MCQs on Ecosystem – CBSE Class 12 Biology (NCERT Based)

Course: CBSE Class 12 Biology
Unit V: Ecology and Environment
Topic: Ecosystem
Strictly as per NCERT | CBSE Board Exam Oriented | Concept-Clearing Explanations

Section A: Ecosystem – Structure and Function (Q1–Q15)

Q1. An ecosystem is defined as

A. a group of plants only
B. a group of animals only
C. interaction between living organisms and their environment
D. a population of a species

Correct Answer: C
Explanation: An ecosystem includes biotic components (living organisms) and abiotic components (physical environment) interacting as a system.

Q2. Which of the following is an abiotic component of an ecosystem?

A. Producers
B. Consumers
C. Decomposers
D. Temperature

Correct Answer: D
Explanation: Abiotic components include non-living factors such as temperature, light, water, and soil.

Q3. Producers in an ecosystem are mainly

A. herbivores
B. carnivores
C. green plants
D. decomposers

Correct Answer: C
Explanation: Green plants produce food using photosynthesis and form the base of food chains.

Q4. Consumers directly dependent on producers are

A. secondary consumers
B. tertiary consumers
C. herbivores
D. decomposers

Correct Answer: C
Explanation: Herbivores feed directly on producers and are primary consumers.

Q5. Decomposers are also called

A. autotrophs
B. heterotrophs
C. saprotrophs
D. producers

Correct Answer: C
Explanation: Decomposers obtain nutrients by breaking down dead organic matter.

Q6. Which organism plays a key role in nutrient recycling?

A. Producers
B. Herbivores
C. Carnivores
D. Decomposers

Correct Answer: D
Explanation: Decomposers recycle nutrients back to the ecosystem.

Q7. Standing crop refers to

A. rate of biomass production
B. total biomass at a given time
C. number of organisms
D. food chain length

Correct Answer: B
Explanation: Standing crop is the amount of biomass present per unit area at a specific time.

Q8. Primary productivity is the rate of

A. energy transfer between trophic levels
B. biomass production by producers
C. decomposition
D. respiration

Correct Answer: B
Explanation: Primary productivity refers to biomass produced by plants through photosynthesis.

Q9. Gross primary productivity (GPP) includes

A. energy stored only
B. energy lost in respiration
C. total energy fixed by plants
D. energy transferred to consumers

Correct Answer: C
Explanation: GPP is the total energy fixed by plants before respiratory losses.

Q10. Net primary productivity (NPP) is

A. GPP + respiration
B. GPP – respiration
C. respiration – GPP
D. total ecosystem productivity

Correct Answer: B
Explanation: NPP is the energy available to consumers after plant respiration.

Q11. Which ecosystem has the highest primary productivity?

A. Desert
B. Grassland
C. Tropical rainforest
D. Tundra

Correct Answer: C
Explanation: Tropical rainforests have ideal conditions for photosynthesis.

Q12. Litter refers to

A. waste materials
B. dead plant remains on soil surface
C. animal excreta
D. inorganic matter

Correct Answer: B
Explanation: Litter consists of dead leaves, twigs and plant debris.

Q13. Which component links biotic and abiotic parts of ecosystem?

A. Producers
B. Consumers
C. Decomposers
D. Herbivores

Correct Answer: C
Explanation: Decomposers convert organic matter into inorganic nutrients.

Q14. Ecosystem stability depends largely on

A. population size
B. biodiversity
C. migration
D. extinction

Correct Answer: B
Explanation: Greater biodiversity increases ecosystem stability.

Q15. Which of the following is a natural ecosystem?

A. Aquarium
B. Crop field
C. Forest
D. Garden

Correct Answer: C
Explanation: Forests develop naturally without human intervention.

Section B: Energy Flow and Food Chains (Q16–Q30)

Q16. Energy in an ecosystem flows

A. cyclically
B. unidirectionally
C. randomly
D. bidirectionally

Correct Answer: B
Explanation: Energy flows from producers to consumers and is lost as heat.

Q17. The ultimate source of energy in an ecosystem is

A. plants
B. consumers
C. sun
D. decomposers

Correct Answer: C
Explanation: Solar energy drives photosynthesis.

Q18. A food chain always begins with

A. decomposer
B. consumer
C. producer
D. herbivore

Correct Answer: C
Explanation: Producers form the base of all food chains.

Q19. A food web represents

A. single food chain
B. interconnected food chains
C. trophic levels
D. energy pyramid

Correct Answer: B
Explanation: Food webs show complex feeding relationships.

Q20. Trophic level refers to

A. habitat
B. feeding position
C. population density
D. niche

Correct Answer: B
Explanation: Each step in a food chain is a trophic level.

Q21. The 10% law of energy transfer was proposed by

A. Lindeman
B. Darwin
C. Odum
D. Elton

Correct Answer: A
Explanation: Lindeman stated that only 10% energy transfers to next level.

Q22. Energy pyramid is always

A. inverted
B. upright
C. spindle shaped
D. irregular

Correct Answer: B
Explanation: Energy decreases at higher trophic levels.

Q23. Which pyramid can be inverted?

A. Energy pyramid
B. Biomass pyramid
C. Productivity pyramid
D. Energy flow

Correct Answer: B
Explanation: Biomass pyramid can be inverted in aquatic ecosystems.

Q24. Secondary consumers are usually

A. herbivores
B. carnivores
C. producers
D. decomposers

Correct Answer: B
Explanation: Secondary consumers feed on herbivores.

