Collecting data - quadrats - Classification and biodiversity – WJEC - GCSE Biology (Single Science) Revision - WJEC

Collecting data - quadrats

Man working within a quadrat, a rectangular plot — Figure caption,
A quadrat is a tool used to isolate a physical area for study

It is often impractical to count all the organisms in a . For example:

animals may move into or out of an area during counting
some may die during the time it takes to complete the counting
it may be difficult to find all the animals during counting

As a result, biologists have developed different ways to estimate the size of a population. This involves taking random samples of the area to be studied. The more samples that are taken, the more valid the results will be and it is important to use techniques which randomise each sample to avoid .

Using a quadrat

A quadrat is usually a 1 m² frame made of wood. It may contain wires to mark off smaller areas inside, such as 5 × 5 squares or 10 × 10 squares. The organisms underneath, usually plants, can be identified and counted. Quadrats may also be used for slow-moving animals, eg slugs and snails.

To take a truly random sample, you should:

Measure out an area to be surveyed.
Use a random method to collect coordinates for the vertical and horizontal axes measured out, eg rolling two 20 sided dice.
Select the first coordinate and move that distance along the \(\text{x}\)-axis.
Select the second coordinate and move that distance along the \(\text{y}\)-axis.
Place your quadrat where the coordinates meet, making sure that the placement is consistent each time.
Count the number of organisms in the quadrat.
Repeat this procedure at least 25 times.

Estimating population size

This example demonstrates where you should place a quadrat, according to coordinates, and which organisms you should and shouldn’t count.

A diagram to demonstrate where you should place a quadrat according to coordinates, and how you should then use the quadrat to count organisms.

Use this equation to calculate the number of grass plants found in the whole area.

\({\text{Total grass plants found in sample}}\times\frac{\text{Total area (}{\text{m}}^{2}{\text{)}}}{\text{Total area sampled (}{\text{m}}^{2}{\text{)}}}\)

Question

Sally wants to know how many flowers are in a 50 m² field. She throws enough quadrats to cover 10 m² and finds 40 flowers. Using the equation, how does she calculate the estimated number of flowers in the whole field?

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Classification and biodiversity – WJECCollecting data - quadrats