6.5 - Indicator Species

Indicator species for air quality

• An indicator species is a species whose presence or absence in an area can be used as a sign of how polluted that area is.
• Some organisms are very sensitive to changes in their environment, so they can only survive where conditions are clean.
• By looking at which of these species are present, and in what numbers, you can judge the level of pollution without measuring the pollutant directly.

Lichens indicate air pollution

• Lichens are the standard indicator of air quality.
• Many lichens are very sensitive to sulfur dioxide, an air pollutant released from sources such as car exhausts and power stations.
• The number and type of lichen show how clean the air is:
  • Plenty of lichen, especially bushy lichen, indicates clean air.
  • Only crusty lichen, or none, indicates dirtier air.

Presence/absence vs counting

• A quick survey of whether lichens are simply present or absent tells you whether the air is polluted, but not by how much.
• Counting how many lichens, or which types, are found in different places lets you compare how air-pollution levels vary from one area to another.