Explain how habitat fragmentation reduces the long-term survival prospects of a population of large mammals occupying a nature reserve that has become isolated from other suitable habitats.

IB DP Biology Higher Level (2023 syllabus) — A4.2 Conservation of biodiversity · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

1. Isolation stops immigration, so gene flow between the reserve and other populations is cut off.
2. The small, isolated group is more affected by genetic drift, which removes alleles and reduces genetic diversity.
3. Inbreeding becomes more common, causing harmful recessive alleles to be expressed and heterozygosity to fall – a classic case of inbreeding depression.
4. With less genetic variation the population cannot adapt quickly to new stresses such as disease or climate change, so its long‑term survival prospects are greatly reduced.

Examiner tips

  • Use the exact terms: isolation, gene flow, genetic drift, inbreeding depression, reduced genetic diversity, extinction risk.
  • Show the causal chain – isolation → no gene flow → drift & inbreeding → lower fitness and adaptability.
  • Keep each point concise and directly linked to the marks scheme.

Mark scheme (4 marks)

  1. Isolation prevents immigration of individuals, reducing gene flow between populations
  2. Small, isolated population experiences increased genetic drift, leading to loss of genetic diversity / alleles
  3. Increased inbreeding results in expression of harmful recessive alleles / reduced heterozygosity, lowering individual fitness (inbreeding depression)
  4. Reduced genetic diversity lowers the population's ability to adapt to environmental change / disease, increasing extinction risk

Key terms in this question

habitat fragmentation

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