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The diagram below shows the… (Class 12 Biology 2026, Q29)

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BiologyCBSE Class 12 · Board exam 2026, set 57/1/1 · 4 marks · Case-based

CBSE Class 12 Biology 2026: question 29

Set in the CBSE Class 12 board exam of 2026, paper 57/1/1, section D, question 29, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q29 · 4 marks · Case-basedCompetency-basedOrganisms and Populations

The diagram below shows the distribution of two barnacle species, Chthamalus and Balanus on a rocky sea shore.
When Balanus is experimentally removed, Chthamalus expands its range in lower intertidal zone.

  1. (a)

    Identify and define the ecological phenomenon demonstrated by this observation.

    1 mark

  2. OR

    (a)

    State the principle that explains the elimination of one species.

    1 mark
  3. (b)

    Two different species can compete for the same resource. Give another example of it.

    1 mark
  4. (c)

    How do species avoid competition in nature ? Explain with an example.

    2 marks

Figure in the paper: Rocky sea shore diagram with Balanus balanoides and Chthamalus stellatus shown. Zones from highest tides to lowest tides: Upper intertidal zone – Balanus dessicates in this zone, allowing Chthamalus to thrive; Middle intertidal zone – where it can survive, Balanus outcompetes Chthamalus for space; Lower intertidal zone – Balanus is subject to predation in the lower tidal zone. See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2026 board paper, set 57/1/1.

Hints

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

Hint 1

What happens to the inferior competitor's range when the superior one is taken away?

Hint 2

Think of an example from South American lakes involving birds and fish.

Hint 3

Animals can share a resource if they use it at different times or places. What is this called?

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 phenomenon is competitive release.

    ½ mark

  2. Definition: a species whose distribution is restricted to a small area, because a competitively superior species is present, expands its range when the superior competitor is removed. Here Chthamalus spreads into the lower zones once Balanus is removed.

    ½ mark

  3. (a) OR: Gause's competitive exclusion principle: two closely related species competing for the same resources cannot coexist indefinitely, and the competitively inferior one is eventually eliminated.

    1 mark

  4. (b) In some shallow South American lakes, visiting flamingos and resident fishes compete for the same food, zooplankton.

    1 mark

  5. (c) Species avoid competition by resource partitioning: they choose different times for feeding or different foraging patterns.

    1 mark

  6. Example: MacArthur showed that five closely related species of warblers living on the same tree avoid competition and coexist. They do this by behavioural differences in foraging, each feeding in a different part of the tree.

    1 mark

Answer: (a) Competitive release (or Gause's competitive exclusion principle); (b) flamingos and fishes competing for zooplankton in South American lakes; (c) resource partitioning, e.g. MacArthur's warblers foraging in different parts of the same tree

Where the marks are

  • Names competitive release (½ mark)
  • Correct definition (½ mark)
  • OR: states Gause's competitive exclusion principle (1 mark)
  • Example of interspecific competition, e.g. flamingos and fish for zooplankton (1 mark)
  • Resource partitioning (1 mark)
  • Example, e.g. MacArthur's warblers (1 mark)

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 paper2026 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.