Describe how the capture-recapture method is used to estimate the size of a population of beetles living in a woodland.

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

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

A student wants to find out how many ground beetles live in a woodland area. The woodland covers a large area and it is not possible to count all the beetles directly.

Model answer (4 marks)

1. Capture a sample of beetles and count them.
2. Mark each beetle in the sample and release them back into the woodland.
3. Wait for the marked beetles to mix with the rest of the population.
4. Capture a second sample and count how many of the beetles are marked; use the proportion of marked beetles to estimate the total population size.

Examiner tips

  • Use the four steps in the correct order; include the key words "capture", "mark", "release", "mix", "second sample".
  • Explain the relationship between the proportion of marked beetles in the second sample and the estimated population size.

Mark scheme (4 marks)

  1. Organisms (beetles) are captured and counted
  2. The captured organisms are marked and released back into the habitat
  3. Time is allowed for the marked beetles to redistribute/mix with the rest of the population before a second sample is captured
  4. The number of marked individuals in the second sample is used to estimate the total population size (larger proportion of marked = smaller population; smaller proportion = larger population)

Key terms in this question

capture-recapture · population

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