Why sample?
Counting and identifying every organism in a large or complex ecosystem is often impractical. Instead, samples can be taken from part of the habitat and used to estimate properties of the wider population.
Abundance is the number of individuals of a species, while distribution describes how that species is spread through an ecosystem.
The sampling method depends on the type of organism and on the pattern of sampling required.
| Method | Suitable organisms | Main use |
|---|---|---|
| Randomly placed quadrats | Non-motile or slow-moving | Sampling an area without deliberately favouring particular locations. |
| Belt transect | Non-motile or slow-moving | Investigating changes in abundance or distribution across changing environmental conditions. |
| Mark-release-recapture | Motile | Estimating the size of a mobile population. |
Random and systematic sampling
In random sampling, sampling locations are determined by chance. This reduces the risk that the investigator deliberately or unconsciously chooses particular parts of the habitat.
It is especially appropriate where the habitat is reasonably uniform or where there is no obvious pattern in species distribution.
In systematic sampling, sampling positions follow a deliberately chosen pattern. Because the arrangement is selected by the investigator, an unsuitable choice could make the sample unrepresentative.
Systematic sampling is useful when a particular spatial change is being investigated; for example, quadrats can be placed at regular intervals along a belt transect.