Quadrats
Biology: Edexcel IGCSE, topic 5.1.6
Quadrats
The population size of immobile (e.g. plants) or slow-moving organisms (e.g. limpets) can be estimated using random samples of quadrats.
Quadrat
- Quadrats are normally 1m2 square frame divided into 100 squares.
- Each square represents 1% of the total area of the quadrat.
- If an individual covers more than half of a square, it is counted as representing 1% of the quadrat.
- Quadrats are used to count the population size within a large area.
Sampling
- If you wanted to investigate the size of a population, it would take hours to count every individual.
- Taking samples (a small section of the total area) allows the population size to be estimated.
- It is important that the samples accurately represent the total population size.
Random sampling
- Random sampling can be applied by dividing the area of interest into a grid and labelling the grid with coordinates.
- A random number generator provides a set of random coordinates.
- The quadrats can be placed using the coordinates.
Transects
- Quadrats could also be placed at regular intervals along a transect.
- A transect is a line across a habitat, usually placed using a tape measure.
- Transects are used when there is a change, or gradient, in abiotic conditions across the habitat.
- Transects are used when looking for changes in the abundance or distribution of a species across the environment (e.g. how abundance changes from open field to forest).
Line transect
- A line transect records the species that makes contact with the tape measure at regular intervals along the transect.
Belt transect
- A belt transect uses quadrats.
- Quadrats are placed at regular intervals along the transect.
- An abundance of different species in the quadrat can be measured by:
- Percentage cover – estimate how much of the quadrat is covered by a particular species
- Frequency – count how many individuals of a particular species are present in the quadrat.
Test yourself on Quadrats
Which of the following statements about quadrats are true?
In what sort of habitat are transects used?