Chapter 12 - Ecosystem

Ecosystem explains how living organisms interact with the physical environment through productivity, decomposition and energy transfer. The current rationalised NCERT Class 12 Biology Chapter 12 covers ecosystem structure and function, primary and secondary productivity, decomposition, food chains, energy flow and ecological pyramids.

Teachoo’s category provides detailed notes, NCERT solutions, energy-flow diagrams, important questions, MCQs and case-based practice for CBSE and NEET.

Ecosystem structure and function

An ecosystem includes a biological community and the abiotic environment with which it interacts. Ecosystems may be natural or artificial, terrestrial or aquatic and small or extremely large.

The biotic components include producers, consumers and decomposers. Abiotic components include light, temperature, water, air, minerals and organic matter.

The physical appearance of an ecosystem is influenced by species composition, distribution and stratification. Forest vegetation, for example, may occur in layers occupied by different organisms.

Major functional aspects include productivity, decomposition, energy flow and interactions across trophic levels.

Productivity

Productivity is the rate of biomass production, not simply the standing quantity of biomass present at a given time.

Primary productivity is the production of organic matter by producers through photosynthesis or chemosynthesis. It is divided into:

• Gross primary productivity – total rate of organic-matter production

• Net primary productivity – organic matter remaining after respiratory loss

The relationship is:

NPP = GPP – R

Net primary productivity represents biomass available for consumption by heterotrophs.

Secondary productivity is the rate at which consumers form new organic matter.

Productivity depends on plant species, photosynthetic capacity, nutrients and environmental conditions. Students should distinguish rate from standing crop and learn the appropriate units.

Decomposition

Detritus includes dead plant and animal material and faecal matter. Decomposers and detritivores process it and release simpler substances.

The principal steps are:

• Fragmentation

• Leaching

• Catabolism

• Humification

• Mineralisation

Detritivores break detritus into smaller particles. Water carries soluble nutrients into deeper soil layers through leaching. Microbial enzymes break complex substances down during catabolism.

Humification forms dark, resistant humus that decomposes slowly and stores nutrients. Mineralisation releases inorganic nutrients from organic matter.

Decomposition is influenced by detritus composition, temperature, moisture and oxygen. Warm, moist and aerobic conditions generally promote decomposition. Material rich in lignin or chitin decomposes more slowly than material rich in nitrogen and soluble sugars.

Energy flow

Sunlight is the principal energy source for most ecosystems. Producers capture only a fraction of incoming solar radiation and convert it into chemical energy.

Energy then moves through trophic levels. Producers occupy the first trophic level, primary consumers the second and higher consumers subsequent levels.

The grazing food chain begins with living producers. The detritus food chain begins with dead organic matter. Food webs connect multiple feeding pathways and make ecological relationships more realistic than a single linear chain.

Energy flow is unidirectional. Energy enters, passes through organisms and is progressively dissipated as heat. It is not recycled in the same manner as matter.

The ten-per-cent law states that only a relatively small fraction of the energy available at one trophic level is transferred to the next. This helps explain why food chains are usually short.

Ecological pyramids

Ecological pyramids represent trophic relationships through:

• Pyramid of numbers

• Pyramid of biomass

• Pyramid of energy

Pyramids of numbers and biomass may be upright or inverted depending on the ecosystem. The pyramid of energy is always upright because usable energy decreases at successive trophic levels.

Ecological pyramids simplify real food webs. They may not adequately represent omnivores, organisms occupying multiple trophic levels or the important role of decomposers.

Important diagrams and calculations

• Ecosystem components

• Gross and net productivity relationship

• Decomposition cycle

• Grazing and detritus food chains

• Trophic levels and energy transfer

• Food web

• Pyramids of number, biomass and energy

Why is Ecosystem important for CBSE and NEET?

CBSE commonly asks process explanations, comparisons, diagrams and productivity calculations. NEET frequently tests decomposition steps, food-chain types, trophic levels, energy transfer and conditions under which pyramids are upright or inverted.

How should students study this chapter?

Follow matter and energy separately. Matter can move through organisms and return through decomposition, while energy flows in one direction and is lost as heat. Draw one ecosystem and mark producers, consumers, decomposers, food chains and trophic levels on it.

Frequently Asked Questions

What is the difference between gross and net primary productivity?

Gross primary productivity is total producer biomass formation. Net primary productivity is what remains after producers use part of that energy in respiration.

Is standing crop the same as productivity?

No. Standing crop is the amount present at a particular time. Productivity is a rate of production over time.

What is the difference between humification and mineralisation?

Humification forms resistant humus. Mineralisation releases inorganic nutrients from organic material.

Why is the energy pyramid always upright?

Energy is lost as heat and through biological work at every trophic transfer, so less usable energy remains at higher levels.

Can the pyramid of biomass be inverted?

Yes. In some aquatic ecosystems, producer biomass at a moment may be lower than consumer biomass because producers reproduce and are consumed rapidly.

Why are food chains generally short?

Only a fraction of energy passes to each higher trophic level, so insufficient energy remains to support many successive levels.