Practical: population size with quadrats

Ecology and the environment · The organism in the environment · note 2 of 6

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Practical: population size with quadratsSpec 4.2

In short

A quadrat is a square frame used to sample the organisms in a small, known area. To compare population size in two areas, place at least 10 quadrats at random coordinates in each area, count the chosen species in each quadrat and calculate the mean number per quadrat. Random placement avoids bias, and more quadrats give a more reliable mean.

To compare the population size of an organism in two different areas, you cannot count every individual. Instead you take samples using a quadrat, a square frame that marks out a small, known area.

Quadrat: A square frame used to sample the organisms in a small, known area.

Method: comparing two areas

Choose the two areas to compare, for example a shaded area under a tree and an open, sunny area of the same lawn.
In the first area, lay two tape measures at right angles along two sides to make a grid.
Use a random number generator to pick two coordinates, one along each tape measure.
Place the quadrat at that point and count the number of the chosen species (for example daisies) inside it.
Repeat for at least 10 quadrats, then calculate the mean number per quadrat.
Repeat the same steps in the second area, using the same size of quadrat and the same number of quadrats.
Compare the two means.

Variables and safety

Independent variable: the area (for example shaded or open).
Dependent variable: the number of the chosen species per quadrat.
Control variables: the size of the quadrat, the number of quadrats, the species counted, the person counting and the time of year.
Safety: wear suitable footwear, wash hands after handling soil or plants, and do not touch plants you cannot identify.

Practical:

Say that quadrats are placed randomly to avoid bias, and that using many quadrats makes the mean more reliable. The same number of quadrats must be used in each area for a fair comparison.

Typical result: if one area has more light or water, the mean number of a plant that needs these will be higher there. The mean from each area is then used to estimate the whole population, as shown in the next section.

Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026

Frequently asked questions

What is the difference between a population and a community?

A population is all the organisms of one species living in the same area, such as all the badgers in a wood. A community is all the populations of different species living in that area, such as the badgers, foxes, oak trees, insects and fungi. If more than one species is mentioned, it is a community.

How do you estimate population size using quadrats?

Place quadrats randomly, count the organisms in each and calculate the mean number per quadrat. Then multiply the mean by the total area divided by the area of one quadrat. For example, a mean of 4 per 0.25 m² quadrat in a 100 m² field gives an estimate of 4 × 400 = 1600.

Why are quadrats placed randomly and how many should you use?

Quadrats are placed randomly, using coordinates from a random number generator, so the sample is not biased by the person choosing where to put them. You should use as many as possible, at least 10, because a bigger sample is more representative, reduces the effect of anomalies and gives a more reliable mean.

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