Ecosystem Class 12 Notes

• Abiotic components — physical factors like temperature, water, light, soil and air.
• Producers (autotrophs) — plants, algae and some bacteria that fix energy via photosynthesis; the base of every ecosystem.
• Consumers (heterotrophs) — herbivores (primary), carnivores (secondary and tertiary) that eat producers or other consumers.
• Decomposers — bacteria and fungi that break down dead organic matter and return nutrients to the soil (saprophytes).
• Examples of ecosystems: forest, grassland, pond, marine and desert; man-made ones include croplands and aquariums.

• Secondary productivity is the rate at which consumers convert ingested food into their own biomass.
• NPP is highest in tropical rainforests (about 2.2 kg C m⁻² yr⁻¹) and lowest in deserts and deep seas.
• Productivity can be measured as kilocalories, biomass or carbon fixed per unit area per unit time.

Energy flows through an ecosystem in a single direction — from the sun into producers, then to consumers — and is never recycled. The producers capture solar energy that enters the ecosystem, but this energy does not flow back to the sun and cannot be reused because energy is lost as heat at each step.

• Food chain — a linear sequence: grass → grasshopper → frog → snake → hawk.
• Food web — many interconnected food chains in an ecosystem; a web is more stable than a single chain.
• Trophic levels — the positions an organism occupies in a food chain (producer, primary consumer, etc.).
• Detritus food chain starts with dead organic matter (detritivores and decomposers); grazing food chain starts with green plants.

• Pyramid of numbers — usually upright, but inverted in some cases (e.g. a single tree supporting many herbivore insects).
• Pyramid of biomass — upright in most terrestrial ecosystems, but inverted in the sea where a large mass of phytoplankton supports smaller zooplankton and fish.
• Pyramid of energy — always upright, because energy decreases at each successive trophic level and can never be greater than the level below.

• Fragmentation — detritus is broken into smaller pieces by detritivores such as earthworms.
• Leaching — water-soluble inorganic nutrients drain into the soil (downward).
• Catabolism — fungi and bacteria digest detritus and release inorganic substances; enzymes break down complex molecules.
• Humification — accumulation of a dark coloured, amorphous humus layer that is resistant to further microbial action.
• Mineralisation — the release of inorganic minerals (e.g. nitrates, phosphates) from the humus.

• Carbon cycle — carbon moves between the atmosphere (CO₂), producers (photosynthesis) and consumers, and returns via respiration, decomposition and combustion.
• Nitrogen cycle — nitrogen fixation (by Rhizobium, Azotobacter, cyanobacteria), nitrification (Nitrosomonas, Nitrobacter), assimilation, ammonification and denitrification (Pseudomonas).
• Phosphorus cycle — a sedimentary cycle; phosphorus moves through rocks, soil, water and organisms, with no atmospheric phase, so it is slower and can limit productivity.

• Primary succession — starts from bare rock or barren land with no soil (e.g. a newly exposed lava or glacier), taking very long to form soil.
• Secondary succession — starts where soil already exists (e.g. after a forest fire or abandoned farmland) and is faster than primary succession.
• Hydrarch succession — begins in water (ponds, marshes) and leads to a terrestrial climax.
• Xerarch succession — begins in dry, bare conditions (e.g. bare rock or sand) and leads to a terrestrial climax.
• Seral stages — the intermediate communities in succession; the final stable community is the climax community.

Ecosystem services are the benefits humans receive from ecosystems — purification of air and water, nutrient cycling, crop pollination, climate regulation and the prevention of soil erosion. These services have enormous economic value and underscore why ecosystems must be conserved.

Example: An ecosystem has a Gross Primary Productivity (GPP) of 10,000 kcal/m²/year and respiration (R) of 3,000 kcal/m²/year. What is its Net Primary Productivity (NPP), and what does it mean?

Solution: NPP = GPP − R = 10,000 − 3,000 = 7,000 kcal/m²/year. This is the energy stored by producers after their own respiration, and it is the amount actually available to consumers and decomposers in the ecosystem.

Example: If a producer stores 10,000 kcal and the 10% law applies, how much energy reaches the third trophic level?

