Wednesday, August 5, 2020

Ecosystem - Notes | Class 12 | Part 2: Productivity, Decomposition



Solar energy is the basic requirement for an ecosystem to function and sustain.

Amount of biomass (organic matter) produced per unit area over a time period by plants during photosynthesis is called primary production. It is expressed in weight (g–2) or energy (kcal m–2).

The rate of biomass production is called productivity. It is expressed in g–2 yr–1 or (kcal m–2) yr–1.

It is divided into gross primary productivity (GPP) and net primary productivity (NPP).
  • Gross primary productivity (GPP): It is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is used by plants in respiration.
  • Net primary productivity (NPP): It is the available biomass for the consumption to heterotrophs (herbivores & decomposers). i.e., NPP is the Gross primary productivity minus respiration losses (R).
NPP = GPP – R 

Secondary productivity: It is the rate of formation of new organic matter by consumers.

Primary productivity varies in different ecosystems because it depends on
  • The plant species inhabiting an area.
  • Environmental factors.
  • Availability of nutrients.
  • Photosynthetic capacity of plants.
Annual net primary productivity of whole biosphere is about 170 billion tons (dry weight) of organic matter. Of this, despite occupying about 70 % of the surface, the productivity of the oceans is only 55 billion tons.


It is the breakdown of complex organic matter by decomposers into inorganic substances like CO2, water and nutrients. 

It is largely an oxygen-requiring process.

Raw material for decomposition is called Detritus. E.g. dead plant remains (leaves, bark, flowers etc.), dead remains of animals, fecal matter etc.

Steps of decomposition 
  • Fragmentation: It is the breakdown of detritus into smaller particles by detritivores (e.g. earthworm).
  • Leaching: Water soluble inorganic nutrients go down into soil horizon and precipitate as unavailable salts.
  • Catabolism: Degradation of detritus into simpler inorganic substances by bacterial and fungal enzymes.
The above three processes occur simultaneously.
  • Humification: Accumulation of humus (dark amorphous substance) in soil. Humus is resistant to microbial action and so decomposes very slowly. Being colloidal, it serves as a reservoir of nutrients.
  • Mineralization: It is the release of inorganic nutrients due to the degradation of humus by some microbes.
Factors influencing decomposition 

1. Chemical composition of detritus:
  • Decomposition is slow in detritus rich in lignin & chitin.
  • It is quicker in detritus rich in nitrogen and water-soluble substances like sugars.
2. Climatic factors (temperature & soil moisture):
  • Warm and moist environment favour decomposition.
  • Low temperature & anaerobiosis inhibit decomposition resulting in buildup of organic materials.
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