Sampling organisms (quadrats and transects) · GCSE Biology

Why we sample

It is usually impossible to count every organism in a habitat, so ecologists count them in small samples and use these to estimate the total.
Samples must be random, so they fairly represent the whole area.

Place quadrats (square frames, such as 0.5 m × 0.5 m) randomly in the area, using coordinates from a random number generator.
Count the number of the species in each quadrat.
Work out the mean number per quadrat.
Estimated population = mean per quadrat × (total area ÷ area of one quadrat).

A transect is a line, such as a tape measure, across an area where conditions change (for example from a shady wood into an open field).
Place a quadrat at regular intervals along the line and count the organisms or estimate percentage cover.
Measure an abiotic factor at each point, such as light intensity, to see how it affects the distribution of the species.

Use a large number of quadrats to make the estimate more accurate.
Place quadrats randomly to avoid bias.
Decide in advance how to count organisms partly inside the quadrat, for example counting only those covering more than half a square.

Quadrat: A square frame used to sample organisms in an area.
Transect: A line across a habitat along which samples are taken.
Random sampling: Placing samples so every part of the area has an equal chance of being chosen.
Distribution: Where organisms are found in an area.
Abundance: How many organisms of a species there are in an area.
Percentage cover: The percentage of a quadrat's area covered by a species.
Mean: The total divided by the number of values.