Questions about Quadrats

Interdependence is a key concept in ecology. Whilst this can be taught theoretically, some practical fieldwork greatly enhances pupils’ understanding. Almost certainly the fieldwork would entail looking at plant abundance and distribution and, of course, some work with quadrats!

A quadrat is a simple device for marking out a small area. For young children at primary school the quadrat is often a convenient way of focusing a pupil’s attention on a particular small area. At secondary level, pupils should understand how quadrats can be used to sample a larger area. By recording information from a number of quadrats placed within a larger study area, they can obtain a representative sample of the whole area, which may be too big to describe in full.

This article describes how quadrats can be used to help pupils at lower secondary level estimate the relative abundance of plant species. All the information given here refers to frame quadrats. (Point quadrats can be tedious and difficult for pupils to use and are probably best avoided at this level.)

1. How is the study area chosen?
2. What size and shape should the quadrat be?
3. What should be recorded within the quadrat?
4. What strategy should be used for placing the quadrats?
5. How many quadrats need to be placed?

1. How is the study area chosen?

Choose an area that is large enough to be representative of the vegetation being investigated. You also need to consider the time available for the study and the number of pupils involved. The area must not be so big that it cannot be sampled adequately or so small that the habitat is damaged by trampling feet.

Generally a plot size of about 20 x 20 m is suitable for a class of 25 to 30 pupils. Correct identification is crucial to all ecological work. If identifications are incorrect, it becomes impossible to explain results. It is best, therefore, within the study area, to limit the selection to a few plants that are easy to recognise. You can easily make identification sheets for students by scanning or photocopying actual specimens.
The fold out chart series produced by the Field Studies Council offers useful help in the identification of plants in a wide variety of habitats including woodland, grassland and heathland.

2. What size and shape should the quadrat be?

In theory any shape of frame can be used but for many measurements you need to know the area of the quadrat so a square quadrat is the most popular.

The size of the quadrat is usually related to the size of the plants being studied. Here are some useful guidelines, given in the Open University Project guide.

• 10 cm x 10 cm quadrats – for very small plants, such as algae or bryophytes on tree trunks or walls
• 25 cm x 25 cm quadrats – for short grassland and other low-growing vegetation
• 50 cm x 50 cm quadrats – for long grass or heathland

Larger quadrats are difficult to handle and for plants such as trees and shrubs it is probably best to mark out plots on the ground with tape measures.

Making the quadrats

The easiest way to make a quadrat, approximately 25 cm x 25 cm, is to bend a metal coat hanger into a square. Cut off the hook and for safety cover the cut end with insulating tape. To make a larger quadrat, purchase stock wire from an ironmonger or farm supplier. You can then bend this into quadrats of any size required. As for the coat hanger, cover the joined ends in insulating tape. Use a brightly covered tape so that quadrats left lying on the ground are more easily found! To make small quadrats, e.g. 10 cm x 10 cm (for using on a flat surface like a wall), draw the shape on an acetate sheet.

3. What should be recorded within the quadrat?

Abundance means the amount of something. Pupils are often asked to make an estimate by eye of the percentage amount of ground covered by each species within the quadrat. This can be time consuming and such subjective measures are very prone to inaccuracies, especially with younger pupils. It is better to carry out one of the quantitative measures described below.
When working with plants, the two measurements of abundance commonly used are:

• the number of individual plants
• the area covered by the overground parts of the selected species

We will look at each in turn together with an example to show how the measure is used.

Measure 1 – The number of individual plants

The pupil counts the number of individual plants of the selected species in each quadrat. The result can be expressed as number of plants per square metre. This measure is known as density.

Example

The chosen study area measured 10 m x 10 m (100 m²).
Pupils placed 8 quadrats, each 50 cm x 50 cm (a total of 2 m²).
A total of 24 daisy plants were found in the quadrats.
So there were 12 daisies per m².

This is an easy concept for pupils to understand but has disadvantages. Individual plants are often not easy to distinguish, e.g. grasses. Even plants that appear separate may be joined underground. No information is obtained about the size of the plants but this may be of great importance ecologically.

Measure 2 – The amount of area covered by the overground parts of the selected species

The greater this area, the more likely a plant is to occur within a quadrat. This measure is known as plant frequency and, ecologically, is a more useful measure than density, as both the size and number of individual plants contribute to the area covered.

Plant frequency is also quicker to measure than density. It is not necessary to measure the number of plants of each species within a quadrat but only to record their presence or absence. A species counts as present if any part of the plant lies within the quadrat.

Results are usually expressed as the number of times a species occurred in the quadrats as a percentage of the total number of quadrats placed.

4. How should the quadrats be placed?

The aim is to remove personal choice as to where the quadrat is placed. “Throwing” a quadrat is not truly random.

We can approach sampling in two different ways: random sampling or systematic sampling. We will look at each in turn to see how it is carried out.

Random sampling

Ideally every place within the sampling area should have an equal chance of being sampled, each time a sample is taken.

To achieve this, place a tape measure along two sides of the area being studied. Then find random coordinates as follows:

• The length of one side of the quadrat forms the sampling interval. Then divide the length of the plot into these intervals e.g. if you use a 10 x 10 m plot and a 50 x 50 cm quadrat, the intervals will be 0, 0.5, 1.0, 1.5. . . 9.5, 10.
• Write the intervals on pieces of paper and put them into a hat.
• Let each pair of students draw out 2 pieces of paper. (Replace the first piece before taking the second.)
• Each student then finds his or her appropriate position along the tape measure. They turn into the plot at a right angle to the tape and walk into the plot until they meet. This is their sampling position.

Systematic sampling

This is most useful when a pattern in the vegetation is being investigated, for example when looking at the change in abundance of plant species across a pathway.

Lay out a tape measure and place quadrats at regular intervals along the tape measure. Make sure you choose an interval that is small enough to demonstrate any changes taking place. You can even place the quadrats end over end.

5. How many quadrats should be placed?

The sample size depends on how much variability is shown by the plants within the study area. For this reason, when working with younger pupils it is often best to try and avoid areas where plants show obvious clumping.

If the area is fairly uniform ensure that at least 2% of the total area has been sampled by the quadrats. This should give a reasonable size sample.

Example

The plot is 20 m x 20 m (400 m²)

50 x 50 cm square quadrats are being used, so there are 4 quadrats to the square metre.

It would take 1600 quadrats to cover the whole area.

To cover 2%, we would need 32 quadrats.

If you have a class with 24 pupils divided into 12 pairs, each pair needs to do at least 3 quadrats. You may wish them to do more as part of the learning process, but make sure the grass is not over-trampled!