It is usually impossible to count every organism in a habitat, so ecologists sample it. They count the organisms in a number of quadrats and use the results to estimate the size of the whole population. Quadrat positions should be chosen using random numbers, for example as coordinates on a grid, so that the sampling is not biased. In a belt transect the quadrats are placed at set distances instead, and a mean is worked out for each distance.
First calculate the mean number of organisms per quadrat: add up all the counts and divide by the number of quadrats. Next find the area of one quadrat. A quadrat that is 0.5 m by 0.5 m has an area of 0.25 m2. The mean number per m2 is the mean per quadrat divided by the area of the quadrat. Finally multiply by the total area of the habitat to estimate the population.
Worked example. Five quadrats of area 0.25 m2 contain 4, 6, 2, 5 and 3 daisies. The total is 20, so the mean is 4 per quadrat. The mean per m2 is 4 ÷ 0.25 = 16. For a field of area 100 m2 the population estimate is 16 × 100 = 1600 daisies. Using more quadrats makes the estimate more reliable. For plants such as grass, where counting individuals is hard, the percentage cover of each quadrat can be estimated instead.