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Key points

  • 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.

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Video

Can you answer these questions based on the video?

1. What helps scientists estimate the number of species in an area?

2. What is a square frame used in sampling called?

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Why sampling is important

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 or its to draw conclusions about the whole area. This type of scientific surveying is called . 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 . Because of this, sampling is essential for .

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

a pangolin on a tree branch
Image caption,
The pangolin was discovered to be critically endangered because sampling revealed their numbers in the wild have reduced significantly
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Quadrats

A 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 .
  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.
a student using a quadrat to record the number of organisms in the grass.
Image caption,
A quadrat can be used when recording the number of daises on the field

Random sampling using quadrats

A photo showing a grid with random squares coloured in

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 脳 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鈥檛 ever land at your feet or beyond your throw, so it isn鈥檛. Sampling also shouldn't harm any species - it is important to leave everything as you found it.

Sampling must not be - 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 脳 10 divisions.
  2. Use a suitable method to generate a pair of random 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.
A photo showing a grid with random squares coloured in

Systematic sampling using quadrats

A photo showing a grid with a symmetrical pattern of squares coloured in

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.

A photo showing a grid with a symmetrical pattern of squares coloured in
a snail on the ground
Image caption,
Systematic sampling would show where the snails are

In this case sampling would be used instead. This is not random. An imaginary line called a 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.

a snail on the ground
Image caption,
Systematic sampling would show where the snails are
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Other equipment used for sampling

Image gallerySkip image gallerySlide 1 of 3, a student uses a pooter - a jar with 2 straws. one straw is placed on the animal and the other is sucked. The animal will be sucked into the jar, Pooter Pooters are small devices using to suck up small animals like ants.
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Test your knowledge

Quiz

Test questions

Can you remember the steps in systematic sampling with a quadrat? Tap Show more to see the answer.

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Play the Atomic Labs game! game

Try out practical experiments in this KS3 science game.

Play the Atomic Labs game!
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More on Ecosystems and habitats

Find out more by working through a topic