Question 1 (2 marks)
Knowledge
Explain why quadrat locations should be random when estimating population size.
Show model answer / marking points
To reduce sampling bias (1) and improve representativeness (1).
Required Practical 07: Field Investigations | AQA GCSE Science | The Science Department
About this practical and its aim
Aim
Estimate the population size of a common species and investigate how an environmental factor affects its distribution.
Introduction
This is a great opportunity to get outside and measure a population size without counting every daisy on a field. Completing this task with a quadrat and measuring a transect line help you to understand these methods and will help you remember these processes. Nothing beats actually having a go then comparing your results to the rest of the class.
The science behind the practical
Quadrats provide samples of organisms in an area. Transects reveal changes in abundance along an environmental gradient.
Key things to remember
Use unbiased random quadrat locations and systematic transects; record the quadrat area and total habitat area accurately.
Equipment and apparatus
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Quadrat
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tape measure
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transect line
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random number generator
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identification key
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clipboard
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suitable outdoor clothing
Method: How to carry out the experiment
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Part 1:
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Measure the area of the field that your class are going to investigate. (record this and take great care of the units).
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Calculate and record the area of your quadrat (again, care with units).
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Throw the quadrat to at least 10 places on the field (random) and each time, count how many daisies are in the quadrat. Record this.
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When you have finished, you can estimate the population size of daisies using the formula
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Estimated population size = (total area of the field / area you measured) x total number of daisies you counted.
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Part 2:
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Set Up a transect line. I recommend starting at the trunk of a large tree then running into the centre of the field.
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Along the line, look at variation of grass density, numbers of daisies and amount of visible soil.
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This gives you an opportunity to see variations that care caused by water availability, light availability (shade caused by the tree) and other factors that you may find where you do your investigation. You can create conclusions based around the relationship between (for example) distance from the tree and the amount of bare soil visible.
Variables and fair testing
| Type of variable | What is measured or controlled? |
|---|---|
| Independent variable | Sampling location or distance along a transect |
| Dependent variable | Abundance, frequency or percentage cover of the selected species |
| Control variables | Quadrat size; counting rules; sampling effort; observer method |
Safety and managing risks
Follow your teacher's instructions and your school's risk assessment. This page is not a substitute for a formal risk assessment.
| Hazard | Risk | Precaution |
|---|---|---|
Uneven outdoor ground |
Trips and falls |
Inspect route and wear appropriate footwear. |
Stinging plants/insects and weather |
Stings, allergic reactions, sun exposure |
Follow outdoor risk assessment and individual medical plans; arrange suitable protection. |
Results, calculations and analysis
Recording, presenting and interpreting results
Record counts from multiple quadrats, calculate mean abundance, and plot abundance against distance along a transect.
Calculations
Estimated population = (mean count per quadrat × habitat area) ÷ quadrat area.
Conclusions, accuracy and evaluation
Drawing a conclusion
An estimated population depends on how representative the sample is; gradients may affect distribution.
Evaluating the investigation
Use more random samples, avoid deliberately favourable spots, and repeat at different times if relevant.
More to learn
Random sampling and transects
Use random quadrats to estimate mean population density; use a transect to investigate changes along a gradient.
Common exam mistake
Use consistent area units when converting mean counts to estimated population.
GCSE examination practice
Answer each question in full before opening the model answer. Check your work against the marking points and look for scientific key words.
Question 2 (3 marks)
Calculation
Mean count is 5 plants per 0.25 m² quadrat. Estimate number in a 40 m² habitat.
Show model answer / marking points
40 ÷ 0.25 = 160 quadrats (1); 160 × 5 = 800 plants (2).
Question 3 (2 marks)
Analysis
What does a transect help you investigate?
Show model answer / marking points
How distribution/abundance changes with position or an environmental gradient (2).
Question 4 (2 marks)
Evaluation
Suggest one improvement to make the investigation more reliable and explain why it helps.
Show model answer / marking points
Repeat measurements and calculate a mean (1), reducing the influence of random variation/anomalies (1). Alternative valid improvement specific to the experiment with explanation can gain both marks.
This page was updated on: 13th June 2026