When investigations are carried out in Ecology it is not normally possible to take measurements of the whole area that is being studied. There are two reasons for this.
It would take too much time. Imagine trying to count all the daisies on a large field.
It would damage the area being studied. Imagine for example how collecting all the caterpillars from an oak tree would affect all the birds that usually feed on them.
Sampling solves this problem. Most ecological investigations are carried out on a sample of the area being studied. The techniques for getting a suitable sample are therefore very important in ecology.
Firstly the sample must be large enough to be representative of the area being studied.
Secondly the way the sample is taken from the area being studied is important. It should be either random or systematic. In A level Biology random samples are used when the area being investigated is uniform or when two different areas are being compared. Systematic samples are used when an investigation is carried out along a known environmental gradient.