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(i) How does alien species… (Class 12 Biology 2025, Q33a)

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BiologyCBSE Class 12 · Board exam 2025, set 57/7/1 · 5 marks · Long answer

CBSE Class 12 Biology 2025: question 33(a)

Set in the CBSE Class 12 board exam of 2025, paper 57/7/1, section E, question 33(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q33(a) · 5 marks · Long answerBiodiversity and Conservation

(i) How does alien species invasion cause a decline in biodiversity ? Explain.
How have the following contributed to biodiversity loss ?
I. Nile Perch
II. Lantana and Eichhornia
III. Clarias gariepinus
(ii) Why have certain regions been declared as biodiversity hotspots by environmentalists of the world ? Name any two such regions in India.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 57/7/1.

Hints

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

Hint 1

Think about what happens when a new species arrives in a place where it has no natural enemies.

Hint 2

Recall the 'Evil Quartet' examples: which lake in Africa lost its cichlid fishes?

Hint 3

For hotspots, focus on two features: the number of species and how many of them are endemic.

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. (i) Alien (exotic) species are introduced into a new area, either deliberately or by accident. Some of them have no natural predators there and become invasive. They out-compete the native species for food and space, which causes those indigenous species to decline or go extinct.

    1 mark

  2. I. Nile Perch: It was introduced into Lake Victoria (East Africa). It led to the extinction of more than 200 species of cichlid fish native to the lake.

    1 mark

  3. II. Lantana and Eichhornia (water hyacinth): These invasive weeds spread rapidly. They caused environmental damage and threaten native species. Eichhornia chokes water bodies, while Lantana displaces native vegetation.

    ½ mark

  4. III. Clarias gariepinus (African catfish): It was brought in for aquaculture. It now threatens the indigenous catfishes of Indian rivers.

    ½ mark

  5. (ii) Biodiversity hotspots are regions with very high species richness and a high degree of endemism, that is, many species found nowhere else.

    ½ mark

  6. Such regions are declared hotspots so that intensive conservation efforts can be concentrated on them, which reduces ongoing species extinctions.

    ½ mark

  7. Hotspots covering India (any two): Western Ghats and Sri Lanka; Himalaya; Indo-Burma.

    1 mark

Answer: (i) Invasive aliens out-compete native species and cause their decline or extinction. Nile perch wiped out more than 200 cichlid species in Lake Victoria; Lantana and Eichhornia damage the environment and threaten native species; Clarias gariepinus threatens indigenous catfishes. (ii) Hotspots have high species richness and high endemism, so they are prioritised for conservation; e.g. Western Ghats–Sri Lanka and Himalaya (also Indo-Burma).

Where the marks are

  • Explains that introduced species become invasive and cause decline or extinction of native species (1 mark)
  • Nile perch caused extinction of more than 200 cichlid species in Lake Victoria (1 mark)
  • Lantana and Eichhornia caused environmental damage and threatened native species (½ mark)
  • Clarias gariepinus threatens indigenous catfishes in Indian rivers (½ mark)
  • Hotspots have high species richness and high endemism, and are declared for conservation (½ + ½ mark)
  • Names any two Indian hotspot regions (½ + ½ mark)

Revise the chapter

Practise this chapterBoard-style questions from Biodiversity and Conservation, one at a time, marked against the answer key and marking points.Back to the full paper2025 board paper, set 57/7/1: 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.