Sampling with quadrats, transects and capture-recapture
Counting every daisy in a field is impossible, so ecologists count a sample and scale up. Random quadrats estimate how many organisms there are; transects show how their distribution changes across a habitat.
Part 1 of 3: Learn it
In short
- Place quadrats at random to avoid bias, and use the mean count per quadrat.
- Population estimate = mean per m² × total area in m².
- A transect is a line of quadrats used to see how a population changes across a habitat.
Where this is in your specification
Spec points: AQA 4.7.2.1 and the field investigation required practical (Combined Trilogy 4.7.2.1), Edexcel Topic 9 (core practical), OCR B4.2
| Board | Topic: Ecology |
|---|---|
| AQA 8461 | 4.7 (8461); 4.7 (8464) |
| Edexcel 1BI0 | Topic 9 |
| OCR J247 | B4.2 and B7 |
| Eduqas C400 | 6.1 to 6.4 |
| Cambridge IGCSE 0610 | 19.1 to 19.4, 20.1 to 20.4 |
| National 5 C807 75 | Life on Earth: 1 Ecosystems, 2 Distribution of organisms, 4 Energy in ecosystems, 5 Food production |
Random quadrats
- Lay two tape measures at right angles along the edges of the area.
- Use random numbers (from a calculator or table) as coordinates for each quadrat.
- Count the organisms in each quadrat, or estimate the percentage cover.
- Find the mean per quadrat, convert it to a number per m², and multiply by the area.
Transects
To see how a species changes across a habitat, for example moving away from a hedge or up a beach, stretch a tape across the area and place a quadrat at regular intervals along it. Record abiotic factors such as light intensity or soil moisture at each point too, so you can link the change in numbers to a cause.
Capture-recapture
For animals that move around, catch a sample, mark them harmlessly and release them. Later, catch a second sample and count how many are marked.
M is the number marked in the first sample, C is the number caught in the second sample and R is the number of marked animals recaptured. It assumes the marks do not come off or make the animals easier to catch, and that the population does not change in between.
Why are quadrats placed at random?
Show the answer
So that the results are not biased by where you choose to look, and the sample is fair for the whole field.
Part 2 of 3: See it worked
Worked examples
Example 1
A student places ten 0.25 m² quadrats at random in a 40 m by 30 m field. Between them they hold 36 clover plants. Roughly how many clover plants grow in the whole field?
- Total area sampled = 10 × 0.25 = 2.5 m²
- Plants per m² = 36 ÷ 2.5 = 14.4
- Field area = 40 × 30 = 1200 m², so 14.4 × 1200 = 17 280
Answer: About 17 000 clover plants.
Example 2
A team traps 40 ground beetles, puts a dot of paint on each and lets them go. A day later they trap 50 beetles, and 8 have paint dots. Estimate how many beetles live in the area.
- N = (M × C) ÷ R
- N = (40 × 50) ÷ 8 = 2000 ÷ 8
Answer: About 250 beetles.
Common mistakes
- Choosing quadrat positions by eye instead of at random.
- Forgetting to convert from a quadrat's area to 1 m² before scaling up.
- Using a transect to estimate the total population. Transects show distribution; random quadrats estimate numbers.
- Giving an over-precise answer to an estimate, such as exactly 17 280 plants.
When would you use a transect rather than random quadrats?
Show the answer
To see how the distribution of a species changes across a habitat, such as along a gradient of light.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
Why are quadrats placed at random?
So that the results are not biased by where you choose to look, and the sample is fair for the whole field.
When would you use a transect rather than random quadrats?
To see how the distribution of a species changes across a habitat, such as along a gradient of light.
Mean count is 6 plants per 0.5 m² quadrat. How many per m²?
12 per m².
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
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