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• Sampling is a type of scientific surveying which can save time and money and give information on the population of animals and plants.

• There are many different types of sampling; the most useful type depends on the habitat and what is being sampled.

To find out how many polar bears there are in the Arctic, they could all be counted individually. However, it would take a long time and the chances of actually finding them all would be quite small. To make this process easier scientists look at a smaller part of the population or its habitat to draw conclusions about the whole area. This type of scientific surveying is called sampling. Obtaining an estimate by sampling is faster, less expensive and, in most cases, gives us all of the information we need.

There are lots of different methods of sampling, but they all involve investigating living organisms in one or more habitats. This information is extremely important. It is the first step in working out which species are nearing extinction. Because of this, sampling is essential for conservation.

It is essential that any sampling does not affect the living things or their habitat.

A quadrat is a square frame which is used in sampling. It is placed in a location and the plants or slow-moving animals inside it are counted.

The following things can be counted within a quadrat:

1. The number of a single species.
2. The number of different species - a measure of biodiversity.
3. The percentage of the quadrat taken up by one species - it is much easier to do this than count all the individual blades of grass, for example.

When counting the number of a single species in a habitat, if the proportion of the habitat you have sampled is known, the total number of that species in that habitat can be estimated. For example, if you counted 43 dandelions on 25% of the school field then you could estimate the whole field by multiplying 43 by four to make 172.

25% = 43

100% (43 × 4) = 172

Most sampling using quadrats is random. This means it has the same chance of being placed in any position within the habitat. You cannot close your eyes, turn around a few times and throw your quadrat. This might seem random but it can’t ever land at your feet or beyond your throw, so it isn’t. Sampling also shouldn't harm any species - it is important to leave everything as you found it.

Sampling must not be biased - for instance, choosing to sample where there are lots of plants. Here is an example of a way sampling could be undertaken without bias.

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 to generate a pair of random coordinates on your grid. For example, you could draw numbers out of a hat.
3. Place the first quadrat on your grid using these coordinates.
4. Count and record the number of each species within this quadrat.
5. Repeat steps 1-4 so that you have a total of at least 10 counts. This will give you more data, which will make the results more accurate.

Sometimes scientists want to link the number of organisms they are sampling to an environmental factor. For example: 'are more snails found nearer the edges of the lake?' Random sampling would give an estimate of the total number but not show where they are in relation to the lake.

In this case systematic sampling would be used instead. This is not random. An imaginary line called a transect is drawn from one part of the habitat to another. The quadrat is placed at regular distances along this line and the species counted as before. The environmental factor that is being investigated is measured at each quadrat position. The link between the organisms and the factor can then be seen.