Class 9 Biology Chapter 12 Ecosystem MCQ with Answers — 30 Solved Questions

The 2024–25 CBSE Class 9 curriculum emphasizes competency-based assessment over memorization. MCQs test three cognitive levels: recall (What is an autotroph?), understanding (Why do decomposers return nutrients to soil?), and application (If 10,000 kJ enters an ecosystem, how much reaches tertiary consumers?). Ecosystem MCQs are particularly valuable because they blend ecology principles with quantitative reasoning—a hallmark of modern CBSE exams. Unlike short-answer questions, MCQs demand precision: a single misread word in an option can change your answer. The Ecosystem chapter covers four interconnected topics: primary productivity (the rate at which autotrophs fix solar energy), decomposition (the biological breakdown of organic matter), energy flow (the unidirectional passage of energy through food chains), and nutrient cycles (the cyclical movement of nitrogen, phosphorus, and carbon). Each topic generates 2–3 high-yield MCQ patterns. By solving 30 strategically designed questions, you internalize not just answers but the underlying logic—exactly what CBSE scorers reward.

These 10 questions test foundational definitions and straightforward recall. **Q1.** Which of the following is an autotroph? (A) Lion (B) Grass (C) Decomposer (D) Earthworm **Answer:** (B) Grass — Autotrophs produce their own food via photosynthesis; all others are heterotrophs. **Q2.** The primary source of energy in an ecosystem is: (A) Soil nutrients (B) Solar radiation (C) Decomposing matter (D) Water **Answer:** (B) Solar radiation — The sun's energy powers all photosynthesis and sustains the entire food chain. **Q3.** Decomposers include: (A) Plants and animals (B) Bacteria and fungi (C) Herbivores and carnivores (D) Insects and birds **Answer:** (B) Bacteria and fungi — Decomposers break down dead organic matter into simpler compounds. **Q4.** Primary productivity is measured as: (A) Biomass of all organisms (B) Energy fixed by autotrophs per unit area per unit time (C) Total calories consumed by herbivores (D) Number of plants in an ecosystem **Answer:** (B) Energy fixed by autotrophs per unit area per unit time — This is the NCERT definition in Class 9. **Q5.** Which organism occupies the second trophic level? (A) Grass (B) Grasshopper (C) Frog (D) Snake **Answer:** (B) Grasshopper — Primary consumers (herbivores) occupy trophic level 2; grass is trophic level 1. **Q6.** In a food chain, energy decreases as we move up because: (A) Autotrophs absorb more sunlight (B) Some energy is lost as heat during metabolism (C) Carnivores are larger than herbivores (D) Decomposers consume excess energy **Answer:** (B) Some energy is lost as heat during metabolism — Only ~10% of energy transfers between trophic levels; 90% is lost. **Q7.** The carbon cycle involves: (A) Only photosynthesis (B) Only respiration (C) Photosynthesis, respiration, and decomposition (D) Only decomposition **Answer:** (C) Photosynthesis, respiration, and decomposition — The carbon cycle includes multiple biological and abiotic processes. **Q8.** Nitrogen-fixing bacteria belong to which trophic level? (A) Producer (B) Primary consumer (C) Decomposer (D) Secondary consumer **Answer:** (C) Decomposer — Nitrogen-fixing bacteria (e.g., Azotobacter, Rhizobium) convert atmospheric N₂ into usable nitrates. **Q9.** In nutrient cycles, the limiting nutrient is often: (A) Carbon (B) Nitrogen or phosphorus (C) Hydrogen (D) Oxygen **Answer:** (B) Nitrogen or phosphorus — These are least abundant and most frequently deficient in ecosystems. **Q10.** Gross Primary Productivity (GPP) minus respiration loss equals: (A) Net Primary Productivity (NPP) (B) Secondary Productivity (C) Decomposer Productivity (D) Total Biomass **Answer:** (A) Net Primary Productivity (NPP) — The equation is GPP − Respiration = NPP. This is the energy available to consumers.

