Ecosystem — Class 12 MCQs with Answers
Class 12 CBSE Biology · Chapter 12
90 practice questions · 30 Easy · 30 Medium · 30 Hard · Updated
Practise the most important Class 12 CBSE Biology questions from Chapter 12, "Ecosystem". You get 9 timed quizzes made from 90 NCERT-based MCQs, with answers and explanations. The questions are split into 30 Easy, 30 Medium and 30 Hard. Warm up on the basics, then move on to the exam-level questions that set top scorers in CBSE & Maharashtra HSC Board exams, MHT-CET and NEET UG apart.
To score well in "Ecosystem", focus on diagrams, key terms and knowing NCERT line by line. Each MCQ here is timed and uses exam-style marking (+4 correct, −1 wrong, 0 skipped). This trains you to stay accurate under time pressure, as real papers need. Every question has a short explanation, so a wrong answer becomes a quick lesson. It is the fastest way to fix gaps before a test.
Use this chapter for focused revision. Start with the Easy set to check your basics on Ecosystem, then move to Medium and Hard to practise applying them. Your accuracy, streaks and XP save automatically. This chapter also adds to your overall Class 12 Biology mastery score. 14 sample questions are solved in full below, with the answer and a worked explanation. Sign in free to start practising.
Key concepts: Ecosystem (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.
Ecosystem — important questions & MCQs with answers (Class 12 Biology)
14 solved questions from this chapter's difficulty levels, each with its answer and explanation. The other 76 are timed and scored when you sign in.
- Q1Easy
Producers in an ecosystem:
A.Green plants and autotrophs✓ CorrectB.CarnivoresC.DecomposersD.ParasitesAnswer: A. 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.
- Q2Easy
Lindeman's 10% law: energy passed to next trophic level:
A.1%B.100%C.50%D.~10%✓ CorrectAnswer: D. ~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.
- Q3Easy
An ecosystem is a functional unit consisting of:
A.Only plantsB.Living organisms interacting with their physical environment✓ CorrectC.Only abiotic factorsD.Only animalsAnswer: B. Living organisms interacting with their physical environment
Explanation: An ecosystem is a functional unit where biotic communities interact with abiotic factors (soil, water, climate) through energy flow and nutrient cycling — not just a collection of plants or animals alone.
- Q4Easy
Gross primary productivity (GPP) is the rate of:
A.DecompositionB.Organic matter consumed by herbivoresC.Total organic matter produced by photosynthesis✓ CorrectD.Respiration onlyAnswer: C. Total organic matter produced by photosynthesis
Explanation: Gross primary productivity (GPP) is the total rate of organic matter (biomass) production by photosynthesis, before subtracting respiratory losses — NPP is what remains after autotrophs respire.
- Q5Easy
In an ecosystem, green plants are:
A.DetritivoresB.ConsumersC.DecomposersD.Producers (autotrophs)✓ CorrectAnswer: D. Producers (autotrophs)
Explanation: Green plants are producers (autotrophs): they synthesize their own food from CO₂, water and sunlight via photosynthesis, forming the base of every trophic pyramid that consumers and decomposers depend on.
- Q6Easy
Net primary productivity (NPP) equals GPP minus:
A.Light energyB.DecompositionC.ConsumptionD.Respiration losses✓ CorrectAnswer: D. Respiration losses
Explanation: NPP = GPP − respiration: net primary productivity is the organic matter left over after producers use part of their gross photosynthetic yield to fuel their own metabolism.
- Q7Medium
Pyramid of energy is always:
A.Same shapeB.InvertedC.RandomD.Upright✓ CorrectAnswer: D. 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.
- Q8Medium
Nitrogen fixation by:
A.Animals onlyB.Plants directlyC.Rhizobium / Cyanobacteria✓ CorrectD.SunlightAnswer: C. 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.
- Q9Medium
The ultimate source of energy for almost all ecosystems is:
A.DecomposersB.Soil mineralsC.The sun✓ CorrectD.WaterAnswer: C. The sun
Explanation: The sun is the ultimate energy source for nearly all ecosystems: producers capture solar energy via photosynthesis, and this fixed energy then flows unidirectionally through every consumer trophic level.
