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Ecosystem: questions with answers

Class 12 Biology, Chapter 12 (NCERT). 15 questions by concept. Open “Show answer” for the answer and why.

How to teach this chapter · Common misconceptions & worked visual for this chapter

Ecosystem structure and components

Question 1

Which statement best explains ecosystem structure and components?

  1. (a)An ecosystem contains only living organisms
  2. (b)An ecosystem includes interacting biotic communities and abiotic environment, organised into trophic and functional roles; energy flows through it while matter cycles.
  3. (c)The process has no biological mechanism and never depends on evidence.
  4. (d)Energy is recycled indefinitely like nutrients
Show answer

(b) An ecosystem includes interacting biotic communities and abiotic environment, organised into trophic and functional roles; energy flows through it while matter cycles.

An ecosystem includes interacting biotic communities and abiotic environment, organised into trophic and functional roles; energy flows through it while matter cycles.

Question 2

A learner claims: An ecosystem contains only living organisms. Which correction is best?

  1. (a)An ecosystem contains only living organisms
  2. (b)Energy is recycled indefinitely like nutrients
  3. (c)Abiotic resources and physical conditions are integral ecosystem components.
  4. (d)The claim is correct because all biological systems behave identically.
Show answer

(c) Abiotic resources and physical conditions are integral ecosystem components.

Abiotic resources and physical conditions are integral ecosystem components.

Carbon cycle

Question 3

Which statement best explains carbon cycle?

  1. (a)Carbon moves among atmosphere, organisms, oceans and rocks through photosynthesis, feeding, respiration, decomposition, dissolution and combustion; fossil-fuel burning adds geologically stored carbon rapidly.
  2. (b)Photosynthesis releases carbon dioxide as its main carbon flux
  3. (c)The process has no biological mechanism and never depends on evidence.
  4. (d)Combustion has no effect on atmospheric carbon
Show answer

(a) Carbon moves among atmosphere, organisms, oceans and rocks through photosynthesis, feeding, respiration, decomposition, dissolution and combustion; fossil-fuel burning adds geologically stored carbon rapidly.

Carbon moves among atmosphere, organisms, oceans and rocks through photosynthesis, feeding, respiration, decomposition, dissolution and combustion; fossil-fuel burning adds geologically stored carbon rapidly.

Question 4

A learner claims: Photosynthesis releases carbon dioxide as its main carbon flux. Which correction is best?

  1. (a)Photosynthesis releases carbon dioxide as its main carbon flux
  2. (b)Combustion has no effect on atmospheric carbon
  3. (c)The claim is correct because all biological systems behave identically.
  4. (d)Photosynthesis removes carbon dioxide into organic matter.
Show answer

(d) Photosynthesis removes carbon dioxide into organic matter.

Photosynthesis removes carbon dioxide into organic matter.

Primary productivity

Question 5

Which statement best explains primary productivity?

  1. (a)The process has no biological mechanism and never depends on evidence.
  2. (b)Gross primary productivity is total photosynthetic fixation; net primary productivity equals GPP minus producer respiration and is the biomass available for growth and consumers.
  3. (c)Primary productivity is measured only as standing biomass
  4. (d)NPP equals GPP plus respiration
Show answer

(b) Gross primary productivity is total photosynthetic fixation; net primary productivity equals GPP minus producer respiration and is the biomass available for growth and consumers.

Gross primary productivity is total photosynthetic fixation; net primary productivity equals GPP minus producer respiration and is the biomass available for growth and consumers.

Question 6

A learner claims: NPP equals GPP plus respiration. Which correction is best?

  1. (a)NPP equals GPP plus respiration
  2. (b)Primary productivity is measured only as standing biomass
  3. (c)Producer respiration is subtracted: NPP = GPP - R.
  4. (d)The claim is correct because all biological systems behave identically.
Show answer

(c) Producer respiration is subtracted: NPP = GPP - R.

Producer respiration is subtracted: NPP = GPP - R.

Decomposition process and controls

Question 7

Which statement best explains decomposition process and controls?

  1. (a)The process has no biological mechanism and never depends on evidence.
  2. (b)Detritus decomposition includes fragmentation, leaching, catabolism, humification and mineralisation. Warm, moist, aerobic conditions generally speed up microbial decomposition. Lignin, cold and dry conditions slow it down.
  3. (c)Cold dry conditions always speed decomposition
  4. (d)Humification immediately releases all inorganic nutrients
Show answer

(b) Detritus decomposition includes fragmentation, leaching, catabolism, humification and mineralisation. Warm, moist, aerobic conditions generally speed up microbial decomposition. Lignin, cold and dry conditions slow it down.

Detritus decomposition includes fragmentation, leaching, catabolism, humification and mineralisation. Warm moist aerobic conditions generally accelerate microbial decomposition, while lignin and cold or dryness slow it.

Question 8

A learner claims: Humification immediately releases all inorganic nutrients. Which correction is best?

  1. (a)Cold dry conditions always speed decomposition
  2. (b)The claim is correct because all biological systems behave identically.
  3. (c)Humification forms resistant dark organic matter; mineralisation releases inorganic nutrients.
  4. (d)Humification immediately releases all inorganic nutrients
Show answer

(c) Humification forms resistant dark organic matter; mineralisation releases inorganic nutrients.

Humification forms resistant dark organic matter; mineralisation releases inorganic nutrients.

Food chains and food webs

Question 9

Which statement best explains food chains and food webs?

