CBSE Class 12 Biology Revision Notes Chapter 14 - Ecosystem

REVISION NOTES FOR CBSE CLASS 12 BIOLOGY CHAPTER- 14

CHAPTER 14: ECOSYSTEM

Ecosystems; Patterns, components; productivity and decomposition; energy flow; pyramids of number, biomass, energy; nutrient cycles (carbon and phosphorus); ecological succession; ecological services- carbon fixation, pollination, seed dispersal, oxygen release (in brief).

Ecosystem: It is the basic functional unit of biosphere in which living organisms interact among themselves and with their surrounding physical environment.

Stratification: Vertical distribution of different species occupying different levels in an ecosystem. Trees occupy top vertical strata, shrubs the second layer. and herbs the third layer and herbs/grasses occupy the bottom layers.

Primary Production: Amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.

Gross Primary Productivity: Rate of production of organic matter during photosynthesis. 

Net Primary Productivity: NPP = GPP-R (Gross primary productivity minus the respiration losses).

Secondary Productivity: Rate of formation of new organic matter by consumers.

Detritus : Dead leaves, twigs, animal remains etc. constitute detritus.

Detrivore: Organisms who break down detritus into smaller particles, e.g., earthworm.

Ecological succession: The successive and orderly replacement of one community by the other community in an area, over a period of time.

Climax community: The stable and final biotic community that develops at the end of ecological succession and is in perfect harmony with its physical environment.

Process of Decomposition: The decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients. This process is called decomposition. Steps of decomposition are:

(i) Fragmentation: Break down of detritus into smaller particles by detritivores (earthworm).

(ii) Leaching: Water soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.

(iii) Catabolism : Bacterial and fungal enzymes degrade detritus in simple inorganic substances.

(iv) Humification: Accumulation of a dark coloured amorphous substance called humus which is highly resistant to microbial action and rich in nutrients.

(v) Mineralisation: The humus is further degraded by some microbes and release of inorganic nutrients occur.

FACTORS AFFECTING DECOMPOSITION :

Decomposition is controlled by:

(a) Chemical composition of detritus: The decomposition

will be slower if detritus is rich in lignin and chitin and will be faster if detritus is rich in nitrogen and water soluble substance (sugar).

(b) Climatic factors: In warm and moist environment, the process of decomposition increases whereas low temperature and anaerobiosis inhibit the decomposition.

Energy Flow: Energy flow is the key function in the ecosystem. The plants (producers) capture only 2-10 percent of the photosynthetically active radiation (PAR). Unidirectional flow of energy is takes place from the sun to producers and then to consumers. About 10% energy flows from one trophic level to another.

Grazing Food Chain (GFC): It begins with producers.

Grass(Producer)→deer(PrimaryConsumer)→Lion

(Secondary consumer)

Detritus Food Chain (DFC): It begins with dead organic matter. It is made up of decomposers (Fungi, Bacteria). They meet their energy and nutrient requirements by degrading detritus. Decomposers are also known as saprotrophs.

Food Web: A number of food chains interconnected with each other forming a web-like pattern.

Ten Percent Law of Energy Transfer: Proposed by Lindeman. At each step of food chain, when energy is transferred from one trophic level to the next trophic level, only 10 percent of energy is passed on to the next trophic level. 

Standing State: Amount of all the inorganic substances present in an ecosystem per unit area at a given time.

Standing Crop: Amount of living material present in different trophic levels at a given time. It is measured as the mass of living organisms or the number in a unit area.

Ecological Pyramids: The sequential graphic representation of an ecological parameter (energy/number/biomass) depicting different trophic levels in a food chain.

(i) Pyramid of Numbers: (Grassland Ecosystem)

(ii) Pyramid of Energy: (Always upright in all Ecosystems)

(iii) Pyramid of Biomass

Ecological Succession: The gradual and fairly predictable change in the species composition of a given area is called ecological succession. The species that invade a bare area is called pioneer species.

The entire sequence of communties that successively change in a given area is called sere. The stable and final biotic community that develops at the end of ecological succession and is in perfect harmony with its physical environment is called climax community.

XERARCH:

HYDRARCH:

Secondary Succession: The secondary succession begins in the area where natural biotic communities have been destroyed (burned or cut forests, land that have been devastated by flood).

1. Previous climax community

2. Forest destroyed by fire/flooded land/abandoned form.

3. Grasses/Pioneer Community

4. Grasses and shurbs.

5. Intermediate Communities

6. Climax Communities.

Nutrient Cycling: Movement of nutrient elements through the various components of an ecosystem also called Biogeochemical cycles.

Carbon cycle: Occurs through atmosphere, ocean, and through living and dead organisms. Considerable amount of carbon returns to atmosphere as CO₂ through respiratory activities. Decomposers also contribute to Carbon di-oxide pool. Burning of wood, forest fire and combustion of organic matter, fossil fuels, volcanic activity also release CO₂, in atmosphere.

Phosphorus cycle: (Sedimentary cycle) Rocks contain phosphorous in the form of phosphates

Comparison between carbon cycle and phosphorus cycle:

CBSE Class 12 Biology Revision Notes Chapter 15 - Biodiversity and its Conservation