Sampling - using quadrats, random and belt transects
Laura Armstrong
Teacher
Recall Questions
This topic requires prior knowledge of ecosystems, population distribution, and the concept of representative sampling in ecological investigations. You can test your knowledge on these below.
What is a habitat?
A place where an organism lives that provides the necessary conditions for survival and reproduction.
What is an abiotic factor?
A non-living factor in the environment that can affect organisms, such as light, temperature, or soil pH.
Why must sample size be large in habitat sampling investigations?
To minimise the effect of anomalies and to make the results more representative.
Why Do We Use Sampling?
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It's impractical to count every individual in a large area, it would take too much time.
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Sampling is used to estimate species abundance or distribution in a habitat. Abundance can be measured as the number of individuals or the percentage cover if distinguishing between individuals is hard.
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Sampling should be unbiased and representative of the whole area. Random sampling reduces bias.
Quadrat Sampling
What is a Quadrat?
- A square frame, usually 0.25 m² or 1 m², used to define a sample area.
How to Use Quadrats:
- Place quadrats randomly using random coordinates (generated with a random number generator).
- Measure:
- Number of individuals
- Species richness
- Species frequency – how often a species occurs
- Percentage cover – the proportion of quadrat area covered by a species.
- Use a large enough sample size to be representative by repeating the placing of the quadrant.
- At least 10 quadrats must be used.
- Calculate a mean from multiple quadrats and the abundance per m².
- Multiply by total area to estimate total population size (if needed).
Belt Transect Sampling
What is a Belt Transect?
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A line laid across a habitat (e.g., tape measure) with quadrats placed at regular intervals (such as every 3 metres) along it. The abundance is then measured in each quadrat placed along the transect.
- This is an example of systematic sampling.
When to Use:
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To study how species distribution changes across a gradient of an abiotic factor (e.g., light, moisture, salinity).
Example
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In a rocky shore ecosystem, use a belt transect from high tide to low tide to observe how lichens or barnacles change with distance from the shore.
A spearmann’s rank correlation coefficient can then be used to test whether the correlation between the distribution of the species and the gradient of the abiotic factor is significant.
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Key Terms
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Quadrat: a square frame used for sampling.
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Transect: a line across a habitat along which measurements are taken.
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Percentage cover: area of quadrat covered by a species.
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Species frequency: the proportion of quadrats containing a specific species.
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Abundance: number or density of individuals of a species.
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Exam Tips
Always describe how random sampling is achieved – e.g., random number generator for coordinates.
Don’t confuse abundance (how many) with distribution (where they are found).
Remember all the different ways abundance can be measured; Number of individuals, species richness, species frequency or percentage cover.
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Describe how you would use a belt transect to investigate the effect of light intensity on the distribution of dandelions across a school field. (4 marks)
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Stretch a measuring tape (transect) from the shaded area to the open sunlight area.
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Place a quadrat at regular intervals (e.g., every 2 m) along the transect.
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In each quadrat, count the number of dandelions or estimate percentage cover.
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Use a light meter to record light intensity at each point.
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Repeat transect or use multiple transects to ensure representative data.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!