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Read the following passage… (Class 12 Biology 2025, Q30)

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BiologyCBSE Class 12 · Board exam 2025, set 57/1/1 · 4 marks · Case-based

CBSE Class 12 Biology 2025: question 30

Set in the CBSE Class 12 board exam of 2025, paper 57/1/1, section D, question 30, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q30 · 4 marks · Case-basedCompetency-basedOrganisms and Populations

Read the following passage and answer the questions that follow :
In nature, we rarely find isolated, single individuals of any species; majority of them live in groups in a well-defined geographical area, share or compete for similar resources, potentially interbreed and thus constitute a population. The population has certain attributes whereas, an individual organism does not. A population at a given time is composed of individuals of different ages. The size of the population tells us a lot about its status in the habitat. Whatever ecological processes we wish to investigate in a population, be it the outcome of competition with another species, the impact of the predator or the effect of pesticide application, we always evaluate in terms of any change in the population size. The size, in nature, could be low or go into millions. Population size, technically called population density (N) need not necessarily be measured in numbers only. The size of a population for any species is not a static parameter. It keeps on changing with time depending on various factors including food availability, predation pressure and adverse weather.

  1. (a)

    The Monarch butterfly is highly distasteful to its predator because of a special chemical present in its body. How does the butterfly acquire this chemical ?

    1 mark
  2. (b)

    If population density at a time t + 1 is 800, Emigration = 100, Immigration = 200, Natality = 200 and Mortality = 150, calculate the population density at time t and comment upon the type of age pyramid that will be formed in this case.

    2 marks
  3. (c)

    What is the difference in a method of measuring population density in an area if there are 200 carrot grass plants to only single huge banyan tree ?

    1 mark

  4. OR

    (d)

    Name two methods to measure the population density of tigers.

    1 mark

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 57/1/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

The chemical is not made by the adult butterfly. Think about what it ate earlier in life.

Hint 2

Use and decide whether the population is growing or shrinking.

Hint 3

When individuals differ hugely in size, numbers alone mislead. For tigers, think of indirect signs they leave behind.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (a) The butterfly acquires the chemical during its caterpillar stage, by feeding on a poisonous weed.

    1 mark

  2. (b) Formula:

    ½ mark

  3. Substitution:

    ½ mark

  4. Comment: Natality plus immigration exceeds mortality plus emigration, so the population is growing. The age pyramid will therefore be expanding (triangular), with a broad base of pre-reproductive individuals.

    1 mark

  5. (c) Counting individuals is not a meaningful measure here, because one huge banyan tree cannot be compared with 200 small plants. The banyan tree is measured by biomass or percent cover, while the carrot grass (Parthenium) is measured by percent cover.

    1 mark

  6. (d) (alternative to (c)) Tiger density is estimated indirectly from pug marks and faecal pellets.

    1 mark

Answer: (a) From a poisonous weed eaten in its caterpillar stage; (b) = 650, expanding age pyramid; (c) banyan tree by biomass/percent cover, carrot grass by percent cover; (d) pug marks and faecal pellets.

Where the marks are

  • Chemical acquired by feeding on a poisonous weed as a caterpillar (1 mark)
  • Correct formula and substitution (½ mark) and = 650 (½ mark)
  • Expanding age pyramid because the population is increasing (1 mark)
  • (c) Banyan tree by biomass/percent cover and carrot grass by percent cover (½ + ½ marks), OR (d) pug marks and faecal pellets (½ + ½ marks)

Revise the chapter

Practise this chapterBoard-style questions from Organisms and Populations, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 57/1/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.