Estimating Population Size with Quadrats

It is rarely possible to count every organism in a habitat, so ecologists estimate population size by counting organisms in samples. For plants and for animals that move slowly or not at all, such as limpets or barnacles, samples are taken with a quadrat.A frame quadrat is a square frame of known area, often 0.25 m² (50 cm × 50 cm) or 1 m², sometimes divided into a grid of smaller squares. The quadrat size should suit the organism: small plants need small quadrats, and shrubs need larger ones.Three measurements can be made in each quadrat:
• Density: the number of individuals of a species per unit area, for example per m². This is used when individuals are easy to count.
• Frequency: the percentage of quadrats in which a species is present. It is quick, but gives no idea of how many there are.
• Percentage cover: the percentage of the quadrat area covered by a species. It is used when individuals are hard to count, such as grasses or mosses.To estimate the size of a population, find the mean density from the samples and multiply by the total area.Worked example
Ten 0.25 m² quadrats contain a total of 45 daisy plants. The field is 2000 m².mean per quadrat = 45 ÷ 10 = 4.5
density = 4.5 ÷ 0.25 = 18 plants per m²
population = 18 × 2000 = 36 000 plantsThe more quadrats are used, the more representative the estimate.