An indicator species is any biological species that defines a trait or characteristic of the environment. For example, a species may delineate an ecoregion or indicate an environmental condition such as a disease outbreak, pollution, species competition or climate change. Indicator species can be among the most sensitive species in a region, and sometimes act as an early warning to monitoring biologists.
Indicators of environmental condition 
Indicator species are also known as sentinel organisms, i.e. organisms which are ideal for biomonitoring. Organisms such as oysters and mussels have been extensively used as biomonitors in marine and estuarine environments. For example, the 'Mussel Watch' programme is a world-wide project using mussels to assess environmental impacts on coastal waters. Their well-documented feeding habits, stationary condition and their role as integral parts of the food chain are some of the main reasons why oysters and mussels are widely used biomonitors. A considerable amount of contaminant concentrations are found in the surficial sediments (i.e. the finer-grained particulate matter, usually muds, silts or clays) of marine and estuarine environments. A major physical process governing the transport of fine particulate material and associated particle-bound contaminants in estuarine environments is resuspension. Strong winds create surface waves, which, in shallow water (<5m), project energy to the water-sediment interface resulting in resuspension of fine sediment from the upper layers of the estuary floor. Once in suspension, fine material may be transported by tidal currents to other parts of the estuary and possibly to the ocean during multiple reworking phases. Mussels and oysters are filter feeders and therefore uptake is by ingestion of particulates in the water column. Sediment resuspension is thus very important in the bioaccumulation process which aids the evaluation of possible adverse biological effects of sedimentary contaminants in marine and estuarine environments. GR - Legend
Lindenmayer et al. suggest 7 alternative definitions of indicator species:
- a species whose presence indicates the presence of a set of other species and whose absence indicates the lack of that entire set of species;
- a keystone species, which is a species whose addition to or loss from an ecosystem leads to major changes in abundance or occurrence of at least one other species
- a species whose presence indicates human-created abiotic conditions such as air or water pollution (often called a pollution indicator species)
- a dominant species that provides much of the biomass or number of individuals in an area
- a species that indicates particular environmental conditions such as certain soil or rock types
- a species thought to be sensitive to and therefore to serve as an early warning indicator of environmental changes such as global warming or modified fire regimes (sometimes called a bioindicator species)
- a management indicator species, which is a species that reflects the effects of a disturbance regime or the efficacy of efforts to mitigate disturbance effects.
See also 
- Animal sentinels
- Biological integrity
- Botanical prospecting for uranium
- Ecological indicator
- Environmental indicator
- Indicator plant
- Indicator value
- Farr, Daniel (2002). Indicator Species. in Encyclopedia of Environmetrics (eds. A H El-Sharaawi and W W Piegorsch), John Wiley & Sons, Ltd. ISBN 978-0-471-89997-6
- Noss, Reed (1990). "Indicators for monitoring biodiversity. A hierarchical approach" (PDF). Conservation Biology 4 (4): 355–364. doi:10.1111/j.1523-1739.1990.tb00309.x. Retrieved 2009-09-14.
- Shrivastava, Rahul (2007). Indicator Species. in Encyclopedia of Environment and Society (ed. Paul Robins), Thousand Oaks, CA: Sage Publications. ISBN 1-4129-2761-7
- Lindenmayer, David B.; C.R. Margules, D.B. Botkin (2000). "Indicators of Biodiversity for Ecologically Sustainable Forest Management" (PDF). Conservation Biology 14 (4): 941–950. doi:10.1046/j.1523-1739.2000.98533.x. Retrieved 2009-09-14.
- Indicator Plant/Indicator Animal species marietta.edu—"The Tropical Rain Forest"
- Biological Indicators of Watershed Health: Biological Integrity - US Environmental Protection Agency
- Canary Database: Index