These questions require understanding relationships and performing energy calculations. **Q11.** If primary productivity of a grassland is 2000 kcal/m²/year, approximately how much energy reaches secondary consumers? (A) 2000 kcal/m²/year (B) 200 kcal/m²/year (C) 20 kcal/m²/year (D) 2 kcal/m²/year **Answer:** (C) 20 kcal/m²/year — Energy transfer is ~10% per trophic level. Herbivores get ~10% (200 kcal), carnivores get ~10% of that (20 kcal). **Q12.** Which of the following statements about decomposition is correct? (A) It only occurs after an organism dies. (B) It releases nutrients bound in organic matter back into the ecosystem. (C) It is powered by sunlight like photosynthesis. (D) It reduces the biodiversity of an ecosystem. **Answer:** (B) It releases nutrients bound in organic matter back into the ecosystem — Decomposition is the critical link recycling nutrients through biogeochemical cycles. **Q13.** In the nitrogen cycle, nitrification converts: (A) N₂ to NO₃⁻ (B) NH₃ to NO₂⁻ and NO₃⁻ (C) NO₃⁻ to N₂ (D) Proteins to nitrogen gas **Answer:** (B) NH₃ to NO₂⁻ and NO₃⁻ — Nitrifying bacteria (Nitrosomonas and Nitrobacter) oxidize ammonia to nitrite, then nitrate. **Q14.** An ecosystem has 100 kg of grass. If 10% energy transfer holds, which is the maximum biomass of tertiary consumers (assuming three trophic levels plus grass)? (A) 10 kg (B) 1 kg (C) 0.1 kg (D) 0.01 kg **Answer:** (C) 0.1 kg — Grass (100 kg) → Herbivores (10 kg) → Carnivores (1 kg) → Top Carnivores (0.1 kg). **Q15.** Which factor would NOT directly increase Net Primary Productivity? (A) Increased sunlight (B) Increased carbon dioxide concentration (C) Increased temperature (moderate range) (D) Increased predator population **Answer:** (D) Increased predator population — Predators consume biomass but do not increase photosynthetic capacity of plants. **Q16.** The phosphorus cycle differs from the nitrogen cycle because: (A) Phosphorus has no gaseous phase in the atmosphere. (B) Phosphorus is more abundant than nitrogen. (C) Phosphorus does not require decomposers. (D) Nitrogen cycles faster than phosphorus. **Answer:** (A) Phosphorus has no gaseous phase in the atmosphere — Phosphorus is always in solid/aqueous form (rocks, soil, water); nitrogen exists as N₂ gas in air. **Q17.** In a marine ecosystem, which trophic level stores the least total energy? (A) Phytoplankton (B) Zooplankton (C) Fish (herbivorous) (D) Large predatory fish **Answer:** (D) Large predatory fish — Energy decreases with each trophic level; apex predators have the smallest biomass and energy. **Q18.** If decomposers are removed from an ecosystem, which would happen first? (A) All plants would die. (B) Nutrients would accumulate in dead organic matter and become unavailable to plants. (C) Herbivores would have more food. (D) Water cycle would stop. **Answer:** (B) Nutrients would accumulate in dead organic matter and become unavailable to plants — Without decomposers, nutrient cycling halts and soil becomes nutrient-poor. **Q19.** The 10% rule in energy flow suggests that feeding on: (A) Plants is more efficient than feeding on herbivores. (B) Herbivores is more efficient than feeding on plants. (C) Carnivores is more efficient than feeding on herbivores. (D) Omnivores is the most efficient strategy. **Answer:** (A) Plants is more efficient than feeding on herbivores — Eating lower trophic levels wastes less energy in transfers; vegetarianism is ecologically efficient. **Q20.** Which statement about the carbon cycle is false? (A) Photosynthesis removes CO₂ from the atmosphere. (B) Respiration returns CO₂ to the atmosphere. (C) Fossil fuels release stored carbon when burned. (D) Decomposition consumes all carbon and prevents its return to the atmosphere. **Answer:** (D) Decomposition consumes all carbon and prevents its return to the atmosphere — Decomposition releases CO₂ back into the air; it does not consume carbon permanently.

