AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
Required practical activity 9: measure the population size of a common…
Full specification wording
Levels of organisation: Required practical activity 9: measure the population size of a common species in a habitat. Use sampling techniques to investigate the effect of a factor on the distribution of this species.
This practical has two linked aims: to estimate the population size of a common species and to investigate how a factor affects its distribution.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 11 of 12
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
This practical has two linked aims: to estimate the population size of a common species and to investigate how a factor affects its distribution. A quadrat, usually a square frame, is placed at random positions in the habitat, for example by using random numbers as coordinates. The number of the chosen species inside each quadrat is recorded, and the mean per quadrat is calculated. Population size is then estimated by scaling the mean up to the whole area: estimated population = mean number per quadrat × total habitat area ÷ quadrat area. To study distribution, quadrats are placed along a transect line across the habitat, and the factor, such as light intensity or soil moisture, is measured at each point. Plotting species number against the factor shows whether distribution is affected.
What earns the marks
- Place quadrats at random positions, for example using random number coordinates, to avoid bias.
- Count the chosen species in each quadrat and calculate a mean number per quadrat.
- Estimate population size using mean number per quadrat × total habitat area ÷ quadrat area.
- Use a transect to sample along a line and measure the chosen factor, such as light intensity, at each sample point.
- Repeat samples and control variables such as quadrat size and sampling method to improve reliability.
- Plot or compare species numbers against the factor to identify a relationship with distribution.
Common mistakes and how to put them right
Choosing quadrat positions by eye where the species looks common.
Put it right by using random numbers to select coordinates.
Forgetting to convert units so quadrat area and habitat area match.
Put it right by converting both to the same unit, such as m², before calculating.
Changing quadrat size or counting method between samples.
Put it right by keeping quadrat size and counting rules constant throughout.
Exam tips
- State clearly that random sampling reduces bias and that repeats improve reliability.
- Show the population estimate calculation with units, for example mean per m² × total area in m².
- When describing a transect, name the factor measured and the instrument used, such as a light meter for light intensity.
Try the lesson
The start of the Levels of organisation: Required practical activity 9: measure the population size of a common species in a habitat. Use sampling techniques to investigate the effect of a factor on the distribution of this species. lesson, made from checked teaching notes for this specification point.
Have a go first
A student uses a quadrat to sample the number of daisies in a field. What is the main reason for placing the quadrat at random?
Listen
This practical has two linked aims: to estimate the population size of a common species and to investigate how a factor affects its distribution. A quadrat, usually a square frame, is placed at random positions in the habitat, for example by using random numbers as coordinates. The number of the chosen species inside each quadrat is recorded, and the mean per quadrat is calculated. Population size is then estimated by scaling the mean up to the whole area: estimated population = mean number per quadrat × total habitat area ÷ quadrat area. To study distribution, quadrats are placed along a transect line across the habitat, and the factor, such as light intensity or soil moisture, is measured at each point. Plotting species number against the factor shows whether distribution is affected. A strong answer earns marks for these points. Place quadrats at random positions, for example using random number coordinates, to avoid bias. Count the chosen species in each quadrat and calculate a mean number per quadrat. Estimate population size using mean number per quadrat × total habitat area ÷ quadrat area. Use a transect to sample along a line and measure the chosen factor, such as light intensity, at each sample point. A common slip: Choosing quadrat positions by eye where the species looks common. Instead, using random numbers to select coordinates. Exam tip: State clearly that random sampling reduces bias and that repeats improve reliability.
See it
Key words
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Random sampling to avoid bias
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Sampling and estimation
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Distribution along environmental gradients
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Reliability, repeats and control variables
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See every definition
- Random sampling to avoid bias
- Placing quadrats at random positions, for example using random number coordinates, so that bias is avoided.
- Sampling and estimation
- Counting the chosen species in each quadrat, calculating a mean number per quadrat, and scaling the mean up to the whole area to estimate population size.
- Distribution along environmental gradients
- Placing quadrats along a transect line across the habitat and measuring the chosen factor, such as light intensity or soil moisture, at each point to see whether distribution is affected.
- Reliability, repeats and control variables
- Repeating samples and keeping variables such as quadrat size and sampling method constant to improve reliability.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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