"The term "indicator species" has three distinct meanings. They are a species, or group of species, that reflect the biotic or abiotic state of an environment; reveal evidence for, or the impactsof, environmental change; or indicate the diversity of other species, taxa, or entire communities within an area. The uses of indicator species in the first two senses of the world are very similar, differing largely in the fact that to indicate change,organisms need to be sampled more than once in the same place and in the same way. Using organisms to indicate the state of, and changes in, the environment has numerous tried and tested applications, from detecting pollution to monitoring recovery of formerly degraded habitats, at many scales, from local to global. The use of indicator species to predict the diversity of other, unstudied taxa for scientific or conservation reasons is much more contentious and may prove tobe impossible with any degree of rigor."

Source: Lawton, J.H.; Gaston, K.J. (2001): Indicator Species. In: Levin, S.A. (ed.): Encyclopedia of Biodiversity. Vol. 3, Academic Press, ISBN: 0-12-226868-7, page 450.