An indicator species, or bioindicator, is a plant or animal species which is used to gather information about an environment or area. The presence of such a species can be a signal, as can their absence. Numerous plant and animal species are used as indicators, along with organisms like lichens and fungi, in environments ranging from mountaintops to the continental shelf.
The presence of an indicator species often defines a specific ecoregion. For example, a certain type of tree may only grow in a very specific area, or a specific mammal may only roam in a particular region. If the species begins to disappear, the ecoregion could be said to be shrinking, and it might raise concerns about the health of the environment in that area. Numerous organizations track the presence of indicator species, especially in vulnerable areas, to see whether or not habitats are shrinking or being damaged.
These species can also be used more generally to gather information about the health of the environment. Some indicator species are chosen specifically because they are extremely sensitive to pollution or human interference, so that researchers tracking those species can identify problems early on. Others may fall sick, suggesting presence of a disease which needs to be addressed, or they may migrate or develop stress behaviors in response to climate change. All of these alterations in the natural order can be observed and tracked to keep an eye on the environment.
Indicator species have been used historically to collect information about the geological composition of many regions. Certain plants grow abundantly in the presence of particular minerals, for example, so the presence of such plants can be a sign that those minerals may be abundant. Other plants like wet, marshy ground, suggesting the presence of a water source when they are abundant. Some plants have also been known to change color in response to the presence of specific minerals and chemicals.
Researchers determine which species in a region are indicator species on the basis of field research, historical records, and the known information about those species. When a government agency uses this type of system, it typically documents the process of determination meticulously, in case decisions made on the basis of an indicator species are challenged by other agencies or citizens.