These test conceptual depth and logic linking two statements. **Q21.** **Assertion (A):** Net Primary Productivity is always less than Gross Primary Productivity. **Reason (R):** Some energy fixed by plants is used in their own respiration. (A) Both A and R are true; R explains A. (B) Both A and R are true; R does not explain A. (C) A is true; R is false. (D) A is false; R is true. **Answer:** (A) Both A and R are true; R explains A — Plants use photosynthetically fixed energy for growth, maintenance, and respiration. NPP = GPP − Plant Respiration; hence A is true and R correctly explains why. **Q22.** **Assertion (A):** Decomposers are heterotrophs. **Reason (R):** They obtain energy by breaking down dead organic matter but cannot manufacture their own food. (A) Both A and R are true; R explains A. (B) Both A and R are true; R does not explain A. (C) A is true; R is false. (D) Both A and R are false. **Answer:** (A) Both A and R are true; R explains A — Decomposers lack chlorophyll and rely on external organic matter for energy; they are, by definition, heterotrophs. **Q23.** **Assertion (A):** In a grazing food chain, only herbivores can be classified as primary consumers. **Reason (R):** Primary consumers always feed directly on producers. (A) Both A and R are true; R explains A. (B) Both A and R are true; R does not explain A. (C) A is true; R is false. (D) A is false; R is true. **Answer:** (A) Both A and R are true; R explains A — Primary consumers occupy trophic level 2 and feed directly on autotrophs (producers). Both statements are consistent and A is explained by R. **Q24.** **Assertion (A):** The pyramid of energy always has a smaller apex than its base. **Reason (R):** Energy is lost at each trophic level as heat and through metabolic processes. (A) Both A and R are true; R explains A. (B) Both A and R are true; R does not explain A. (C) A is true; R is false. (D) Both A and R are false. **Answer:** (A) Both A and R are true; R explains A — Only ~10% of energy transfers upward. Heat loss and maintenance respiration ensure the pyramid is always inverted; R explains the mechanism behind A. **Q25.** **Assertion (A):** The phosphorus cycle does not have an atmospheric reservoir like the nitrogen or carbon cycles. **Reason (R):** Phosphorus exists only in solid form in rocks and soil, never as a gas. (A) Both A and R are true; R explains A. (B) Both A and R are true; R does not explain A. (C) A is true; R is false. (D) Both A and R are false. **Answer:** (A) Both A and R are true; R explains A — Phosphorus forms no gaseous molecules under normal Earth conditions; it cycles through rock weathering, soil, water, and organisms—never the atmosphere. R correctly explains why A is true. **Q26.** **Assertion (A):** Nitrogen-fixing bacteria increase soil fertility by converting atmospheric N₂ into nitrates. **Reason (R):** Plants cannot directly absorb gaseous nitrogen from the atmosphere. (A) Both A and R are true; R explains A. (B) Both A and R are true; R does not explain A. (C) A is true; R is false. (D) Both A and R are false. **Answer:** (A) Both A and R are true; R explains A — Plants lack the enzyme nitrogenase and cannot fix N₂. Bacteria like Rhizobium and Azotobacter convert N₂ to NH₃ and NO₃⁻, which plants absorb. R explains why A is true. **Q27.** **Assertion (A):** If a food chain has 4 trophic levels and the primary productivity is 10,000 kcal/m²/year, the tertiary consumers can have a maximum of 10 kcal/m²/year. **Reason (R):** Energy transfer between trophic levels is approximately 10%. (A) Both A and R are true; R explains A. (B) Both A and R are true; R does not explain A. (C) A is true; R is false. (D) A is false; R is true. **Answer:** (A) Both A and R are true; R explains A — Level 1 (Producers): 10,000 kcal. Level 2 (Primary consumers): 1,000 kcal. Level 3 (Secondary consumers): 100 kcal. Level 4 (Tertiary consumers): 10 kcal. R explains the mechanism. **Q28.** **Assertion (A):** Decomposers respire more biomass than any other group in an ecosystem. **Reason (R):** Decomposers feed on all dead organic matter from all trophic levels. (A) Both A and R are true; R explains A. (B) Both A and R are true; R does not explain A. (C) A is true; R is false. (D) Both A and R are false. **Answer:** (A) Both A and R are true; R explains A — Decomposers access energy from dead organisms, feces, and shed material across all trophic levels. Their biomass and respiration can exceed that of individual consumer groups. R correctly explains A. **Q29.** **Assertion (A):** Gross Primary Productivity in an aquatic ecosystem is often lower than in a terrestrial forest. **Reason (R):** Aquatic ecosystems receive less sunlight due to water absorption and turbidity. (A) Both A and R are true; R explains A. (B) Both A and R are true; R does not explain A. (C) A is true; R is false. (D) A is false; R is true. **Answer:** (A) Both A and R are true; R explains A — Water absorbs wavelengths and particulates reduce light penetration, limiting photosynthesis in water compared to land (where up to 50% of incident light can be used). R explains why aquatic GPP tends to be lower. **Q30.** **Assertion (A):** A food web is more stable than a simple food chain because multiple feeding pathways buffer against species loss. **Reason (R):** If one species becomes extinct in a food web, organisms can rely on alternative food sources, whereas in a chain, extinction of one species collapses the entire chain. (A) Both A and R are true; R explains A. (B) Both A and R are true; R does not explain A. (C) A is true; R is false. (D) Both A and R are false. **Answer:** (A) Both A and R are true; R explains A — Food webs provide redundancy and alternative pathways for energy transfer. Loss of one species is less catastrophic because consumers have backup food sources. R mechanistically explains why A is true.

