What does index species mean?

Index species is a species whose presence (or abundance) in a location serves as an indicator of some environmental condition within a specific location or an indicator of the health of the ecosystem.

Examples:

- Lichens serve as an index species indicating air pollution. Lichens' health is strongly tied to the quality of their environment with many lichens sensitive to the levels of air pollution around them.

- Fish: certain species serve as indices of contamination and degradation in coastal waters.