In this we will discuss about:- 1. Introduction to Productivity in Ecosystem 2. Concepts of Productivity 3. Environmental Factors.
Introduction to Productivity in Ecosystem:
The rate of biomass production is called productivity. The portion of fixed energy, a trophic level passes on to the next trophic level is called production.
Productivity in ecosystems is of two kinds, i.e., primary and secondary. Green plants fix solar energy and accumulate it in organic forms as chemical energy. As this is the first and basic form of energy storage, the rate at which the energy accumulates in the green plants or producers is known as primary productivity.
Productivity is a rate function, and is expressed in terms of dry matter produced or energy captured per unit area of land, per unit time. It is more often expressed as energy in calories/cm2/yr or dry organic matter in g/m2/yr (g/m2 x 8.92 = lb/acre). Hence, the productivity of different ecosystems can be easily compared.
Primary productivity has two aspects:
(i) Gross and
(ii) Net.
The total solar energy trapped in the food material by photosynthesis is referred to as gross primary productivity (GPP).
However, a good fraction of gross primary productivity is utilised in respiration of green plants. The amount of energy-bound organic matter created per unit area and time that is left after respiration is net primary productivity (NPP).
Net productivity of energy = Gross productivity — Energy lost in respiration.
The rates at which the heterotrophic organisms resynthesise the energy-yielding substances are called secondary productivity. Here, the net primary productivity (NPP) results in the accumulation of plant biomass, which serves the food of herbivores and decomposers.
It is notable that the food of consumers has been produced by the primary producers, and secondary productivity depicts only the utilisation of this food for the production of consumer biomass. Secondary productivity is the productivity of animals and saprobes in ecosystem.
Concepts of Productivity:
a. Standing crop,
b. Materials removed, and
c. Production rate.
a. Standing Crop:
This is abundance of organisms existing in the area at any one time. It may be expressed in terms of number of individuals, as biomass of organisms, as energy content or in some other suitable terms. Measurement of standing crop reveals the concentration of individuals in various populations of the ecosystem.
b. Materials Removed:
The second concept of productivity is the materials removed from the area per unit time. It includes the yield to man, organisms removed from the ecosystem by migration, and the material withdrawn as organic deposit.
c. Production Rate:
The third concept of productivity is the production rate, at which the growth processes are going forward within the area. The amount of material formed by each link in the food chain per unit of time per unit area or volume is the production rate.
Environmental Factors Affecting the Productivity in Ecosystem:
1. Solar radiation and temperature.
2. Moisture, i.e., leaf water potential, soil moisture, fluctuation of precipitation, and transpiration.
3. Mineral nutrition, i.e., uptake of minerals from the soil, rhizosphere effects, fire effects, salinity, heavy metals and nitrogen metabolism.
4. Biotic activities, i.e., grazing, above ground herbivores, below ground herbivores, predators and parasites and diseases of primary producers.
5. Impact of human populations, i.e., populations of different sorts, ionising radiations, such as atomic explosions, etc.
6. In aquatic systems, productivity is generally limited by light, which decreases with increasing water depth. In deep oceans nutrients often become limiting for productivity. Nitrogen is most important nutrient limiting productivity in marine ecosystems.
The largeness of primary productivity depends on the photosynthetic capacity of producers and the existing environmental conditions, such as solar radiation, temperature and soil moisture.
In tropical conditions, primary productivity may remain continuous throughout the year, provided adequate soil moisture remain available.
While in temperate regions, primary productivity is limited by cold climate and a short snow- free growing period during the year.
Primary productivity of the major ecosystems of the world is as follows:
