Investigating Populations

• Sampling is used to give reasonably accurate estimates for the abundance of a species
• The larger the sample size, the more reliable the data
• Sampling can be undertaken in two different methods: random or systematic
• Random sampling is used for uniform areas
• Systematic sampling is used for areas with changes in abiotic/biotic factors
• Point quadrats are used to measure frequency
• Frame quadrats are used to calculate percentage cover
• Things to consider with quadrats: size of the quadrat, number of samples, position of each quadrat
• Random sampling

  1. set out grid using two tape measures
  2. obtain random pairs of coordinates
  3. place the corner of the quadrat at the given coordinates
  4. count the number of organisms
  5. record data
  6. repeat until you get a running mean
• A minimum of 20 tests must be undertaken to make sure the results are reliable because it is enough to carry out statistical tests and calculate a running mean
• Quadrats along a belt transect

  Lay out tape measure and place quadrats on alternating sides along the length of the tape measure at set distance intervals
• Disadvantages of belt transects

  • if distances between the quadrats are too large, the sampling is insufficient because other factors could also change
• Frequency of a species = number of quadrats containing the species/total number of quadrats examined x100
• Advantages of using frequency = provides a quick idea of the species present and their general distribution
• Disadvantages of using frequency = no account of organism size or percentage cover
• Advantages of percentage cover = useful if species is abundant or difficult to count, data is collected quickly, individuals plants don't need counting
• Disadvantages of percentage cover = overlapping organisms are counted once, doesn't state the number of individuals
• Mark-release-recapture

  1. capture a number of organisms
  2. carefully mark them in a way that doesn't hinder their chance of detection
  3. release and allow time to re-integrate into the population
  4. collect 2nd sample
  5. count and record the number of marked/unmarked organisms
  6. use MRR formula to estimate population size
• Estimated total population = (number in first sample x number in second sample) divided by the number marked in the second sample
• Assumptions that the mark-release-recapture method relies on:

  • proportion of marked to unmarked individuals is the same for both samples
  • marked individuals distribute evenly before second sample
  • population has a definite boundary so no immigration/emigration
  • few births/deaths
  • method of marking doesn't affect chance of survival
  • mark/label isn't lost or rubbed off