7.4.4 Estimating the Size of a Population

• Estimated because it is too difficult and time-consuming to count all of the species
• Sampling Methods

  • Random Sampling
  • Systematic Sampling
• Methods to Estimate Population Size

  • Quadrats 
  • Belts Transects
  • Mark-release-recapture
• Quadrats = Use to sample slow or non-motile organisms
• Method: Quadrats

  1. Place 2 tape measures at right angles to create a gridded area to take samples from
  2. Use a random number generator to generate coordinates to take samples from (avoids bias)
  3. Place quadrat on the coordinate to either count abundance of species or calculate percentage cover
  4. Repeat (at least 30 times) to get a representative sample of the whole population
• Abundance of species = number of species in the whole quadrats
• Percentage cover = how much of the quadrat is covered
• Belt Transect = Can be used instead of random sampling
• Method: Belt Transect

  1. Tape measure placed through non-uniform ecosystem eg a rocky beach 
  2. Place quadrat at intervals along transect to measure the impact of changes in the environment
• Frequency = how many squares the species is present in
• Density = every individual species is counted
• Mark-Release-Recapture = Used for motile organisms
• Method: Mark-Release-Recapture

  1. Animals are trapped and marked with a substance that must be non-toxic (cannot kill them) , weather-resistant (cannot be washed off)  and not impacted by predation or reproduction (increase or decrease population size)
  2. Released and left to distribute marked animals
  3. They are recaptured a few days later and a 2nd sample is taken. This is repeated at least 10 times
• Population Size = (size of 1st sample x size of 2nd sample)/ number of marked recaptured
• Assumptions of mark-release-recapture

  • Gives enough time for organisms to distribute themselves randomly
  • No migration
  • No births or deaths