Sampling Practical

• Quadrat


  A square frame used to sample an area of habitat
• Random number generator


  Used to choose sample sites to avoid sampling bias
• Method for choosing sample sites

  1. Mark up a grid on a map or a to-scale drawing of the area being studied

  2. Label the grid with coordinates

  3. Use a random number generator to choose a set of coordinates

  4. This is done to avoid sampling bias which could lead to over or under-estimation
• It is important that the area is big enough to get a representative estimate for the specific habitat ecosystem
• Quadrats


  • Contents within quadrats can be counted using different measurements
  • Population density, percentage cover and species frequency are all different ways of counting and recording the number of different species and individuals present within a quadrat
• Calculating population density (number of individuals per m2)

  1. Place a quadrat at the generated coordinate

  2. Count the number of individuals in each quadrat

  3. Use a running mean to determine the number of quadrats required to get a representative sample

  4. To calculate the estimated population size for the whole area or habitat, divide the whole area by the area of one quadrant, then multiply this value by the mean number of individuals per quadrat
• Calculating the abundance of a species using percentage cover (%)

  1. Place a grid quadrat (a square frame split into 100 smaller squares) at this coordinate

  2. Count the number of squares in each quadrat within which the species occupies over half the square

  3. This method is subjective and therefore the same person should make the estimate for all samples to control this variable
• Calculating the abundance of a species using frequency (%)

  1. Place a frame quadrat at multiple coordinates generated

  2. Count the number of quadrats that contain the species

  3. If 3 out of 10 quadrats contain the species the frequency is 30%
• Species density


  Indicates how many individuals of that species there are per unit area
• Percentage cover


  The number of squares the species is found in is equivalent to its percentage cover in that quadrat
• Species frequency


  The probability that the species will be found within any quadrat in the sample area
• Quadrats and transects


  • Can only be used for sessile and immobile species (e.g. plants and slow-moving animals)
• Counting species abundance


  • Some species can be counted directly, others that are very small or in high numbers require abundance to be calculated using percentage cover or frequency techniques
• Frequency technique


  Shows how common a species is but does not give information on the estimated number of individuals or the size of the population
• Percentage cover and frequency

  When used together, give a good picture of the distribution of a species
• If a species had a high mean percentage cover but a low frequency

  It would suggest the species lives in groups in preferred areas of the habitat
• Mark-release-capture method


  Estimating population sizes of mobile animals
• Mark-release-capture method for a single species

  1. First large sample taken, individuals caught, counted and marked

  2. Marked individuals returned to habitat to mix with population

  3. Second large sample captured, marked and unmarked individuals counted

  4. Proportion of marked to unmarked used to calculate population estimate
• Assumptions for mark-release-capture method

  • Marked individuals given time to disperse and mix back in
  • Marking doesn't affect survival rates of marked individuals
  • Marking remains visible throughout sampling
  • Population stays the same size during study period