Estimating population sizeSpec 4.2
You cannot count every organism in a large area, so you count the organisms in a sample of quadrats and scale up the mean to the whole area.
estimated population = mean number per quadrat × (total area ÷ area of one quadrat)
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.
- 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.
Maths skill:
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.