OCR AS Biology

Revision Notes

4.2.2 Sampling to Determine Biodiversity

Sampling to Determine Biodiversity

  • Measuring the different levels of biodiversity within an ecosystem can be challenging
  • 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
    • Non-random
  • 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 non-random 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

Random sampling

  • Some ecosystems are very complex with large numbers of different species of different sizes
  • For the sake of logistics, random 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 are square frames made of wood or wire
    • They can be a variety of sizes eg. 0.25m2 or 1m2
    • They are placed on the ground and the organisms within them are recorded
    • They can be used to measure the distribution and abundance of plants or slow-moving animals

Quadrat in use, downloadable IGCSE & GCSE Biology revision notes

Using a quadrat to investigate population size or distribution

  • 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 or percentage cover of all the different species present can be recorded

Percentage Cover in Quadrat, downloadable AS & A Level Biology revision notes

Using a quadrat to investigate the percentage cover of two species of grass. There may be some squares lacking any species and other squares with multiple species – this means the total percentage cover of a single quadrat can sometimes be over or under 100%

  • Quadrats are suitable for sampling plants or slow-moving animals
  • For many animal species, however, it is not possible to use quadrats to measure their distribution and abundance
  • In these cases, other techniques involving other items of equipment are necessary, including:
    • Sweeping nets: these are large, strong nets with a fine material (very small holes) that are used to catch flying insects and insects that live in long grass by sweeping the net back and forth through the grass
    • Pitfall traps: these are cans or jars that are buried in the ground that are used to catch ground-dwelling (often nocturnal) insects and other invertebrates as they fall into the trap
    • Pooters: these are small plastic or glass containers with two tubes sticking out that are used to suck up small insects and other small invertebrates. The first tube is placed over the insect and the second tube is used by the scientist to create suction
    • Tullgren funnel: these are funnels with a light bulb above and a container below that are used to collect invertebrates that live in leaf litter or soil. The leaf litter or soil is placed in the funnel and the light and heat forces the invertebrates to move down until they drop into the container
    • Kick-sampling: this technique is used to catch freshwater invertebrates living in streams or rivers. A net in placed on the stream-bed so that the water is flowing into it and the stream-bed just above the net is churned up by the scientist (using their foot) for a set period of time. The invertebrates are carried by the stream into the net

Invertebrate sampling techniques (1), downloadable AS & A Level Biology revision notesExample of how a pitfall trap can be used.


Invertebrate sampling techniques (2), downloadable AS & A Level Biology revision notes

An example of how a pooter can be used.

Invertebrate sampling techniques (3), downloadable AS & A Level Biology revision notes

An example of how a Tullgren funnel can be used.


Invertebrate sampling techniques (4), downloadable AS & A Level Biology revision notesExample of how kick-sampling is done.

Non-random sampling

  • Random sampling is not always possible or may take an impractically long time. In these cases, non-random sampling may be more suitable
  • There are three main types of non-random sampling:
    • Opportunistic sampling
    • Stratified sampling
    • Systematic sampling
  • Opportunistic sampling involves picking and choosing sampling locations based on various non-random factors. For example, students on a field trip may be told they can only collect samples from locations that are nearby, easy to reach, and safe
  • Stratified sampling involves matching the number of sampling locations in a particular habitat with the relative proportion of area that habitat covers in the whole area being studied. For example, if 10% of a dense woodland being sampled is actually made up of grassy clearings, then 90% of the sample locations should be in the woodland habitat and 10% in the grass habitat
  • Systematic sampling is used when there is a clear change in the physical conditions across the area being studied
    • For example, there may be changes in altitude, soil pH or light intensity
    • Methods using transects can help show how species distribution changes with the different physical conditions in the area
    • A transect is a line represented by a measuring tape, along which sample are taken
  • For a line transect:
    • Lay out a measuring tape in a straight line across the sample area
    • At equal distances along the tape, record the identity of the organisms that touch the line. For example, every 2m
    • This produces qualitative data
  • For a belt transect:
    • Place quadrats at regular intervals along the tape and record the abundance or percentage cover of each species within each quadrat
    • This produces quantitative data

Line and Belt Transects, downloadable AS & A Level Biology revision notes

A line transect and belt transect is carried out in the field


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