A conservation team is monitoring a population of water voles living along a riverbank. They carry out a mark-recapture study over two seasons and notice that the estimated population size has declined significantly between the first and second season. Explain how a reduction in genetic diversity within this water vole population could reduce its long-term sustainability.

OCR A-Level Biology A (H420) — 6.6 Populations and sustainability · Explain · 4 marks · View as Markdown

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

Water voles are one of the UK's most rapidly declining native mammals, largely due to habitat loss and predation by introduced American mink. Conservation efforts include captive breeding programmes and habitat restoration schemes along river corridors.

Model answer (4 marks)

A smaller population has fewer alleles and less variation at each gene locus.

This increases the likelihood of inbreeding, which can expose harmful recessive alleles and reduce fitness.

With reduced genetic variation the population’s ability to adapt to new pathogens or changing climate is limited.

If a novel disease or environmental change occurs, the genetically similar individuals may all be susceptible, risking total population loss.

Examiner tips

  • Use the exact terms: alleles, inbreeding, recessive alleles, adaptation, genetic diversity, long‑term sustainability.
  • Show the causal chain: small size → fewer alleles → inbreeding → reduced adaptation → risk of extinction.
  • Keep each point concise – one sentence per mark.
  • Avoid unnecessary words like ‘in this essay’.

Common mistakes

  • Confusing genetic diversity with population size alone.
  • Using vague terms such as ‘variation’ without linking to alleles.
  • Failing to mention the risk of a novel disease or environmental change.

Mark scheme (4 marks)

  1. A smaller population has fewer alleles / less variation in alleles at each gene locus
  2. Increased likelihood of inbreeding, leading to expression of harmful recessive alleles
  3. The population has reduced capacity to adapt to environmental changes such as new pathogens or changing climate
  4. A novel disease or environmental change could eliminate the entire population because individuals are genetically similar / susceptible

Key terms in this question

mark-recapture · genetic diversity · sustainability

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