Lichens consist of fungal and algal parts, living in symbiosis with each other. The fungus provides the main body of the lichen while the algae provides a means to manufacture food by the prescence of chlorophyll. Both organisms depend on each other and because two organisms rely on conditions being good, they suffer greatly if the air quality is poor. There are over 15000 species of lichen and the ones which grow on trees and walls are very susceptible to atmospheric pollution- especially sulphur dioxide in the air.
Different lichens will withstand different levels of sulphur dioxide concentrations and a survey of lichen species can establish the levels of atmospheric pollution in an area.
In polluted areas, or in areas where pollution has increased, the population of lichens of certain species will decrease while those of others increase because these can tolerate more pollution. So, botanists can use lichen species as air quality indicators.
The reasons for this are many but the main one is that the fungus part of the lichen will depend on the ability of the algal component to manufacture food by obtaining carbon dioxide and water. Carbon dioxide is obtained from the atmosphere and too much results in the algal component of the lichen growing much better than the fungal part. Symbiosis is therefore put out of balance and the relationship is destroyed. The algae cannot survive alone, and the fungi get an over production of nutrients in their tissues. Naturally, lichens are widely distributed with different species being more numerous on different materials such as wood, limestone, granite and other rocks.
Another way in which lichens indicate air quality is to do with global warming. There are more species in colder parts of the world than in warmer parts and lichens depend on cooler conditions for their survival. However, with the increasingly warmer southern areas in the northen temperate zones, the areas in which many species of lichen thrive is shrinking.
Lichens reproduce by spores from the fungi and algal cells and the two must meet in order for a new lichen of the same species to grow. In some species, small parts break off and are blown by the wind to establish new colonies. However, for a short time in the reproductive cycle the fungal spores andalgal spores are separate and conditions must be right for each part in order to meet and form a new lichen. If conditions change so either of the parts cannot survive, no new lichen is formed and it is this that is perhaps one of the greatest impacts of a reduction of air quality.
It has been noticed that lichens need undisturbed, clean conditions and there is a marked decrease in the populations of lichen even in places they were once dominant like on stone walls and in graveyards. Now, only the sheltered sides of stones are lichen covered because the other sides are open to the air currents and pollutants carried with these.
Lichens are pioneers, often being the first organisms to establish and begin the weathering of rock material and so create conditions for higher plants to move in and become established. Poor air quality, even in far flung places of the earth, means this is no longer happening at such a rate.
We should take note of what nature is telling us- if there are no pioneers, then climax vegetation stands little chance of establishing itself and we only stand to lose what we once took for granted.