BIOLOGY · Stage 8 · Chapter 7: The Health of the Environment
Biology 7.7 Sampling your ecosystems — Quadrats and Transects
5 min read 6 parts Lesson 7 of 8
Lesson summary
7.7 Sampling your ecosystems — Quadrats and Transects — Alhussan International Academy — Biology (Stages 7–9) notes: Describe how to use quadrats and transects to estimate population size and to show how organisms are distributed.Sampling — it is impossible to count every organis…
- A good sample
- Unsuitable for plants hard to count individually, such as grass
- Random sampling
- Counting rules
- Sources of error
- Other methods
Section 1 of 6Why sample
- Sampling
- it is impossible to count every organism in an ecosystem, so small areas are sampled and the results used to estimate the whole.
- A good sample
- it is large enough and taken without bias, so it represents the whole habitat.
Section 2 of 6What a quadrat is
- Quadrat = an area used to study the numbers and distribution of organisms in an ecosystem.
- It is often a square frame of known area, such as 0.25 m², 0.5 m by 0.5 m or 1 m by 1 m.
- Uses
- it samples plants, slow-moving or stationary animals such as molluscs on a rocky shore, and evidence of animals such as droppings and nests.
- Unsuitable for plants hard to count individually, such as grass
- estimate percentage cover instead.
Section 3 of 6Using a quadrat to estimate a population
- Choose the area and measure its total size.
- Divide the area into a grid and use a random number generator to give the coordinates of each sample.
- Place a quadrat at each position and count the organisms of the species inside it.
- Take enough samples, at least three and preferably ten, to reduce the effect of chance.
- Calculate the mean number per quadrat.
- Estimate the population = mean number per quadrat × (total area ÷ area of one quadrat).
- Random sampling
- it avoids bias, such as choosing only the most interesting places.
- Counting rules
- it does not matter how organisms touching the frame are counted, but the same rule must be used for every sample.
Section 4 of 6Using a transect
- Transect = a line across a habitat along which samples are taken.
- Use
- it is used when conditions change gradually, for example up a beach from the sea shore, outwards from a pond edge or from the shade of a hedge into a field.
- Method
- lay a measuring tape in a straight line and place a quadrat at regular intervals, such as every 2 metres.
- Count the organisms and measure an abiotic factor such as light or soil moisture at each point.
- Plot the results against distance to see whether numbers change as the abiotic factor changes.
| Method | Use |
|---|---|
| Random quadrats | Estimate population size in an even habitat |
| Transect | Show change in distribution along a gradient |
Section 5 of 6Improving reliability
- Take many samples and repeat the survey.
- Use the same size of quadrat and record at the same time of day.
- Sources of error
- miscounting plants that overlap the frame edge, and unrepresentative sampling.
- Other methods
- pitfall traps for ground insects, nets to sweep vegetation and pooters to collect small animals.
Section 6 of 6Using sampling against an invasive species
- Identify the species
- it is important to recognise the species before sampling, for example an invasive earpod wattle seedling on heathland.
- Evidence
- the sample results give a community evidence that a species is invading, so a removal plan can be made.
- Monitoring
- repeat the sampling after removal to check whether more seedlings appear.
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