Class 12 Biology


Ecosystem: It is a functional unit of nature, where living beings interact among themselves and also with the surrounding physical environment.

Structure and Function of Ecosystem

Stratification: Vertical distribution of different species occupying different levels is called stratification.

Aspects of Ecosystem

  1. Productivity
  2. Decomposition
  3. Energy flow
  4. Nutrient cycling.


Primary Production: The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis is called primary production. Primary production is expressed in terms of weight (g-2 or energy (kcal m-2. When we need to compare the productivity of different ecosystems, then primary productivity is expressed in terms of g–2 yr–1 or (kcal m–2 yr–1).

Gross Primary Productivity: The rate of production of organic matter during photosynthesis is called Gross Primary Productivity (GPP) of the ecosystem.

Net Primary Productivity: When respiratory losses (R) is subtracted from GPP, we get Net Primary Productivity (NPP). Net primary productivity is the available biomass for the consumption of heterotrophs.


Secondary Productivity: The rate of formation of new organic matter by consumers is called secondary productivity.

The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter. Of this, the productivity of the oceans are only 55 billion tons.


Decomposition: Conversion of complex organic matter into inorganic substances by decomposers is called decomposition.

Detritus: It is the raw material for decomposition. It is composed of dead remains of plants and animals; including faecal matter.

Steps of decomposition are; fragmentation, leaching, catabolism, humification and mineralization.

  1. Fragmentation: Detritivores break down detritus into smaller particles. This process is called fragmentation. Earthworm is an example of detritivore.
  2. Leaching: The loss of water soluble plant nutrients from soil is called leaching. During this process, water soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
  3. Catabolism: Degradation of detritus into simpler organic substances by bacterial and fungal enzymes is called catabolism.
  4. Humification: Formation of humus is called humification. Humus is a dark coloured amorphous substance. It is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Humus serves as a reservoir of nutrients.
  5. Mineralization: Degradation of humus to release inorganic nutrients is called mineralization.

Decomposition is largely an oxygen-requiring process. The rate of decomposition is affected by chemical composition of detritus and climatic factors. Decomposition rate is slower if detritus is rich in lignin. Decomposition rate is quicker if detritus is rich in nitrogen and water-soluble substances like sugars. Warm and moist environment favors decomposition, but low temperature and anaerobiosis inhibit decomposition.

Energy Flow

Less than 50% of the incident solar radiation is photosynthetically active radiation (PAR). Plants capture only 2 – 10% of the PAR. All organisms are directly or indirectly dependent for food on producers. So, there is unidirectional flow of energy from the sun to producers and then to consumers. This obeys the First Law of Thermodynamics.

Ecosystems need a constant supply of energy to synthesize the required molecules to counteract the universal tendency towards increasing disorderliness. This obeys the Second Law of Thermodynamics.

Grazing Food Chain: The food chain which begins with producers is called grazing food chain. A simple grazing food chain (GFC) is depicted below:

Grass → Goat → Man

Producer → Primary Consumer → Secondary consumer

Detritus Food Chain: The food chain which begins with dead organic matter is called detritus food chain (DFC).

GFC is the major conduit for energy flow in the aquatic ecosystem. On the other hand, DFC is the major conduit for energy flow in a terrestrial ecosystem. Some of the organisms of DFC are prey to the GFC animals. In a natural ecosystem, some animals are omnivores.

Trophic Level: The specific place which an organism occupies in a food chain is called the trophic level of that organism. Producers belong to the first trophic level, herbivores belong to the second trophic level, and carnivores belong to the third trophic level.

Standing Crop: Each trophic level has a certain mass of living material at a particular time. This amount of mass of living material is called standing crop. The standing crop is measured in terms of the mass of living organisms (biomass) or the number in a unit area. The biomass of a species is expressed in terms of fresh or dry weight. Measurement in terms of dry weight is more accurate.