Explain how the capture-recapture method can be used to monitor changes in a population of water voles living along a riverbank over several years, and why accurate population estimates are important for conservation.

OCR GCSE Biology A: Gateway Science (J247) — B6.1 Monitoring and maintaining the environment · Explain · 4 marks · View as Markdown

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

Water voles are a protected species in the UK whose populations have declined sharply due to habitat loss and predation by invasive American mink. Conservation organisations regularly monitor water vole populations along riverbanks to inform management decisions.

Model answer (4 marks)

1. Capture a sample of water voles and mark each animal with a harmless, identifiable tag before releasing it back into its habitat.
2. Allow a suitable period for the marked voles to mix fully with the rest of the population.
3. Capture a second sample and record how many of the captured animals are marked.
4. Use the proportion of marked animals in the second sample to calculate the total population size, and compare these estimates over successive years to detect changes.

Accurate population estimates let conservationists recognise whether the water‑vole population is falling, stable or increasing, enabling timely and appropriate management actions to protect the species and its riverbank habitat.

Examiner tips

  • Use the word ‘capture‑recapture’ and explain each step in order; include the key phrase ‘proportion of marked individuals’ for the calculation.
  • Show that accurate estimates help detect population trends and justify management decisions.

Common mistakes

  • Confusing the marking method with a harmful technique; students may say ‘kill and count’. Failing to mention the need for a mixing period between samples. Not linking accurate estimates to the ability to detect population decline and inform conservation actions.

Mark scheme (4 marks)

  1. Animals are captured, marked in a way that does not harm them, and released back into the habitat
  2. Time is allowed for marked individuals to redistribute/mix with the rest of the population before a second sample is captured
  3. The proportion of marked individuals in the second sample is used to estimate the total population size
  4. Accurate population estimates allow conservationists to identify whether the population is declining, so that appropriate conservation management can be put in place to protect the species/habitat

Key terms in this question

capture-recapture · population · conservation

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