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Class 12 · Biology · Chapter 12

Ecosystem: Class 12 Important Questions with Answers

72 important questions for Class 12 Biology Chapter 12, Ecosystem, the way they come in the board exam: MCQs, assertion–reason, case-based and short and long answer questions, with answers. Taken from our Class 12 Biology notes for 2026-27.

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Jump to: Practice set: test yourself20 MCQsAssertion–ReasonAnswers to the practice set

Practice set: test yourself

Try every question before looking at the answers below. 64 or more out of 80 means you are ready for the board.

A. Fill in the blanks (1 mark each)

  1. Vertical distribution of species is ______.
  2. GPP minus respiration equals ______.
  3. Earthworms cause ______ of detritus.
  4. The dark colloidal product of humification is ______.
  5. Release of nutrients from humus is ______.
  6. Mass of living material at a level at a time is ______.
  7. Decomposers are also called ______.
  8. The pyramid of ______ is always upright.

B. True or false? (1 mark each)

  1. Decomposition is mainly anaerobic.
  2. Chitin-rich detritus decomposes slowly.
  3. Oceans are the most productive part of the biosphere.
  4. Energy flow is unidirectional.
  5. A sparrow can occupy two trophic levels.
  6. In water, DFC is the main energy conduit.
  7. Dry weight is a better measure of biomass.
  8. Ecological pyramids include saprophytes.

C. Very short answer (1 mark each)

  1. Give the units of productivity.
  2. Name the four functional aspects of an ecosystem.
  3. Name two decomposers of a pond.
  4. What is detritus?
  5. Which ecosystem does not use solar energy?
  6. What fraction of PAR do plants capture?
  7. Name the first trophic level.
  8. Give an example of an omnivore.
  9. Which pyramid is inverted in the sea?
  10. Give one limitation of ecological pyramids.

D. Short answer I (2 marks each)

  1. Distinguish between GPP and NPP.
  2. Why are oceans less productive?
  3. Give two features of humus.
  4. How do temperature and moisture affect decomposition?
  5. Differentiate GFC and DFC.
  6. What is the 10 per cent law?
  7. Why is the pyramid of energy always upright?
  8. Why is the biomass pyramid inverted in the sea?
My score (fill in after checking the answers)
SectionA /8B /8C /10D /16E /18F /20G (bonus)Total /80
My marks

Rough work

E. Short answer II (3 marks each)

  1. Describe the pond as an ecosystem.
  2. Explain the five steps of decomposition.
  3. Explain the factors controlling the rate of decomposition.
  4. Explain trophic levels and standing crop.
  5. Describe pyramids of number in a grassland and a tree.
  6. Give three limitations of ecological pyramids.

F. Long answer (5 marks each)

  1. Describe energy flow in an ecosystem with a diagram; explain the 10 per cent law.
  2. What are ecological pyramids? Describe the three types with examples and exceptions.

G. HOTS (higher-order thinking)

  1. Why is there no 10 per cent limit on the number of decomposers?
  2. Why does organic matter build up in bogs and cold regions?
  3. How many trophic levels can humans occupy?
  4. Why does the energy flow obey the second law of thermodynamics?

