Indicator species

Indicator species are organisms used to assess the level of pollution of an ecosystem.

Indicator species are particularly sensitive to specific environmental changes, and their presence, abundance, or behaviour tells us how clean or polluted their environment is.

Indicators of water pollution

Water pollution arises from the release of substances, such as untreated sewage, into rivers, lakes, and seas.

Many aquatic invertebrates cannot survive in polluted water due to decreased oxygen levels, so their presence or absence serves as an indicator of the level of water pollution.

1Bloodworms and sludgeworms - indicate a high level of water pollution, as these organisms thrive in polluted water due to their tolerance to adverse conditions.

2Freshwater shrimps and stoneflies - indicate a clean water environment, as these organisms are very sensitive to pollution.

Indicators of air pollution

The primary source of air pollution is the combustion of fossil fuels in vehicle engines and power stations.

Sulphur dioxide, released when fuel contains sulphur compounds, contributes to acid rain.

1. Lichens - organisms that thrive in exposed areas like rocks or tree bark. Lichens are sensitive to air pollutants dissolved in rainwater so they can be used as indicators of air pollution, particularly sulfur dioxide.

The presence and type of lichens reflect the cleanliness of the air:

• Bushy lichens thrive in clean air.
• Leafy lichens tolerate mild pollution.
• Crusty lichens are adapted to more polluted environments.
• Absence of lichens often signals heavy pollution with sulphur dioxide.

2Blackspot fungus on roses - its occurrence on roses indicates poor air quality, particularly in urban areas.