Q25. Detritus food chain begins with

A. producers
B. consumers
C. dead organic matter
D. herbivores

Correct Answer: C
Explanation: Detritus food chain starts with dead remains.

Q26. Which ecosystem shows inverted biomass pyramid?

A. Forest
B. Grassland
C. Pond
D. Desert

Correct Answer: C
Explanation: Aquatic ecosystems have small phytoplankton biomass.

Q27. Food webs increase ecosystem

A. instability
B. simplicity
C. stability
D. extinction

Correct Answer: C
Explanation: Alternate food sources increase resilience.

Q28. Loss of energy at each trophic level occurs mainly as

A. growth
B. reproduction
C. heat
D. biomass

Correct Answer: C
Explanation: Energy is lost during respiration as heat.

Q29. Maximum energy is available at

A. top consumers
B. secondary consumers
C. producers
D. decomposers

Correct Answer: C
Explanation: Producers capture maximum solar energy.

Q30. Which statement is correct?

A. Matter flows unidirectionally
B. Energy is recycled
C. Energy flows one way
D. Nutrients are lost forever

Correct Answer: C
Explanation: Energy flow is unidirectional; matter is recycled.

Section C: Ecological Pyramids and Nutrient Cycling (Q31–Q50)

Q31. Ecological pyramids represent

A. population growth
B. energy flow
C. trophic structure
D. succession

Correct Answer: C
Explanation: Pyramids show trophic level relationships.

Q32. Pyramid of numbers represents

A. biomass
B. energy
C. number of organisms
D. productivity

Correct Answer: C
Explanation: It shows numerical strength at each level.

Q33. Which pyramid is always upright?

A. Numbers
B. Biomass
C. Energy
D. All

Correct Answer: C
Explanation: Energy pyramid cannot be inverted.

Q34. Nutrient cycling refers to

A. energy flow
B. movement of nutrients
C. population growth
D. succession

Correct Answer: B
Explanation: Nutrients move cyclically between biotic and abiotic components.

Q35. Carbon cycle involves exchange of carbon between

A. soil and rocks only
B. atmosphere, biosphere and oceans
C. plants only
D. animals only

Correct Answer: B
Explanation: Carbon cycles through air, water, plants and animals.

Q36. Which process releases CO₂ into atmosphere?

A. Photosynthesis
B. Respiration
C. Nitrogen fixation
D. Decomposition only

Correct Answer: B
Explanation: Respiration releases carbon dioxide.

Q37. Nitrogen fixation converts

A. nitrate to nitrogen
B. ammonia to nitrate
C. atmospheric nitrogen to ammonia
D. nitrite to nitrate

Correct Answer: C
Explanation: Nitrogen fixation makes nitrogen usable.

Q38. Which bacteria fix atmospheric nitrogen?

A. Lactobacillus
B. Rhizobium
C. Nitrosomonas
D. Nitrobacter

Correct Answer: B
Explanation: Rhizobium fixes nitrogen symbiotically.

Q39. Phosphorus cycle differs from carbon cycle because it

A. involves atmosphere
B. lacks gaseous phase
C. is faster
D. involves respiration

Correct Answer: B
Explanation: Phosphorus has no significant gaseous phase.

Q40. Which ecosystem process returns nutrients to soil?

A. Photosynthesis
B. Respiration
C. Decomposition
D. Consumption

Correct Answer: C
Explanation: Decomposers release nutrients back into soil.

Q41. Biogeochemical cycles maintain

A. energy flow
B. nutrient balance
C. population size
D. biodiversity only

Correct Answer: B
Explanation: Cycles ensure continuous availability of nutrients.

Q42. Sedimentary cycles include

A. carbon cycle
B. nitrogen cycle
C. phosphorus cycle
D. oxygen cycle

Correct Answer: C
Explanation: Phosphorus cycle is sedimentary.

Q43. Which human activity disrupts carbon cycle?

A. Photosynthesis
B. Respiration
C. Deforestation
D. Decomposition

Correct Answer: C
Explanation: Deforestation reduces carbon fixation.

Q44. Eutrophication results from excess

A. carbon dioxide
B. nitrogen and phosphorus
C. oxygen
D. hydrogen

Correct Answer: B
Explanation: Excess nutrients cause algal blooms.

Q45. Which organism plays a role in nitrification?

A. Rhizobium
B. Nitrosomonas
C. Azotobacter
D. Cyanobacteria

Correct Answer: B
Explanation: Nitrosomonas converts ammonia to nitrite.

Q46. Ecosystem services include

A. food production
B. climate regulation
C. nutrient cycling
D. all of these

Correct Answer: D
Explanation: Ecosystems provide multiple life-support services.

Q47. Which ecosystem is self-sustaining?

A. Aquarium
B. Garden
C. Pond
D. Crop field

Correct Answer: C
Explanation: Natural ecosystems are self-sustaining.

Q48. Which cycle is affected by excessive fertilizer use?

A. Carbon cycle
B. Oxygen cycle
C. Nitrogen cycle
D. Water cycle

Correct Answer: C
Explanation: Excess nitrogen causes nutrient imbalance.

Q49. Stability of ecosystem increases with

A. fewer species
B. monoculture
C. complex food webs
D. limited nutrients

Correct Answer: C
Explanation: Complex interactions enhance stability.

Q50. Ecosystem functioning mainly depends on

A. number of species
B. energy flow and nutrient cycling
C. migration
D. succession only

Correct Answer: B
Explanation: Energy flow and nutrient cycles sustain ecosystems.

✅ End of MCQs – Ecosystem (Class 12 Biology, NCERT)