What are indicator species and what are they used for?

Indicator species are organisms whos presence or absense may indicate something about the environement. For example, an increase in organisms that are not dependant on oxygen to survive (also known as anaerobic organisms) may indicate that the environment has low levels of oxygen, or, in a coral reef, a decrease in organisms that feed on coral may indicate that there is a reduction in coral cover. Indicator species are important as they may be used to detect small changes in an ecosystem, such as increased pollution, which may be hard to detect without such species.