Explain how habitat fragmentation reduces biodiversity in a population of large mammals.

IB DP Biology Standard 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. Fragmentation isolates subpopulations, cutting off gene flow between them.
2. Small isolated subpopulations are more vulnerable to genetic drift, which reduces genetic diversity.
3. Inbreeding increases homozygosity, causing expression of deleterious recessive alleles (inbreeding depression).
4. Reduced population size and genetic diversity lower the population’s ability to adapt to environmental change, raising extinction risk.

Examiner tips

Use the exact terms: gene flow, genetic drift, inbreeding depression, extinction risk.
Show the causal chain: isolation → reduced gene flow → drift → loss of diversity → inbreeding → extinction risk.
Keep each point concise and separate with numbers or bullets.

Common mistakes

Confusing habitat loss with fragmentation; they are different processes.
Failing to mention inbreeding depression or genetic drift explicitly.
Writing vague statements like “population declines” without linking to genetic mechanisms.

Mark scheme (4 marks)

Fragmentation isolates subpopulations, reducing gene flow between them
Small isolated subpopulations are more vulnerable to genetic drift, reducing genetic diversity
Inbreeding increases the frequency of homozygosity, leading to expression of deleterious recessive alleles (inbreeding depression)
Reduced population size and genetic diversity lowers the ability of the population to adapt to environmental change, increasing extinction risk

Key terms in this question

habitat fragmentation