Fieldwork: quadrats, transects and population estimates – GCSE Combined Science Biology

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Ecologists want to know how many of a plant or animal live in a habitat, and why they are found where they are. It would take far too long to count every daisy on a school field, so they take a sample and use it to estimate.

This matters because conservation, farming and planning all need information about populations. It is also one of the required practicals, and the calculations are marks you can pick up with a little practice.

By the end you will be able to describe how to sample with a quadrat and with a transect, calculate a mean and a population estimate, and interpret the results.

Why we take a sample

It is not practical to count every organism in a large area. Instead, we sample a small part of the habitat and use it to estimate the whole. For this to work, the sample has to be representative, which means it has to give a fair picture of the whole area.

Random sampling avoids bias. If you choose where to put the quadrats yourself, you might pick the places with more plants. Random placement gives every part of the area an equal chance of being sampled.

A larger sample is better because it reduces the effect of chance. Ten quadrats give a more reliable mean than two. The data is also more reliable if the work is repeated.

Using a quadrat

A quadrat is a square frame, often 1 m by 1 m (area 1 m²) or 0.5 m by 0.5 m (area 0.25 m²). Lay two long tape measures at right angles along the sides of the area to make a grid. Use a calculator or random number table to choose two numbers for a position, then place the quadrat there.

Count the number of the organism you are studying inside the quadrat. Record the number. Move on to the next random position. Repeat for a good number of quadrats, such as 10 or 20.

When plants are too close together to count individually, such as grass or moss, estimate the percentage cover instead. This is the percentage of the quadrat covered by the plant. Always use the same quadrat size and the same rule for plants on the edge of the frame.

Calculating a mean and estimating a population

Find the mean number per quadrat: add up all the counts and divide by the number of quadrats. For counts of 4, 6, 8, 5 and 7, the total is 30 and the mean is 30 ÷ 5 = 6.

Then work out the number per square metre. If each quadrat is 0.25 m², the mean per m² is 6 ÷ 0.25 = 24. To estimate the whole population multiply by the total area. For a field of 400 m², the estimate is 24 × 400 = 9600.

In one line: population estimate = (mean number per quadrat ÷ area of one quadrat) × total area. The answer is only an estimate because it comes from a sample.

Transects

Sometimes you want to find out how the numbers of organisms change across a habitat, for example from the edge of a path into a wood, or up a seashore. Lay a tape measure across the area. This is a transect.

In a line transect you record the organisms that touch the line at set intervals. In a belt transect you place a quadrat at set intervals along the line (for example every 2 m), count the organisms in each, and so sample a belt of known width. A belt transect gives more information than a line transect.

Record at the same distances each time, use the same size quadrat, and repeat the transect if you can. The data can be plotted as a graph of distance against number or percentage cover.

Linking results to abiotic factors

To explain a pattern, measure an abiotic factor at each point on the transect. Use a light meter for light intensity, a soil moisture meter for moisture, a thermometer for temperature and a pH probe for the soil pH.

Look for a pattern. If the number of daisies falls as you move into the shade of a tree, and the light intensity also falls, the results suggest that light affects where daisies grow. A correlation like this does not prove that light is the cause, so other factors such as soil moisture need to be checked.

Judging the investigation

To judge the quality of the investigation, ask whether the sample was large enough, whether the quadrats were placed at random, and whether the measurements were made at the same time of day and in similar weather.

Say how it could be improved: use more quadrats, repeat the transect on a different day, or measure more abiotic factors.

Take care of yourself outdoors: check for hazards such as uneven ground and stinging plants, wear suitable clothing, do not touch or taste plants, and wash your hands afterwards.

Recap

A sample is used to estimate a population. Random placement avoids bias and a larger sample reduces the effect of chance.

Population estimate = (mean per quadrat ÷ quadrat area) × total area. A transect shows how organisms change across a habitat.

Measure abiotic factors along the transect to explain the pattern, but remember that a correlation does not prove cause.