Solution: Only 10% transfers at each step. Primary consumer receives 10% of 10,000 = 1,000 kcal; secondary (third trophic level) receives 10% of 1,000 = 100 kcal. This rapid loss explains why food chains are short and why the energy pyramid is always upright.

Exercises (NCERT)

1. 1
   What is an ecosystem? Name its two components.
2. 2
   What is stratification in an ecosystem?
3. 3
   Define gross primary productivity (GPP).
4. 4
   Define net primary productivity (NPP).
5. 5
   What is the relationship between GPP, NPP and respiration?
6. 6
   What is the 10% law?
7. 7
   Why is the number of trophic levels limited?
8. 8
   What is an ecological pyramid? Name its three types.
9. 9
   Which pyramid is always upright?
10. 10
    What is decomposition? Name its steps.
11. 11
    What are the factors affecting decomposition?
12. 12
    What is nutrient cycling? Name its two types.
13. 13
    What is the phosphorus cycle? Is it gaseous or sedimentary?
14. 14
    What is ecological succession?
15. 15
    Differentiate between primary and secondary succession.
16. 16
    What is a pioneer species?
17. 17
    What are ecosystem services? Give two examples.
18. 18
    What is the pyramid of biomass in a sea ecosystem?
19. 19
    Why is the energy flow unidirectional?
20. 20
    What is the role of detritivores in an ecosystem?

Answers at a glance

1. 1
   An ecosystem is a functional unit of nature where living organisms interact with each other and the physical environment; its components are biotic and abiotic.
2. 2
   Stratification is the vertical distribution of different species occupying different levels.
3. 3
   GPP is the total amount of organic matter produced by plants through photosynthesis.
4. 4
   NPP is the organic matter available to consumers after respiration losses.
5. 5
   NPP = GPP − R, where R is respiration loss.
6. 6
   The 10% law states that only about 10% of the energy is transferred from one trophic level to the next.
7. 7
   Because most energy is lost as heat at each transfer, only a few trophic levels are supported.
8. 8
   An ecological pyramid is a graphical representation of trophic structure; its three types are number, biomass and energy.
9. 9
   The pyramid of energy is always upright.
10. 10
    Decomposition is the breakdown of dead organic matter; its steps are fragmentation, leaching, catabolism, humification and mineralisation.
11. 11
    Decomposition is affected by temperature, oxygen, and the chemical composition of detritus (and moisture).
12. 12
    Nutrient cycling is the movement of nutrients through the ecosystem; its two types are gaseous and sedimentary cycles.
13. 13
    The phosphorus cycle is sedimentary (it has no appreciable gaseous phase).
14. 14
    Ecological succession is the gradual and directional change in the species composition of an area over time.
15. 15
    Primary succession begins on a bare area with no soil; secondary succession begins where soil already exists after a disturbance.
16. 16
    A pioneer species is the first organism to colonise a bare area; e.g. lichens.
17. 17
    Ecosystem services are the benefits people get from ecosystems; e.g. purification of air and water and pollination.
18. 18
    In a sea ecosystem, the pyramid of biomass is inverted.
19. 19
    Energy flow is unidirectional because energy captured by autotrophs flows to consumers and is lost as heat.
20. 20
    Detritivores break down detritus into smaller particles and help in decomposition.

MCQ Questions

2 Mark Questions

1. 1
   What is an ecosystem? Name its two components.
2. 2
   Define GPP and NPP.
3. 3
   What is the 10% law?
4. 4
   What is ecological succession?
5. 5
   What are ecosystem services?

3 Mark Questions

1. 1
   Describe the structure of an ecosystem.
2. 2
   Explain the relationship between GPP, NPP and respiration.
3. 3
   Describe the types of ecological pyramids.
4. 4
   Describe the steps of decomposition.
5. 5
   Differentiate between primary and secondary succession.

5 Mark Questions

1. 1
   Describe productivity and energy flow in an ecosystem.
2. 2
   Describe the steps of decomposition and the factors affecting it.
3. 3
   Describe nutrient cycling and ecological succession.