- Q10Medium
Decomposition is faster when the detritus is:
A.Rich in nitrogen and water-soluble substances, in a warm moist climate✓ CorrectB.Rich in lignin and chitinC.Dry and coldD.Anaerobic and acidicAnswer: A. Rich in nitrogen and water-soluble substances, in a warm moist climate
Explanation: Decomposition speeds up when detritus is nitrogen-rich and contains water-soluble substances, in a warm, moist climate that favours microbial enzyme activity — the opposite of lignin- or chitin-rich detritus, which resists breakdown.
- Q11Medium
The pyramid of biomass in a pond/sea ecosystem can be inverted because:
A.Energy increases upwardB.Producers always weigh moreC.There are no consumersD.Small phytoplankton (producers) have less standing biomass than the consumers they support✓ CorrectAnswer: D. Small phytoplankton (producers) have less standing biomass than the consumers they support
Explanation: In pond and sea ecosystems, the biomass pyramid inverts because tiny, fast-reproducing phytoplankton hold little standing biomass at any instant, even though their high turnover rate supports much larger consumer biomass above.
- Q12Hard
Decomposition rate is highest under:
A.Warm and moist conditions✓ CorrectB.Cold and dryC.FrozenD.Arid desertAnswer: A. 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.
- Q13Hard
Net Primary Productivity (NPP) =
A.GPP + RespirationB.GPP × RespirationC.GPP − Respiration✓ CorrectD.Respiration onlyAnswer: C. 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.
- Q14Hard
If producers fix 10,000 J of energy, the energy available to a secondary consumer (per the 10% law) is about:
A.10 JB.1000 JC.100 J✓ CorrectD.10,000 JAnswer: C. 100 J
Explanation: By the 10% law, 10,000 J at producers gives 1,000 J to primary consumers, then 10% of that — 100 J — to secondary consumers; each transfer divides the available energy by ten.
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Start this chapter free →Ecosystem — FAQs
What are the key concepts in Class 12 Biology Ecosystem?+
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. Key ideas include Ecosystem structure, Trophic levels, Grazing vs detritus food chain, Ten percent law.
What does Class 12 Biology Chapter 12 (Ecosystem) cover on XamBaaz?+
It has 90 NCERT-based MCQs on "Ecosystem": 30 Easy, 30 Medium and 30 Hard. Together they make 9 timed quizzes, and you never get the same set twice. Every question has an instant explanation. They help you prepare for CBSE & Maharashtra HSC Board exams, MHT-CET and NEET UG.
Are these "Ecosystem" questions free to practise?+
Yes. Sign in with Google to practise "Ecosystem" free. Full unlimited access is ₹999/year on a launch offer until 1 December 2026. No chapter is charged separately.
How should I revise "Ecosystem" for the exam?+
Start with the Easy quiz to check your basics, then try Medium and Hard to practise applying them. There are 9 timed quizzes on this chapter, so you can come back for a fresh set instead of one you have seen. Read each explanation, retry the questions you miss, and track your accuracy until it stays high.
Are these "Ecosystem" MCQs available with answers?+
Yes. 14 sample questions are shown here in full, each with the correct option and a step-by-step "Why" explanation. Sign in free with Google to start practising, with instant scoring.
Is there negative marking in the "Ecosystem" quizzes?+
Yes. The timed quizzes use exam-style marking: +4 for a right answer, −1 for a wrong one and 0 for a skip, the same negative marking as NEET UG. MHT-CET and CBSE board papers have no negative marking. Our mocks for those are scored their way.
What are the important questions from Ecosystem (Class 12 Biology)?+
The questions that matter most test Ecosystem structure, Trophic levels, Grazing vs detritus food chain, Ten percent law. This page shows 14 solved important MCQs with answers and explanations; all 90 questions on the chapter are available as timed quizzes once you sign in.
Is there an online quiz for Ecosystem?+
Yes — Class 12 Biology Ecosystem has timed online quizzes at Easy, Medium and Hard levels, with instant scoring and a worked explanation on every question. The first quiz on the chapter is free.