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A species that by its presence or absence shows the type of environmental conditions that prevail. For example, the absence of lichen growth on trees is taken to indicate atmospheric pollution by sulphur dioxide. As some lichen genera, e.g. Usnea, are more sensitive to pollution than others, e.g. Lecanora, the degree of pollution can be ascertained by the relative abundances of the different lichens. Other examples of indicator species are Gypsophila species, which indicate alkaline conditions, and Sphagnum mosses and wavy hair grass (Deschampsia flexuosa), which indicate very acid conditions.
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