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The Woodland Education Centre

ECOLOGICAL SAMPLING METHODS

WHY DO WE NEED TO SAMPLE?

quadrats.jpg (60172 bytes)left: A quadrat is used to sample the species present in a habitat.

If we want to know what kind of plants and animals are in a particular habitat, and how many there are of each species, it is obviously (usually) impossible to go and count each and every one present. It would be like trying to count different sizes and colours of grains of sand on the beach. This problem is usually solved by taking a number of samples from around the habitat, making the necessary assumption that these samples are representative of the habitat in general. In order to be reasonably sure that the results from the samples do represent the habitat as closely as possible, careful planning beforehand is essential.

The usual sampling unit is a quadrat. Quadrats normally consist of a square frame, the most frequently used size being 1m^2 (see picture, above). The purpose of using a quadrat is to enable comparable samples to be obtained from areas of consistent size and shape. Rectangular quadrats and even circular quadrats have been used in some surveys. It does not really matter what shape of quadrat is used, provided it is a standard sampling unit and its shape and measurements are stated in any write-up. It may however be better to stick to the traditional square frame unless there are very good reasons not to, because this yields data that is more readily comparable to other published research. (For instance, you cannot compare data obtained using a circular quadrat, with data obtained using a square quadrat. The difference in shape of the sampling units will introduce variations in the results obtained.)

Choice of quadrat size depends to a large extent on the type of survey being conducted. For instance, it would be difficult to gain any meaningful results when using a 0.5m^2 quadrat in a study of a woodland canopy. The pattern of distribution of species should also be considered when deciding on quadrat size, because different results will be obtained using different quadrat sizes, depending on whether individuals are regularly distributed, randomly distributed, or clustered together in patches. (It may often be difficult to decide if there is a pattern of distribution).

Small quadrats are much quicker to survey, but are likely to yield somewhat less reliable data than large ones. However, larger quadrats require more time and effort to examine properly. A balance is therefore necessary between what is ideal and what is practical. As a general guideline, 0.5 - 1.0m^2 quadrats would be suggested for short grassland or dwarf heath, taller grasslands and shrubby habitats might require 2m quadrats, while quadrats of 20m^2 or larger, would be needed for woodland habitats.

There are three main ways of taking samples.

1. Random Sampling.

2. Systematic Sampling (includes line transect and belt transect methods).

3. Stratified Sampling.

1. RANDOM SAMPLING

Random sampling is usually carried out when the area under study is very large, or there is limited time available. When using random sampling techniques, large numbers of samples/records are taken from different positions within the habitat. A quadrat frame is most often used for this type of sampling. The frame is placed on the ground (or on whatever is being investigated) and the animals, and/ or plants inside it counted, measured, or collected, depending on what the survey is for. This is done many times at different points within the habitat to give a large number of different samples.

In the simplest form of random sampling, the quadrat is thrown to fall at ‘random’ within the site. However, this is usually unsatisfactory because a personal element enters into the throwing and it is rarely completely random. True randomness is an important element in ecology, because statistics are widely used to process the results of sampling. Many of the common statistical techniques used are only valid on data that is truly randomly collected. It must also be noted that this technique would only be possible if quadrats of small size were being used. It would be impossible to throw anything larger than a 1m2 quadrat and even this might pose problems.

A better method of random sampling is to map the area and then to lay a numbered grid over the map. A (computer generated) random number table is then used to select which squares to sample in. (Random number Table). For example, if we have mapped our habitat , and have then laid a numbered grid over it as shown (Figure - below) , we could then choose which squares we should sample in by using the random number table.

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A numbered grid map of an area to be sampled

If we look at the top of the first column in the random number table, our first number is 20. Moving downwards, the next two numbers in the random number table would be 74 and 94, but our highest numbered square on our grid is only 29 (Figure above). We would therefore ignore 74 and 94 and move on to the next number which is 22. We would then sample in Square 22. Continuing down the figures in this column, we would soon come across the number 20 again. As we have already selected this grid for sampling we would similarly ignore this number and continue on to the next. We would continue in this fashion until we had obtained enough samples to be representative of the habitat. There are other methods for selecting numbers from a random number table, but this is the simplest.

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