AQA AS Biology

Revision Notes

4.6.6 Random Sampling

Apparatus & Techniques: Random Sampling

  • Measuring the different levels of biodiversity within an ecosystem can be a tasking job
  • Finding out which species live in an ecosystem and the size of the populations requires the identification and cataloguing of all organisms present to build a species list
  • This is possible for areas that are very small or where the species are very large like trees
  • However, for larger and more complex ecosystems like rainforests, it is simply impossible to find, identify and count every organism that exists there
  • When this is the case different samples of the area can be taken and used to make an estimate for the total species numbers in the area


  • Sampling is a method of investigating the abundance and distribution of species and populations
  • There are two different types of sampling:
    • Random
    • Systematic
  • In random sampling the positions of the sampling points are completely random or due to chance
    • This method is beneficial because it means there will be no bias by the person that is carrying out the sampling that may affect the results
  • In systematic sampling the positions of the sampling points are chosen by the person carrying out the sampling
    • There is a possibility that the person choosing could show bias towards or against certain areas
    • Individuals may deliberately place the quadrats in areas with the least species as these will be easier and quicker to count
    • This is unrepresentative of the whole area
  • When a sampling area is reasonably uniform or has no clear pattern to the way the species are distributed then random sampling is the best choice

Frame quadrats

  • Some ecosystems are very complex with large numbers of different species of different sizes
  • For the sake of logistics, sampling is often used to estimate the distribution and abundance of species
    • The distribution of a species describes how it is spread throughout the ecosystem
    • The abundance of a species is the number of individuals of that species
  • When carrying out sampling, square frames called quadrats can be used to mark off the area being sampled
  • Quadrats of different sizes can be used depending on what is being measured and what is most suitable in the space the samples are being made in
  • Quadrats must be laid randomly in the area to avoid sampling bias
    • This random sampling can be done by converting the sampling area into a grid format and labelling each square on the grid with a number
    • Then a random number generator is used to pick the sample points
  • Once the quadrat has been laid on the chosen sample point the abundance of all the different species present can be recorded


  • Quadrat
  • Random number generator


  • Mark up a grid on a map or a to-scale drawing of the area being studied and label the grid with coordinates
    • It is important that the area is big enough to get a representative estimate for the specific habitat/ecosystem
  • Use a random number generator to choose a set of coordinates
    • To avoid sampling bias which could lead to over or under-estimation (either subconsciously choosing areas that are easier to access or that look like they contain individuals)

Calculating Population Density (number of individuals per m­­­2):

  • Place a quadrat at the generated coordinate
  • Count the number of individuals in each quadrat
  • Use a running mean to determine the number of quadrats required to get a representative sample:
    • Calculate the mean number of individuals per quadrat for the first two quadrats found, then the mean of the first three, then the mean of the first four –there is no change in mean
  • Multiply the mean number of individuals per quadrat by the whole area to calculate the estimated population size

Calculating the Abundance of a Species using Percentage Cover (%):

  • Usually used for plants
  • Place a grid quadrat at this coordinate
    • Each square of a grid quadrat is equivalent to 1% cover
  • Count the number of squares in each quadrat within which the species occupies over half the square
    • If 30 squares contain the species, the percentage cover is 30%
    • 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 (%):

  • Place a frame quadrat at multiple coordinates generated
  • Count the number of quadrats that contain the species
    • If 3 out of 10 quadrats contain the species the frequency is 30%

Results from quadrats

  • The results from the quadrats can be used to calculate the predicted frequency and density of a species within an area
  • Species frequency is the probability that the species will be found within any quadrat in the sample area
    • The number of quadrats that the species was present in is divided by the total number of quadrats and then multiplied by 100
    • For example, if bluebells were found in 18 out of 50 quadrats the species frequency would be (18/50) x 100 = 36%
  • Species density indicates how many individuals of that species there are per unit area
    • The number of individuals counted across all quadrats is divided by the total area of all the quadrats
    • For example, if 107 bluebells were found across 50 quadrats that are 1m2 each the species density would be 107/50 = 2.14 individuals per m2
  • It can sometimes be difficult to count individual plants or organisms. When this is the case percentage cover of the species within the quadrat can be estimated instead
    • The quadrat is divided into 100 smaller squares. The number of squares the species is found in is equivalent to its percentage cover in that quadrat
    • For example, if grass is found in 89 out of 100 squares in the quadrat then it has a percentage cover of 89%


  • Quadrats and transects can only be used for sessile and immobile species (eg. plants and slow-moving animals)
  • Some species can be counted to find their abundance but others that are very small or in high numbers require abundance to be calculated using percentage cover or frequency techniques
    • The frequency technique shows how common a species is but it 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
    • This can be used to answer questions such as: does the species prefer light or dark/wet or dry/crowded or sparse/low or high pH / exposed or shaded environments?

Exam Tip

In the exam, you could be asked to design an appropriate experiment to investigate the distribution of a species or to note how the overall characteristics of organisms differ across a habitat. For example, calculating whether the size of leaves differs between different environments within a habitat or whether data from random sampling suggests whether a species grows in groups or individually.

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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