Sampling: quadrats and transects

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.

Worked examples

Example 1

The mean number of daisies in a 0.5 m × 0.5 m quadrat was 7. The field is 1,200 m². Estimate the number of daisies in the field.

  1. The quadrat's area: 0.5 × 0.5 = 0.25 m²
  2. Daisies per m²: 7 ÷ 0.25 = 28
  3. Multiply by the area of the field. 28 × 1200 = 33600
  4. So there are about 33,600 daisies.

Answer: 33,600

Example 2

40 woodlice were caught, marked and released. Next day 50 were caught, and 8 of them were marked. Estimate the population.

  1. Population = first catch × second catch ÷ marked in second catch.
  2. Put the numbers in. 40 × 50 ÷ 8 = 250
  3. The marked woodlice are 8 out of 50 in the second sample, so 40 must be 8/50 of the whole population.

Answer: 250

Example 3

Quadrats were placed every 2 m along a transect from a tree into an open field. The number of daisies rose from 1 to 12. Suggest a reason.

  1. Under the tree it is shady; in the open field there is more light.
  2. The number of daisies rises as the light increases.
  3. Daisies need light for photosynthesis, so they grow better further from the tree.
  4. Other factors, such as soil moisture, could also change, so measure them too.

Answer: light intensity increases away from the tree's shade, and daisies grow better in more light

Example 4

Why should quadrats be placed at random rather than where the student chooses?

  1. A student might choose places with more, or fewer, plants without meaning to.
  2. Random coordinates give every part of the area the same chance of being sampled.
  3. So the mean is a fair estimate for the whole field.

Answer: to avoid bias, so the sample represents the whole area