Sampling: quadrats and transects · Year 13
Ecologists cannot count every organism in a habitat, so they take samples. Quadrats placed at random give an estimate of how many plants there are; quadrats along a transect show how organisms change across a habitat; and capture–recapture estimates the number of animals that move about.
Key facts
- A quadrat is a square frame, often 0.5 m × 0.5 m (0.25 m²) or 1 m × 1 m. Count the organisms of one species inside it.
- Place quadrats at random, for example using random numbers as coordinates on a grid of two tape measures, so the sample is not biased. Use many quadrats and find the mean.
- Estimated population = mean number per m² × total area. Change the mean per quadrat into per m² by dividing by the quadrat's area.
- A transect is a line across a habitat, such as from a hedge into a field. Quadrats placed along it show how the distribution of a species changes, which can be compared with abiotic factors such as light.
- Capture–recapture: population = (number marked in the first sample × number caught in the second sample) ÷ number marked in the second sample.
- Capture–recapture assumes the marks do not harm the animals, the marked animals mix back in, and no animals are born, die, arrive or leave between samples.
- Percentage frequency = quadrats containing the species ÷ total quadrats × 100. The index of diversity, d = N(N − 1) ÷ Σn(n − 1), compares how varied communities are.