Using quadrats in sampling

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:

  1. Number of an individual species - the total number of individuals of one species (eg daisies) is recorded.
  2. Species richness - the number of different plant or animal species is recorded but not the number of individuals within a species.
  3. Percentage cover - the percentage of the quadrat area that is covered by one species (eg grass). This is easier to estimate if a quadrat has wires making smaller sections. Percentage cover rather than number of individuals is used when estimating plant frequencies if it is difficult to identify individual plants, such as grasses or moss.
A school student testing a selected quadrat

Random sampling

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.

curriculum-key-fact
Most sampling is random, but systematic sampling can be used if there is a trend or pattern across the habitat, such as distance up a beach, or altitude on a hillside. If you are using the wrong kind of sampling method for your experiment, this can lead to biased results.

Random sampling using quadrats

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.

  1. When you have chosen a sampling area, first divide it up into a grid, for instance, having 10 × 10 divisions.
  2. Use a suitable method – you could draw numbers out of a hat – to generate a pair of random coordinates on your grid.
  3. Place the first quadrat on your grid using these coordinates.
  4. Count the number of different species within this quadrat (the species richness).
  5. Repeat steps 1-4 so that you have a total of at least 10 counts.

Using quadrats and transects in systematic sampling

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.

Kite diagram showing grasses and dandelions over 25m. Grasses rise slightly and fall slightly over long period then rise and fall steeply to 25m. Dandelions rise steeply, then fall gradually to 20m.
Question

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.