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A plant or animal species that is sensitive to a particular environmental factor, such as soil acidity or atmospheric pollution, and whose presence or absence is thus indicative of that factor.

For example, certain lichens will not tolerate atmospheric pollution, and consequently their absence indicates that pollution.
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.

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