Quadrats are square frames of wire usually 0.25 m2. They may contain further wires to mark off smaller areas inside, such as 5 cm × 5 cm or 10 cm × 10 cm squares.
These are placed on the ground to look at the plants or slow-moving animals within them. When looking at plants in a quadrat, the following sampling can be used:
Random sampling using a quadrat involves the placing of quadrats at random coordinates. Regardless of whether you are investigating the number of individual species, the diversity of species or the percentage cover in different areas, you would use random sampling.
Sampling of the area you are studying must be random. It must show no bias – for instance, choosing to sample where there are lots of plants.
Sometimes we want to see if the number of species or percentage cover changes within an area. This is often as a result of a change in an abiotic factor such as soil pH.
An example of this is an investigation into whether the growth of seaweed depends upon the distance it is found on the seashore from the tide. We would use systematic sampling as we are looking to link a linear change (in this case the number of hours the seaweed is covered by the tide).
A quadrat could be placed at regular distances, for example every five metres, along an imaginary line called a transect, which would run down the shore. Systematic sampling would be used along the transect to link changes in species to abiotic factors, such as immersion by water, temperature fluctuations and light intensity, all of which are influenced by the tide.
The results from transects can be drawn into kite diagrams. The width of the bar from the middle at any distance shows how many individuals were observed at that point.
At what distances were the most grasses and the most dandelions seen?
The most grasses were seen at 20 metres. The most dandelions were seen at five metres.