4.2 - Abiotic & Biotic Factors

Estimating population size by capture-recapture

• Capture-recapture (also called mark-release-recapture) estimates how many animals of one species live in an area.
• It's used for animals that move about - such as woodlice, snails, beetles or butterflies - which can't simply be counted in a quadrat.

The five steps

1. Capture a first sample of the animals (e.g. with a net, a pooter or a pitfall trap) and count them.
2. Mark each one harmlessly - for example a small spot of non-toxic paint on the underside, where predators can't see it.
3. Release the marked animals back where they were caught.
4. Leave time (often a day or two) for them to mix back in with the rest of the population.
5. Capture a second sample from the same area in the same way. Count both the total number caught and how many of them are marked.

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Formula for population size

The idea behind it is that the fraction of the second sample that's marked is the same as the fraction of the whole population that was marked in the first sample.

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Worked example - Population of woodlice

40 woodlice are caught, marked and released. The next day, 60 woodlice are caught and 12 of them are marked. Estimate the population size.

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Assumptions for a reliable estimate

• No population change - No births, deaths or migration in or out between the two samples, so the population stays the same size.
• Marks stay on - The marks aren't lost, for example washed off by rain.
• Marks don't affect survival - A mark that makes an animal easier for predators to spot leaves fewer marked animals to recapture, so the number marked in the second sample is too small and the estimate comes out too high.
• Marked animals mix evenly - They've had time to mix back in with the rest of the population before the second sample is taken.