Estimating the number of organismsSpec 9.6
In short
Population size is estimated by sampling, because you cannot count every organism in a large area. Use the equation: estimated population = mean number per quadrat × (total area ÷ area of one quadrat). Many randomly placed quadrats give a more reliable estimate, and percentage cover is used for plants that are hard to count, such as grass.
You cannot count every organism in a large area, so you take samples and use them to estimate the total. A larger number of quadrats, placed randomly, gives a more reliable estimate.
Estimating a population
A student counted daisies in five 0.25 m² quadrats placed randomly in a field of area 100 m². The counts were 3, 5, 4, 6 and 2. Estimate the number of daisies in the field.
- Mean per quadrat = (3 + 5 + 4 + 6 + 2) ÷ 5 = 20 ÷ 5 = 4
- Number of quadrat areas in the field = 100 ÷ 0.25 = 400
- Estimated population = 4 × 400 = 1600
Answer: 1600 daisies
Plants that are difficult to count individually, such as grass, can be measured by percentage cover: the percentage of the quadrat covered by the species.
Other points to understand about sampling:
- Take as many quadrats as possible, because a bigger sample is more representative and reduces the effect of anomalies.
- Place quadrats randomly, to avoid bias.
- Calculate a mean from the raw data. A bar chart or frequency table is useful for presenting the results.
Check the units. If the quadrat is 0.5 m by 0.5 m its area is 0.25 m², not 0.5 m². This is the most common slip in estimate questions.
Quick check
How do you stop bias when placing quadrats?
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Use random numbers to choose the coordinates.
What does a belt transect show?
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How the distribution of organisms changes along an environmental gradient.
Write the equation for estimating a population from quadrat data.
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Mean per quadrat × (total area ÷ area of one quadrat).
Why use many quadrats?
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To give a more representative and reliable sample, and reduce the effect of anomalies.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026