20 MCQs

  1. Not a functional aspect of an ecosystem:
    (a) energy flow (b) decomposition (c) stratification (d) productivity
  2. NPP equals
    (a) GPP − R (b) GPP + R (c) R − GPP (d) GPP × R
  3. Annual NPP of the biosphere is about
    (a) 55 bn t (b) 550 bn t (c) 17 bn t (d) 170 bn t
  4. Rate of formation of new organic matter by consumers:
    (a) GPP (b) secondary productivity (c) NPP (d) standing crop
  5. Fragmentation is done by
    (a) bacteria (b) detritivores (c) fungi (d) plants
  6. Soluble nutrients going down the soil is
    (a) catabolism (b) mineralisation (c) humification (d) leaching
  7. Humus is
    (a) colloidal, a nutrient reservoir (b) quickly decomposed (c) light-coloured (d) crystalline
  8. Mineralisation releases
    (a) organic matter (b) humus (c) inorganic nutrients from humus (d) lignin
  9. Decomposition is fastest in detritus rich in
    (a) lignin (b) cellulose and lignin (c) chitin (d) nitrogen and sugars
  10. PAR is about
    (a) 100% (b) less than 50% (c) 2–10% (d) 1%
  11. Main energy conduit on land:
    (a) GFC (b) parasitic chain (c) DFC (d) none
  12. Saprotrophs are
    (a) decomposers (b) producers (c) herbivores (d) carnivores
  13. The second trophic level is
    (a) producers (b) herbivores (c) carnivores (d) decomposers
  14. Standing crop is measured as
    (a) energy flow (b) productivity (c) biomass or number (d) GPP
  15. Energy transferred to next level:
    (a) 1% (b) 90% (c) 50% (d) 10%
  16. Pyramid of biomass in sea is
    (a) inverted (b) upright (c) spindle (d) absent
  17. Pyramid of energy is
    (a) always upright (b) sometimes inverted (c) always inverted (d) spindle
  18. A big tree with insects gives an inverted pyramid of
    (a) energy (b) productivity (c) biomass (d) number
  19. Producers convert about what % of sunlight into NPP?
    (a) 10 (b) 1 (c) 50 (d) 20
  20. Not included in ecological pyramids:
    (a) herbivores (b) producers (c) saprophytes (d) carnivores

1 (c) · 2 (a) · 3 (d) · 4 (b) · 5 (b) · 6 (d) · 7 (a) · 8 (c) · 9 (d) · 10 (b) · 11 (c) · 12 (a) · 13 (b) · 14 (c) · 15 (d) · 16 (a) · 17 (a) · 18 (d) · 19 (b) · 20 (c)

Assertion–Reason (6)

(a) Both true, R explains A. (b) Both true, R does not explain A. (c) A true, R false. (d) A false, R true.

  1. A: Energy pyramid is always upright. R: Energy is lost as heat at each transfer.
  2. A: Humus decomposes very slowly. R: It is highly resistant to microbial action.
  3. A: In terrestrial ecosystems most energy flows through the GFC. R: Decomposers are absent on land.
  4. A: Biomass is better measured as dry weight. R: Water content varies widely.
  5. A: Decomposition is slow in cold, waterlogged soils. R: Low temperature and anaerobiosis inhibit microbes.
  6. A: Oceans have low productivity. R: Oceans cover about 70% of the surface.

Key: 1 (a) · 2 (a) · 3 (d) · 4 (a) · 5 (a) · 6 (b)

Answers to the practice set

A. 1. stratification 2. NPP 3. fragmentation 4. humus 5. mineralisation 6. standing crop 7. saprotrophs 8. energy

B. 1. F 2. T 3. F 4. T 5. T 6. F 7. T 8. F

C. 1. g m−2 yr−1 or kcal m−2 yr−1 2. productivity, decomposition, energy flow, nutrient cycling 3. bacteria and fungi (flagellates) 4. dead plant and animal remains including faeces 5. deep-sea hydrothermal ecosystem 6. 2–10% 7. producers 8. crow (cockroach) 9. biomass 10. same species at two levels not considered (or no food web, no saprophytes)

D. 1. GPP is the total rate of organic matter production in photosynthesis; NPP = GPP − respiration, available to heterotrophs 2. Light decreases with depth and nutrients (especially nitrogen) are limiting 3. Dark, amorphous, colloidal, resistant to microbes, decomposes slowly, reservoir of nutrients (any two) 4. Warm and moist favour microbial activity and fast decomposition; cold and dry or waterlogged slow it 5. GFC starts from green plants, major in water; DFC starts from dead matter, decomposers, major on land 6. Only about 10% of energy at one level passes to the next 7. Energy is lost as heat at every step, so a higher level never has more 8. Phytoplankton have a small standing crop but multiply fast, supporting a larger standing crop of fishes and zooplankton

Answers to E, F, G, case study (short, long and HOTS answers, step by step) are in the notes pack

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