CBSE MCQs are designed to catch careless reading. Here are the top traps in Ecosystem questions: **Trap 1: Confusing Trophic Levels with Organism Type** Students often pick 'Herbivore' when the question asks for 'trophic level 2.' The answer should be 'Primary Consumer' (the role), not the type. Always read: "Which occupies..." vs. "Which is..." — position vs. identity. **Trap 2: Energy Transfer % Mistakes** A common wrong answer is "50% energy transfer" or "100% energy transfer." The NCERT-defined rule is ~10%. If you see 20%, 33%, or 90%, verify using the 10% logic before selecting. **Trap 3: Mixing Up Decomposition & Respiration** Decomposers respire, but not all respiration is decomposition. Decomposition specifically refers to breaking down dead organic matter. Respiration is any organism's metabolic process. These are not synonyms. **Trap 4: Assuming All Cycles Are Gaseous** The carbon and nitrogen cycles have atmospheric phases (CO₂, N₂). Phosphorus does not. A question asking "Which cycle does NOT include a gaseous phase?" has phosphorus as the trap-proof answer. If you see "phosphorus has a gaseous reservoir," reject it immediately. **Trap 5: Reversing Producer & Consumer Definitions** Some options flip: "Producers consume organic matter" or "Consumers produce their own food." These are false. Producers fix energy; consumers ingest it. Read each definition carefully; one misread word invalidates your answer. **Trap 6: Confusing GPP, NPP, and Secondary Productivity** GPP is total energy fixed by plants. NPP is GPP minus plant respiration (energy available to consumers). Secondary Productivity is energy fixed by consumers (herbivores, carnivores). Wrong option example: "NPP is energy used by all heterotrophs"—this is not NPP; it's the energy available to consumers, which is less than NPP because some NPP is lost before reaching consumers. **Trap 7: Overstating Ecosystem Fragility** A trap option in reasoning sections: "If one organism dies, the ecosystem collapses." This is false unless you're in a specialized ecosystem (e.g., only one pollinator). Real ecosystems have redundancy. The correct logic is: "Biodiversity increases stability." **Trap 8: Misreading Assertion–Reason Links** Both statements can be true individually, but one does NOT explain the other. Example: A = "Grass is green." R = "Chlorophyll absorbs light." Both true, but R does not explain why grass looks green (absorption ≠ reflection colour perception). Choose (B), not (A). **How to Avoid:** Read every option twice. Underline the subject and the predicate. Check if the logic matches NCERT definitions, not intuition. When in doubt, use the 10% rule and the concept of nutrient cycling to filter out false claims.

