Previous year questions: Organisms and Populations

Board pattern for this chapter

Questions come as short definitions, one-mark MCQs and assertions, and three-mark explanations of an interaction with an example. All ten questions below are rewritten from actual CBSE board papers between 2022 and 2024. Marks are printed in each heading, and the year tells you which paper to open for the original.

Q1 (CBSE Board 2022, 2 marks)

An ecologist finds a single banyan tree and 20 Parthenium weeds in a forest. What is the best method to measure the population density of the banyan compared with the Parthenium, and why?

Answer: For the Parthenium, counting the number of plants is the best measure, since individual plants are small and easy to count. For a single banyan tree with a large canopy, count alone would make its density look tiny and understate its role in the community. Per cent cover or biomass is the better measure for the banyan, because it captures the space and material the tree occupies rather than the number of individuals. The comparison is about ecological role: one tree that shades a large area matters more than its single unit suggests.

Q2 (CBSE Board 2022, 2 marks)

Write the different age groups represented in an age pyramid and mention the characteristics of a declining age pyramid.

Answer: An age pyramid shows three groups, the pre-reproductive, the reproductive and the post-reproductive ages, and usually separates males and females. A declining population has a narrow base, meaning fewer individuals in the pre-reproductive group, and the pyramid is urn-shaped, wider in the middle. With fewer young people coming into the reproductive group, the population shrinks over time. The shape is what matters, not the absolute height of any bar.

Q3 (CBSE Board 2025, 2 marks)

Explain the pollination mechanism involved in the co-evolution of the orchid Ophrys and its insect pollinators.

Answer: Ophrys uses sexual deceit. One petal of the flower resembles the female bee in size, colour and markings. A male bee mistakes the petal for a female, attempts to copulate with it, and in the process is dusted with pollen. When the same bee visits another Ophrys flower and repeats the behaviour, the pollen is transferred and the flower is pollinated. No nectar reward is given. The relationship is co-evolved, so a change in the female bee's colour pattern would reduce pollination unless the orchid changed too.

Q4 (CBSE Board 2023, 3 marks)

Explain brood parasitism and the co-evolution of mutualists, with one example each.

Answer: Brood parasitism works by placing eggs in another bird's nest: the parasitic bird lays them in a host nest and leaves incubation to the host, as the cuckoo does with the crow. The parasitic eggs have evolved to match the host's eggs in size and colour, which lowers the chance of detection. Co-evolution of mutualists means that two partners that depend on each other evolve together, so a change in one is followed by a matching change in the other. Fig trees and their pollinator wasps are an example: each fig species is pollinated only by its partner wasp, and the wasp depends on the fig fruit for laying eggs.

Q5 (CBSE Board 2023, 1 mark)

In which population interaction are both partners benefited?

(A) predation (B) commensalism (C) amensalism (D) mutualism

Answer: Option (D), mutualism. In mutualism both interacting species gain, as in lichens, where the fungus and the photosynthesising partner both benefit, or the fig and its wasp.

Q6 (CBSE Board 2023, 1 mark)

Which of the following examples show commensalism? (i) an orchid growing on a mango tree (ii) the cuckoo bird and the crow (iii) Cuscuta growing on a Nerium tree (iv) barnacles on the back of a whale (A) (i) and (ii) (B) (i) and (iv) (C) (ii) and (iii) (D) (ii) and (iv)

Answer: Option (B), (i) and (iv). An orchid on a mango tree gains support while the mango is unaffected, and barnacles on a whale gain transport while the whale is unaffected; both are (+ , 0) relations. The cuckoo and crow pair is brood parasitism and Cuscuta is a parasite, so those are (+ , -) relations.

Q7 (CBSE Board 2024, 3 marks)

Predation is called a detrimental interaction. Explain any three positive roles that a predator plays in an ecosystem, with an example each.

Answer: Predators transfer energy from prey to higher trophic levels, which is how the energy fixed by plants moves through the community. They keep prey populations under control, so prey does not reach densities that destabilise the ecosystem; removing the starfish Pisaster allowed more than ten invertebrate species to become extinct through competition within a year. Predators also help maintain species diversity by reducing the intensity of competition among prey species. Biological pest control uses this regulating role, as when a moth from its native range controlled the prickly pear cactus in Australia. In each case the predator does something the prey itself cannot: it moves energy, holds numbers down, or relaxes competition between prey species.

Q8 (CBSE Board 2024, 3 marks)

Describe how the relationship between a fig tree and its wasp is a spectacular example of mutualism.

Answer: The fig and the wasp show a one-to-one relationship in which a given fig species is pollinated only by its partner wasp species. The female wasp enters the fig fruit and uses it as an egg-laying site, and its larvae feed on some of the developing seeds. While searching for a suitable place to lay eggs, the wasp pollinates the fig inflorescence. The fig gives up a few seeds as food for the wasp's larvae, and the wasp transfers pollen between fig flowers. Both partners gain, so this is mutualism.

Q9 (CBSE Board 2024, 1 mark)

In an interaction where one species is harmed and the other is unaffected, the interaction is called

(A) mutualism (B) predation (C) amensalism (D) competition

Answer: Option (C), amensalism. In amensalism the harm is one-sided and the other species gains nothing and loses nothing, which corresponds to a minus-zero relationship. Keep the three signs in order: amensalism is minus-zero, commensalism is plus-zero and competition is minus-minus. If the wording says one is harmed and the other is unaffected, the answer is amensalism; competition would hurt both.

Q10 (CBSE Board 2024, 1 mark)

State what commensalism is and give one example.

Answer: Commensalism means that one partner gains while the other neither gains nor loses. A cattle egret feeding near grazing cattle is an example: the egret catches insects that the cattle flush out of the vegetation, while the cattle are unaffected.

How to use these answers

Cover the answer, read only the heading, and write the answer in the margin. Then compare. If the facts match but the order differs, the answer is still fine. Aim for the length the marks ask for: one named interaction plus a line of explanation for a one-mark question, and a full process for a three-mark one. If a named example is missing, that is the part to revise, because the board rewards examples more than explanation in this chapter. Two-mark answers in particular are scored for the named pair, not for a long definition. Keep each answer close to the length the marks suggest.

What the board rewards

For this chapter, a definition with an example earns a two-mark answer, and a three-mark answer needs the process plus its ecological significance. When a question gives an interaction, name the sign combination as well; examiners accept it as evidence that you know the classification. For graph questions, identify the curve first: J for exponential and sigmoid for logistic. When a question supplies a diagram of an age pyramid, say the shape in words before naming the type, because the shape carries the explanation.

A short revision order

Revise this chapter in three passes, and keep one worked example ready for each interaction: First pass: the attributes and density measures, since they supply the vocabulary for everything later. Second pass: the two growth equations, with the meaning of r and K written out in a line each. Third pass: the six interactions with one example for each and the sign combination. Then read the three-mark answers above aloud, because the board answers to interaction questions are short and structured, and hearing them helps you keep to that length.