Previous year questions: Ecosystem

Board pattern for this chapter

The ecosystem chapter supplies one-mark MCQs and assertions, two-mark comparisons, and three-mark questions on pyramids or food chains. All ten items below are rewritten from official CBSE papers between 2023 and 2026. Marks are printed in each heading, and the year tells you which paper to open for the original.

Q1 (CBSE Board 2023, 2 marks)

Ecological pyramids give important information, but they have limitations. List any two.

Answer: Two limitations: a pyramid does not account for a species that occupies two or more trophic levels at the same time, and it assumes a simple food chain although a food web is what exists in nature. A third limitation, worth adding if marks remain, is that saprophytes get no place in ecological pyramids even though they are important in the ecosystem. Two named limitations are enough for the two marks, so keep the list short and specific. For a three-mark question, add the third standard limitation about saprophytes. The order does not matter; a numbered list is clearer than one long sentence. Marks are awarded per correct limitation named, so two short, distinct points beat one long explanation that carries both ideas loosely.

Q2 (CBSE Board 2024, 3 marks)

(a) How is the grazing food chain different from the detritus food chain? (b) Give one example of a situation where the detritus food chain is connected to the grazing food chain.

Answer: (a) The grazing food chain starts with a producer and moves through herbivores to carnivores. The detritus food chain starts with dead organic matter and is run by decomposers such as fungi and bacteria, which break the detritus down and absorb the products. (b) Detritus feeders can be eaten by animals of the grazing chain, for example when crows or other birds eat earthworms and insects found in decaying litter. That link joins the two chains into a food web.

Q3 (CBSE Board 2024, 3 marks)

(a) What does an ecological pyramid represent? (b) Ecological pyramids may be upright or inverted. Justify with suitable examples.

Answer: (a) An ecological pyramid represents the feeding relationship between successive trophic levels, expressed as number, biomass or energy, with producers at the base and the top consumer at the apex. (b) In a grassland, numbers and biomass fall from producers to carnivores, so the pyramids are upright. In a sea, the biomass pyramid is inverted because a small standing crop of fast-turning-over phytoplankton supports a much larger biomass of zooplankton and fish. The pyramid of energy is always upright, since energy is lost as heat at every transfer.

Q4 (CBSE Board 2025, 2 marks)

An ecological pyramid of biomass in a sea has 40 units of standing crop of phytoplankton supporting 90 units of zooplankton, which in turn support 120 small fishes. Describe the pyramid and state why it takes this shape.

Answer: The pyramid is inverted. With phytoplankton at the base showing the smallest standing crop, zooplankton above with a larger one and fishes at the top with the largest, each bar gets broader upwards. This happens because phytoplankton have a very high turnover rate: they are eaten as fast as they grow, so a small standing biomass can support a larger biomass of consumers above it.

Q5 (CBSE Board 2025, 2 marks)

Correctly depict, also indicating the trophic levels, and describe the ecological pyramid of number with 32 birds dependent on 20 insects feeding on one banyan tree.

Answer: Draw three bars one above the other. The bottom bar is the producer, one banyan tree, at the first trophic level; the middle bar is the primary consumers, 20 insects, at the second trophic level; the top bar is the secondary consumers, 32 birds, at the third trophic level. The numbers rise from 1 to 20 to 32, so the bars widen upwards and the pyramid is inverted in its lower part. Describe it as an inverted pyramid of numbers, which happens when a single large producer supports many consumers.

Q6 (CBSE Board 2026, 1 mark)

Assertion (A): A terrestrial ecosystem sends most of its energy through the grazing food chain. Reason (R): The grazing and detritus chains can join at some points.

Answer: Assertion A is false, but Reason R is true. On land the detritus route carries most of the energy, not the grazing route. The two chains can connect at some levels, for example when animals of the grazing chain eat detritus feeders, so the reason is correct on its own.

Q7 (CBSE Board 2026, 2 marks)

(a) What is primary productivity of an ecosystem, and how is it expressed? (b) Explain the equation NPP = GPP - R.

Answer: (a) Primary productivity is the rate at which producers fix solar energy into organic matter per unit area over a period. It is expressed as weight per unit area per year, for example grams per square metre per year, or as energy per unit area per year, such as kilocalories per square metre per year. (b) The equation says that net primary productivity is gross primary productivity minus the respiration losses of the plants, R. GPP is the total organic matter produced in photosynthesis, and a considerable part of it is used up by the plants in respiration. What remains, NPP, is the biomass available to heterotrophs, so the equation separates total production from the share that can be eaten.

Q8 (CBSE Additional PQ 2022-23, 2 marks)

Biomass is expressed in dry weight or fresh weight. Which is the more accurate unit for standing crop, and why?

Answer: Dry weight is more accurate. Fresh weight includes the water held by the organisms, and water content changes with the surroundings, the season and the condition of the tissue. Dry weight measures only the organic material, so biomass values from different places and times can be compared fairly.

Q9 (CBSE SQP 2022-23, 1 mark)

Which food chain is the major conduit for energy flow in terrestrial and aquatic ecosystems respectively?

(A) Grazing / Grazing (B) Detritus / Detritus (C) Detritus / Grazing (D) Grazing / Detritus

Answer: Option (C), detritus for the terrestrial ecosystem and grazing for the aquatic one. On land the detritus route carries most of the energy, and in water the grazing route is the main one.

Q10 (CBSE Additional PQ 2023-24, 2 marks)

(a) Study an ecological pyramid of numbers and suggest two situations in which it applies. (b) Give two reasons why the percentage of energy transferred to consumers falls as the food chain lengthens.

Answer: (a) A pyramid of numbers applies where numbers fall steadily with trophic level, as in a grassland with many grass plants, fewer herbivores and still fewer carnivores, and where a single large tree supports many insects and birds, which gives an inverted shape. (b) Energy is lost at each transfer: part of it is used in respiration and released as heat, and part remains in the tissues that are not eaten. Since about 90 per cent is lost at each step, the share passed on to the next level keeps falling.

Common mistakes in this chapter

Most lost marks here come from three habits. Students write NPP as the total production, forgetting that respiration must be subtracted. They list the decomposition steps in a strict line, when the first three act together. And they call the sea biomass pyramid abnormal rather than explaining its turnover. Each of these is easy to correct with one careful reading of the topic pages.

What the board rewards

Answers here are judged on named processes and named pyramids. For a comparison question, use a table or two clearly separated paragraphs, and keep the basis of the comparison the same in both columns. For a pyramid question, say first whether it is upright or inverted, then give the reason, then quote the example. A diagram is not needed in the answer, but naming the trophic levels in order earns credit for understanding the structure. Numericals on GPP and NPP are straightforward once the relation is written down, so write the relation first. If the question gives productivity in grams per square metre per year, keep those units all the way through; changing units halfway is a common source of lost marks.

Two answers that carry the whole chapter

If time is short before the exam, write two answers in full. The first is "describe the processes of decomposition"; it tests five named steps and the factors that control them. The second is "ecological pyramids, their types and limitations"; it tests the three kinds, the inverted cases and the four limitations. Between them these two answers cover most of the marks this chapter can ask for, and the remaining questions are short variants of the same material.