Ecosystem MCQs span multiple concepts (productivity, decomposition, energy flow, nutrient cycles). A single question can integrate all four. Here's a time-optimized approach for a 30-question quiz (45–60 minutes total): **Step 1: Categorize by Difficulty (2 minutes)** Scan all 30 questions. Mark Easy (straightforward recall), Medium (require calculation or linking), Hard (assertion–reason). This preview reduces anxiety and builds confidence. **Step 2: Solve Easy Questions First (12 minutes)** Aim for 1 minute per Easy MCQ. These are low-hanging fruit and consolidate foundational knowledge. If you complete all 10 Easy MCQs in 10 minutes, you have a buffer. Do NOT re-read Easy answers; move on. **Step 3: Medium MCQs with Calculation (20 minutes)** Spend 1.5–2 minutes per Medium MCQ. When you encounter an energy calculation: 1. Write the starting energy on paper (e.g., 2000 kcal). 2. Apply 10% step-by-step for each trophic level. 3. Cross-check against options. Discard non-numeric or illogical answers (e.g., 5000 kcal when input is 2000 kcal). Skip if stuck for >2 minutes; return later. **Step 4: Hard / Assertion–Reason MCQs (12–15 minutes)** These demand logic, not speed. Spend 1–1.5 minutes per question: 1. Read Assertion once. Decide True or False. 2. Read Reason once. Decide True or False. 3. Ask: Does Reason explain Assertion? (Yes → A; No → B). 4. Do NOT second-guess. Trust your NCERT knowledge. If unsure, eliminate obviously wrong options (e.g., if both A and R are clearly true, (C) and (D) are out). **Step 5: Review & Gaps (10 minutes)** Return to skipped Medium and Hard MCQs. Re-read the question stem once, not the options. Apply fresh reasoning. If still uncertain, make an educated guess using option frequency (if you've chosen (B) five times, (A) might be next). **Pro Tips:** - **Avoid rereading full option sets.** After your first read, reread only the correct answer for confirmation. - **Use keywords.** In assertion–reason, if you see "all," "never," or "always," verify with a counterexample. - **10% rule is your anchor.** Any energy calculation defaults to ~10% transfer. If an option violates this, reject it. - **Mark passage questions.** If a question refers to data (e.g., "In a given ecosystem..."), underline the data before reading options. Prevents misreading numbers. - **Nitrogen & phosphorus cycles.** Keep their key difference in mind: N has a gaseous phase; P does not. This filters ~30% of hard options instantly. With this strategy, you should complete 30 MCQs in 50 minutes, leaving 10 minutes for panic-proofing. Confidence is 80% of CBSE success; time discipline builds confidence.

Solving 30 MCQs once is not revision; it's practice. True retention requires spaced, interleaved practice. Here's a science-backed plan: **Day 1: Learn & Solve (Easy MCQs Only)** Read NCERT Chapter 12 once (60 min). Solve the 10 Easy MCQs (10 min). Write reasons in your own words (5 min). Accuracy: Aim for 100%. Time: 75 min. **Day 2: Medium MCQs & Energy Calculations** Review your Day 1 Easy answers (5 min). Solve 10 Medium MCQs without a calculator (15 min). Hand-calculate energy transfers on paper. Check answers (5 min). Accuracy: Aim for 80%+. Time: 25 min. **Day 3: Hard / Assertion–Reason MCQs** Solve 10 Hard MCQs (20 min). For each wrong answer, reread the NCERT section it tests. Write a two-sentence explanation of why both statements matter (10 min). Accuracy: Aim for 70%+. Time: 30 min. **Day 4: Full 30-MCQ Simulation Under Timed Conditions** Solve all 30 in one sitting (60 min). Simulate an exam: no phone, no referring to NCERT mid-quiz. Afterward, identify weak topics (e.g., "I struggled with phosphorus cycle"). Reread only those sections (15 min). **Day 5–7: Spaced Retrieval** Every second day, randomly pick 5 MCQs from the three sets. Solve without reviewing the answers. This forces your brain to rebuild connections, strengthening long-term memory. Time: 5 min per day. **Weekly: Cross-Link with Other Chapters** Ecosystem connects to Chapter 13 (Biodiversity & Conservation). A question like "Why does habitat loss reduce energy flow?" blends both chapters. Solving such integrated questions boosts exam readiness. Dedicate 10 min weekly to cross-chapter MCQs. **Accuracy Milestones:** - Easy: 100% by Day 1. - Medium: 80%+ by Day 2. - Hard: 70%+ by Day 3. - Full quiz: 75%+ by Day 4. - Spaced retrieval: Maintain 85%+ by Day 7. If you fall short, revisit the specific NCERT section and repeat that MCQ set. Consistency beats cramming. Start a 3-day free trial at cbsetutor.ai to unlock AI-guided feedback on every MCQ—identify your specific weak spots and get personalized explanations in seconds, not hours.