Using Quadrats

Frame quadrats

This is a square frame which is used to outline a sample area.

The simplest way to collect data using a frame quadrat is to make a species list for each quadrat. Each species is either present or absent. This allows you to calculate the percentage frequency of each species for the whole sample area.

If a species is present, you can also record how much of there it is. There are two ways of describing the amount of each species within a frame quadrat: percentage cover and local frequency.

Calculating percentage frequency

Percentage frequency is the probability that a species will be found within a single quadrat.

\(\mathsf{\% \;frequency = \frac{number\; of\; quadrats\; in\; which\; the\; species\; is\; found}{total\; number\; of\; quadrats}\; \times\;100}\)

Example

An investigation was made into the difference in vegetation between a grazed and an ungrazed section of the fixed dunes. A random sample using 30 frame quadrats is taken in each area. In the grazed area, bird’s-foot trefoil was present in 18 of the 30 quadrats; in the ungrazed area, bird’s-foot trefoil was present in 6 of the 30 quadrats.

\(\mathsf{\% \;frequency\;in\;ungrazed\;area = {18}{20}\; \times\;100\;=\;60\%}\) \(\mathsf{\% \;frequency\;in\;ungrazed\;area = {6}{30}\; \times\;100\;=\;20\%}\)

The number of quadrat samples taken affects the reliability of the results. The number must be large enough to minimise the effect of anomalies, but not too large that it cannot realistically be carried out in the time available. lt must also be sufficient for any statistical tests you are using. In practice, 10 quadrat samples is the absolute minimum for each sample area.

Percentage cover and local frequency

Percentage cover is faster but is more subjective. Simply estimate what percentage of the area enclosed by the quadrat is occupied by each species. One drawback of this method is that plants in flower tend to be over-estimated while low-growing plants are under-estimated.

Local frequency is easiest to measure if you have a gridded quadrat of 10 x 10 squares. For each species, count the number of squares which are at least half-occupied. The final figure (which will vary between 1 and 100) is the local frequency. This method is more time consuming than estimating percentage cover, but the results should be more accurate, as there is less risk of over-estimating some plants and under-estimating others.

Point quadrats

A point quadrat is a frame shaped like a T. The bar of the T has ten holes in it, and to sample vegetation a long pin much like a knitting needle is stuck through each hole. The different plants that the pin ‘hits’ as it is pushed towards the ground are identified and counted.

To collect data on abundance, count the first hit made by the pin on each different plant. Count the first hit only.

Local frequency

Local frequency can be calculated for each species.

\(\mathsf{Local\; frequency = \frac{total\; number\; of\; hits\; of \;a\; species }{total\; number\; of\; pin\; drops}\; \times \; 100}\)

Example

For example, if at a single station sea holly is hit 2 times out of 10 pin drops

\(\mathsf{Local\; frequency = \frac{2}{10} \times 100 = 20\%}\)

The local frequency of sea holly is 20%

Local composition

If you have classified each species into communities (e.g. pioneers, mobile sand dunes, dune slack plants, etc.) you can use point quadrat data to calculate percentage composition. This shows what percentage of plants at each station are from each community.

\(\mathsf{Local\; composition = \frac{\%\; frequency\; of\; a\; community }{\%frequency\;of\; all\; communities}\; \times \; 100}\)