Capture/recapture Techniques (WJEC GCSE Biology: Combined Science)

Higher Tier Only

• Quadrat sampling is only useful for sessile organisms
• The capture/recapture technique can be used to estimate the number of individuals in a population of motile animals
  • This technique is also known as mark-release-capture
• This technique works as follows:
  1. Capture a sample of the population of interest and mark the animals using a non-harmful method
     • E.g. A small amount of non-toxic paint can be applied
  2. Return the marked animals to their habitat and allowed to mix with the rest of the population
  3. When a sufficient amount of time has passed for mixing to occur capture another sample
  4. Count the number of marked and unmarked individuals within the second sample
  5. Use the following equation to calculate an estimate of the population size

N = (n₁ × n₂) ÷ m₂

• • Where:
    • N = population estimate
    • n₁ = number of marked individuals released after the first sample
    • n₂ = total number of individuals in the second sample
    • m₂ = number of marked individuals in the second sample

Assumptions made when using capture/recapture

• When using the capture/recapture technique there are a few assumptions that are made, including:
  • The population stays the same size during the study period
    • E.g. there are no deaths or migrations into or out of the main population in between samples
  • The marking doesn't affect the survival rates of the marked individuals
    • E.g. the marking process doesn't make the individuals more visible and therefore more likely to be predated