Sampling Organisms (SQA National 5 Biology): Revision Note
Exam code: X807 75
Quadrats
Sampling basics
Sampling can be defined as:
collecting a series of small data sets that can be used to represent an entire population
Sampling is a useful technique in large and complex habitats where it is not possible to find, identify, and count every organism
In order for sampling to be representative it is essential that:
samples sites are distributed randomly within a habitat to avoid bias
bias might occur, e.g. if a student chooses to take samples in a particular spot because the organisms look easier to count
enough samples are taken to represent the entire habitat; generally the higher the number of samples the more representative the results will be
Sampling with quadrats
A quadrat is a square frame that is placed within the area to be studied to provide a sample
Quadrats can be used to study sessile organisms
Different sizes of quadrat can be used depending on the species being studied:
a 1 m² quadrat can be used to study small organisms, e.g. herbaceous plants in a grassland
a 400 m² quadrat can be used to study large organisms, e.g. trees
Quadrats are placed on the ground and the species within can be assessed, e.g. by measuring:
abundance:
counting the number of individuals of a single species
determining the percentage of a quadrat that is covered by a species
species diversity: counting the number of different species
Quadrats can be placed in a habitat:
randomly: to investigate abundance across an entire habitat
along a transect: to show how distribution changes with an abiotic variable
Limitations of quadrat sampling
Sampling with quadrats has several limitations that can bias results or lead to error unless the survey is carefully planned; examples include:
Limitation | Potential error | Solution |
|---|---|---|
Sample too small | Sample does not accurately represent the area being studied | Carry out more quadrat samples |
All of the randomly generated samples fall in the same area of the site | Sample does not accurately represent the area being studied | Carry out more quadrat samples until the area has been sampled evenly |
Too many individuals are present for easy counting | Counting is inaccurate | Record percentage cover instead of abundance |
Some individuals fall partly inside and partly outside the quadrat | Counting is inaccurate | Adopt a consistent approach, e.g. count all individuals that are more than half within the quadrat |
Identifying species can be difficult | Counting is inaccurate | Use a species identification key or photo guide to identify organisms |
Small organisms may be hidden beneath larger organisms within a quadrat | Counting is inaccurate | Use a pencil or stick to move organisms carefully out of the way to check for individuals underneath |
Pitfall traps
Quadrats are useful for sampling sessile organisms, but sampling of mobile organisms requires other techniques, e.g. the use of pitfall traps
Pitfall traps are used to sample invertebrate populations that are active on the ground
They are usually made from cans or jars that are buried so that the lip of the container is level with the ground
The container collects ground-dwelling insects and other invertebrates as they fall into the trap
Some researchers may add antifreeze liquid (ethylene glycol) to the container to kill and preserve the invertebrates; this prevents sample invertebrates from escaping and also stops predatory insects from eating prey insects
Limitations of pitfall trap sampling
Pitfall traps, while useful for surveying ground-dwelling invertebrates, have limitations that can lead to error if not considered when designing investigations
Limitation | Potential error | Solution |
|---|---|---|
The placement of traps may be biased | Results from traps don't accurately represent sample area | Use a grid and random number generator to place traps randomly |
Animals escape or climb out | Inaccurate number of organisms collected | Use smooth-sided cups and / or a funnel insert Pour ethylene into the bottom of the trap to kill organisms and prevent them from leaving |
Trap may fill with water during wet weather | Organisms may be prevented from falling in or may be washed away, resulting in an inaccurate count | Fit rain covers Add drainage holes Standardise weather conditions for sampling |
Predation inside traps | Inaccurate number of organisms collected | Add a refuge for prey organisms, e.g. twigs and leaf litter Check trap often and collect organisms |
Only traps active, ground-dwelling species | A full picture of the community and its interactions cannot be formed | Combine with other sampling methods (e.g. sweep-netting) to sample other niches |
External scavengers or predators may consume trapped organisms | Inaccurate number of organisms collected | Use mesh covers or grilles to limit entry of larger animals |
In addition to the potential sources of error surrounding pitfall traps, there are also some ethical concerns around their use; recommendations to reduce these issues include:
use non-lethal methods where possible
check traps often, and release organisms after counting
avoid setting traps in extreme weather conditions
handle and release organisms carefully
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