Indicator species are
organisms that show a certain
tolerance or
intolerance to
pollution or other changes in the
water and
environment.
Macroinvertebrates are used as indicator species because it is well known what the
values of tolerance are for
specific species.
A
sample is taken of the water. Water of good quality will contain all species, from tolerant to intolerant. Poor quality water will not contain intolerant species, such as
stone flies, or very few of them, since they are not able to tolerate the change in quality.
By examining what
species are found one can
categorize the quality of the water.