Ecosystem — Class 12 MCQs with Answers

Ecosystem — important questions & MCQs with answers (Class 12 Biology)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Producers in an ecosystem:

   AGreen plants and autotrophsBCarnivoresCDecomposersDParasites
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   Answer: Green plants and autotrophs

   Explanation: Producers are autotrophs (green plants, algae, some bacteria) that fix solar energy into food via photosynthesis; carnivores and decomposers only consume or break down organic matter already made.

2. Q2 of 6Easy

   Lindeman's 10% law: energy passed to next trophic level:

   A1%B100%C50%D~10%
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   Answer: ~10%

   Explanation: Lindeman's 10% law: only about a tenth of energy at one trophic level transfers to the next, the rest lost as metabolic heat — this steady tenfold loss is why food chains rarely exceed 4-5 links.

3. Q3 of 6Medium

   Pyramid of energy is always:

   ASame shapeBInvertedCRandomDUpright
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   Answer: Upright

   Explanation: The energy pyramid is always upright because each transfer loses roughly 90% as heat (second law of thermodynamics), so producers always hold more energy than the consumers above — unlike biomass or numbers, which can invert.

4. Q4 of 6Medium

   Nitrogen fixation by:

   AAnimals onlyBPlants directlyCRhizobium / CyanobacteriaDSunlight
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   Answer: Rhizobium / Cyanobacteria

   Explanation: Only certain prokaryotes fix atmospheric N₂ into ammonia: the root-nodule symbiont Rhizobium in legumes and free-living cyanobacteria such as Nostoc and Anabaena; plants and animals lack the nitrogenase enzyme needed.

5. Q5 of 6Hard

   Decomposition rate is highest under:

   AWarm and moist conditionsBCold and dryCFrozenDArid desert
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   Answer: Warm and moist conditions

   Explanation: Decomposition is fastest in warm, moist conditions because microbial (bacterial and fungal) enzyme activity peaks there; cold, dry or waterlogged detritus decomposes slowly and can accumulate as peat.

6. Q6 of 6Hard

   Net Primary Productivity (NPP) =

   AGPP + RespirationBGPP × RespirationCGPP − RespirationDRespiration only
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   Answer: GPP − Respiration

   Explanation: NPP = GPP − R: net primary productivity is what's left after autotrophs spend part of their gross photosynthetic output (GPP) on their own respiration (R); NPP is the food energy actually available to herbivores.

Ecosystem notes: key concepts (Class 12 Biology)

Ecosystem: its structure (producers, consumers, decomposers) and function; food chains, webs and trophic levels; energy flow by the 10% law and pyramids of number, biomass and energy; productivity (GPP, NPP); decomposition; nutrient cycling (carbon, phosphorus); ecological succession; and ecosystem services.

Ecosystem structure

A functional unit of an interacting biotic community and abiotic environment; producers (autotrophs), consumers (heterotrophs) and decomposers are its parts.

Trophic levels

Steps in a food chain — producers (T1), herbivores (T2), carnivores (T3+); an organism's position is its feeding rank. Interlinked chains form a food web.

Grazing vs detritus food chain

GFC starts with living producers eaten by herbivores; DFC starts with dead organic matter (detritus) consumed by detritivores and decomposers.

Ten percent law

Lindeman's law — only about 10% of the energy at one trophic level passes to the next; the rest is lost mainly as heat in respiration.

Ecological pyramids

Graph of number, biomass or energy across trophic levels; the pyramid of energy is always upright, while number/biomass can be inverted (numbers on a single tree, biomass in the sea).

GPP and NPP

Gross primary productivity = total organic matter fixed by producers; Net primary productivity = GPP − respiration and is the biomass available to consumers.

Secondary productivity

The rate of formation of new organic matter by consumers (heterotrophs).

Decomposition

Breakdown of detritus into inorganic nutrients via fragmentation, leaching, catabolism, humification and mineralisation; fastest when warm, moist and N-rich.

Standing crop

The amount of living biomass (or its energy equivalent) present in an ecosystem at a given time.

Carbon cycle

Carbon enters the biotic world by photosynthesis and returns as CO₂ via respiration, decomposition and combustion; oceans are the largest carbon reservoir.

Phosphorus cycle

A sedimentary cycle with rock as the reservoir; unlike the carbon cycle it lacks a significant gaseous (atmospheric) phase.

Ecological succession

Gradual, predictable change in community composition ending in a stable climax; primary succession (bare rock, lichen pioneers) is slower than secondary.

Hydrarch and xerarch

Hydrarch succession starts in water and xerarch in dry areas; both converge toward the same mesic climax community.

Ecosystem services

Products and processes ecosystems provide — pollination, climate regulation, nutrient cycling, pollution control and aesthetic value.