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CraftExam
medium difficulty7 min

Loss of Biodiversity and the Evil Quartet

Loss of biodiversity is the reduction or disappearance of genetic variation, species or ecosystems due to factors such as habitat loss, fragmentation, overexploitation, alien species invasion and co-extinctions.

In one line: Home lost, hunted too much, outsiders invade, partners disappear.

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Understand it

  • Biodiversity loss affects populations, food webs and ecosystem services.
  • The four major causes are often called the evil quartet.
  • Habitat loss and fragmentation reduce available living space and split populations into smaller isolated groups.
  • Overexploitation removes organisms faster than they can reproduce.
  • Alien species may become invasive and outcompete, prey upon or disturb native species.
  • Co-extinction occurs when the extinction of one species causes another dependent species to disappear, such as a host-specific parasite or a plant-pollinator pair.
  • In exams, marks usually depend on naming the cause and explaining the biological effect, not just listing threats.

Example

When a forest is converted into agricultural land, habitat loss reduces native species. If a pollinator species disappears, a plant depending only on that pollinator may also decline, showing co-extinction risk.

Rule to remember

Key biological rule: extinction risk rises when population size, habitat continuity and species interactions are disrupted.

The evil quartet summarises the major drivers.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

If a case mentions an introduced fish reducing native fish populations, identify alien species invasion and explain that the introduced species may compete with or prey upon native species.

Method you can reuse

  1. 1Identify the pressure described in the question.
  2. 2Match it to one of the evil quartet causes.
  3. 3Explain how it affects population size, reproduction, competition or species interaction.
  4. 4Add a specific example when possible.
  5. 5End by linking the effect to extinction risk or ecosystem imbalance.

Visual tip

A flow diagram can show human activity leading to habitat loss or fragmentation, then small isolated populations, reduced gene flow and higher extinction risk. Labels should include habitat loss, fragmentation, alien species, overexploitation and co-extinction.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

Why can habitat fragmentation increase extinction risk even when some habitat remains?

Think first, then show answer
Habitat fragmentation increases extinction risk because populations become small and isolated. Such populations have less breeding opportunity, reduced gene flow and higher vulnerability to local disturbances.

Common wrong answer

Writing only 'human activity causes extinction' is too vague because it does not identify the specific driver or the biological mechanism.

Don't mix this up

Students list pollution or climate change generally but ignore the NCERT-tested evil quartet and the cause-effect link for each cause.

Exam tip

For a long answer, write all four causes with one mechanism and one example. Use the term habitat fragmentation separately from habitat loss.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

Commonly asked as 'Explain the evil quartet', case-study identification, assertion-reason on co-extinction, or short notes on habitat loss and fragmentation.

Answer plan

Define biodiversity loss, name the evil quartet, explain each cause in one sentence with a biological effect, add one example and close by linking loss to reduced ecosystem stability.
  1. 1

    Enough for 1 mark

    Loss of biodiversity is the reduction or disappearance of genetic variation, species or ecosystems due to factors such as habitat loss, fragmentation, overexploitation, alien species invasion and co-extinctions.

  2. 2

    Add for 2 marks

    Example: When a forest is converted into agricultural land, habitat loss reduces native species. If a pollinator species disappears, a plant depending only on that pollinator may also decline, showing co-extinction risk.

  3. 3

    Add for 3 marks

    Biodiversity loss affects populations, food webs and ecosystem services.

Practice

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