  1. (a)Grazing chains begin with producers, detritus chains with dead organic matter, and interconnected chains form food webs that represent multiple feeding pathways.
  2. (b)A detritus food chain begins with a living green producer
  3. (c)Each organism has exactly one feeding link
  4. (d)The process has no biological mechanism and never depends on evidence.
Show answer

(a) Grazing chains begin with producers, detritus chains with dead organic matter, and interconnected chains form food webs that represent multiple feeding pathways.

Grazing chains begin with producers, detritus chains with dead organic matter, and interconnected chains form food webs that represent multiple feeding pathways.

Question 10

A learner claims: A detritus food chain begins with a living green producer. Which correction is best?

  1. (a)Each organism has exactly one feeding link
  2. (b)The claim is correct because all biological systems behave identically.
  3. (c)It begins with dead organic matter used by decomposers and detritivores.
  4. (d)A detritus food chain begins with a living green producer
Show answer

(c) It begins with dead organic matter used by decomposers and detritivores.

It begins with dead organic matter used by decomposers and detritivores.

Ecological pyramids

Question 11

Which statement best explains ecological pyramids?

  1. (a)Every ecological pyramid must be upright
  2. (b)The process has no biological mechanism and never depends on evidence.
  3. (c)An energy pyramid can be inverted in the ocean
  4. (d)Pyramids of number, biomass and energy represent different trophic properties. Energy pyramids are always upright, whereas number or biomass pyramids can be inverted in particular systems.
Show answer

(d) Pyramids of number, biomass and energy represent different trophic properties. Energy pyramids are always upright, whereas number or biomass pyramids can be inverted in particular systems.

Pyramids of number, biomass and energy represent different trophic properties. Energy pyramids are always upright, whereas number or biomass pyramids can be inverted in particular systems.

Energy flow and transfer efficiency

Question 12

Which statement best explains energy flow and transfer efficiency?

  1. (a)All producer energy reaches herbivores
  2. (b)The process has no biological mechanism and never depends on evidence.
  3. (c)Energy lost as heat returns to producers
  4. (d)Solar energy captured by producers moves through trophic levels with substantial respiratory loss; the ten-percent law is an approximate transfer rule, not perfect recycling.
Show answer

(d) Solar energy captured by producers moves through trophic levels with substantial respiratory loss; the ten-percent law is an approximate transfer rule, not perfect recycling.

Solar energy captured by producers moves through trophic levels with substantial respiratory loss; the ten-percent law is an approximate transfer rule, not perfect recycling.

Ecological succession

Question 13

Which statement best explains ecological succession?

  1. (a)The process has no biological mechanism and never depends on evidence.
  2. (b)Secondary succession is always slower than primary
  3. (c)Succession is directional community change after formation or disturbance. Primary succession begins without developed soil and is usually slower. Secondary succession begins where a prior community and soil or propagules remain.
  4. (d)Primary succession begins on mature soil
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(c) Succession is directional community change after formation or disturbance. Primary succession begins without developed soil and is usually slower. Secondary succession begins where a prior community and soil or propagules remain.

Succession is directional community change after formation or disturbance. Primary succession begins without developed soil and is usually slower; secondary succession begins where a prior community and soil or propagules remain.

Phosphorus cycle

Question 14

Which statement best explains phosphorus cycle?

  1. (a)Phosphorus cycles mainly through rocks, soil, water and organisms, with weathering as a major source and sedimentation as a sink; it lacks a large gaseous atmospheric phase.
  2. (b)The process has no biological mechanism and never depends on evidence.
  3. (c)Weathering permanently removes phosphate from ecosystems
  4. (d)Phosphorus has a dominant atmospheric gas reservoir
Show answer

(a) Phosphorus cycles mainly through rocks, soil, water and organisms, with weathering as a major source and sedimentation as a sink; it lacks a large gaseous atmospheric phase.

Phosphorus cycles mainly through rocks, soil, water and organisms, with weathering as a major source and sedimentation as a sink; it lacks a large gaseous atmospheric phase.

Ecosystem services

Question 15

Which statement best explains ecosystem services?

  1. (a)The process has no biological mechanism and never depends on evidence.
  2. (b)Ecosystem services include only market goods
  3. (c)Ecosystems supply provisioning, regulating, supporting and cultural benefits such as food, pollination, nutrient cycling, climate regulation, soil formation and recreation. Putting a value on these benefits does not mean they can be replaced.
  4. (d)Technology can replace every ecosystem service without cost
Show answer

(c) Ecosystems supply provisioning, regulating, supporting and cultural benefits such as food, pollination, nutrient cycling, climate regulation, soil formation and recreation. Putting a value on these benefits does not mean they can be replaced.

Ecosystems supply provisioning, regulating, supporting and cultural benefits such as food, pollination, nutrient cycling, climate regulation, soil formation and recreation; valuation does not imply they are replaceable.

Common misconception in Ecosystem

Open full misconception guide →

Does all the energy fixed by plants become new biomass?

Wrong

All of the energy that plants fix by photosynthesis stays in their new biomass.

Why it feels right. I count the energy coming in and overlook what plants use to stay alive.

Right

Net primary productivity is gross primary productivity minus the plants' respiration.

Worked example

Energy fixed, used in respiration and added to biomass, in kJ per square metre per year.
  1. Suppose plants fix 1,000 kJ per square metre in a year and use 400 kJ per square metre for respiration.
  2. The energy added to new biomass is 600 kJ per square metre that year.