Field Investigations and Sampling
Get to grips with Field Investigations and Sampling for GCSE Biology with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. The required practical uses quadrats and transects to estimate population size and to study how organisms are distributed.
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Topic overview
This required practical measures the population size of a common species in a habitat, and relates the distribution of a species to an abiotic factor.
Quadrats are used to sample a habitat. A quadrat is placed randomly, and the number of organisms inside it is counted. Random placement matters because choosing where to put the quadrat would bias the results. Random numbers are used to generate coordinates on a grid.
A transect is used when investigating how distribution changes across a habitat, for example from a path into open grass. A line is laid out and quadrats are placed at regular intervals along it, recording both the species present and the abiotic factor being investigated.
Revision notes
Using quadrats
Place the quadrat randomly, using random numbers to generate grid coordinates.
Count the number of organisms inside, or estimate percentage cover. Repeat many times and calculate a mean. Random placement avoids bias, and repeats improve reliability.
Estimating population size
Mean number per quadrat × total area ÷ area of one quadrat gives an estimate of the total population.
For example, a mean of 4 daisies per 0.25 m² quadrat over a 200 m² field gives 4 × 200 ÷ 0.25 = 3200 daisies.
Using transects
A transect is used to investigate how distribution changes across a habitat.
Lay a line across the area, place quadrats at regular intervals along it, and record both the organisms present and the abiotic factor, such as light intensity or soil moisture. This shows how distribution relates to the factor.
Key points
- Quadrats sample the organisms in a habitat.
- Quadrats must be placed randomly to avoid bias.
- Use random numbers to generate coordinates.
- Repeat many times and calculate a mean.
- A transect shows how distribution changes across a habitat.
- Record the abiotic factor alongside the species.
Worked examples
Example 1
Explain why quadrats must be placed randomly. [2 marks]
Model answer
If the person chose where to place them they might unconsciously select areas with more or fewer organisms (1 mark), which would bias the results and make the estimate of population size inaccurate (1 mark).
Example 2
A mean of 6 plants is found per 0.25 m² quadrat in a field of 400 m². Estimate the total population. [3 marks]
Model answer
Number of quadrat areas in the field = 400 ÷ 0.25 = 1600 (1 mark). Total population = 6 × 1600 (1 mark) = 9600 plants (1 mark).
Example 3
Explain when a transect should be used rather than random quadrats. [2 marks]
Model answer
A transect is used when investigating how the distribution of a species changes across a habitat (1 mark), for example along a gradient from shade into full sunlight, where random sampling would not show the pattern (1 mark).
Common mistakes
Choosing where to place quadrats.
Placement must be random, generated using random numbers, to avoid bias.
Taking only one quadrat reading.
Many repeats and a mean are needed for a reliable estimate.
Forgetting to scale up by area.
The mean per quadrat must be multiplied by how many quadrat areas fit the total area.
Using a transect for a uniform habitat.
Transects are for investigating change across a gradient.
Exam tips
- Say random numbers generate the coordinates.
- Show the area scaling clearly in population calculations.
- Explain when a transect is appropriate.
- Mention repeats and a mean for reliability.
Key terms
- Quadrat
- A square frame used to sample organisms in a habitat.
- Transect
- A line along which samples are taken at intervals.
- Random sampling
- Sampling without bias, using random coordinates.
- Population size
- The total number of individuals of a species in an area.
Related topics
Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.