate and a short snow-free growing period during the year.