Required Practical: Population Sampling and Distribution — AQA GCSE Biology
This required practical uses quadrats and transects to estimate the size and distribution of populations.
Using quadrats (abundance)
A quadrat is a square frame placed on the ground to sample organisms.
- Place quadrats at random positions (e.g. using random number coordinates) to avoid bias.
- Count the number of a species (or estimate percentage cover) in each quadrat.
- Calculate the mean number per quadrat.
- Estimate the total population: $$\text{total} = \frac{\text{mean per quadrat} \times \text{total area}}{\text{area of one quadrat}}$$
Using transects (distribution)
A transect is a line across an area. Place quadrats at regular intervals along the line and record the species and abundance at each point. This shows how distribution changes across the area — for example, from a path into a field, or down a seashore.
Reliable data
- Take many samples and calculate a mean.
- Sample randomly for quadrat counts to avoid bias.
- Use the same size quadrat throughout.
- Identify which variable you are investigating (e.g. distance from a path).
Handling data
You should be able to calculate mean, median and mode, and use your sample to estimate the population size of the whole area.
Exam tips
- Quadrats measure abundance; place them randomly; transects show distribution.
- Learn the population estimate calculation (mean × total area ÷ quadrat area).
- Use many random samples and a mean for reliability.
- Be able to find mean, median and mode from sample data.