A bioindicator is a plant or animal species that is known to be particularly tolerant or sensitive to pollution. Based on the known association of an organism with a particular type or intensity of pollution, the presence of the organism can be used as a tool to indicate polluted conditions relative to unimpacted reference conditions. Sometimes a set of species or the structure and function of an entire biological community may function as a bioindicator. In assessing the impacts of pollution, bioindicators are frequently used to evaluate the "health" of an impacted ecosystem relative to a reference area or reference conditions. Field-based, site-specific environmental evaluations based on the bioindicator approach generally are complemented with laboratory studies of toxicity testing and bioassay experiments.

The use of individual species or a community structure as bioindicators involves the identification, classification and quantification of biota in the affected area. While many species...