Lichen and Air Pollution

You'll find a lot less lichen on the trees and sidewalks of the UW campus than you would in the forests outside of Seattle. This has a lot to do with the air quality in Seattle.

• Lichens obtain most their water and nutrients from the air. So, they respond dramatically to air quality, explaining why lichen diversity is highest in the most pristine habitats. In fact, certain lichen species grow primarily (or even exclusively) in undisturbed habitats
• Lichens can be used to measure air quality: they are often one of the first organisms to respond to pollution. 
• Pollution can destroy the symbiotic balance, cause metabolic interferences, or alter the balance of nutrition favoring either the fungi or the cyanobacteria composing the lichen organism.
• Usually, these detrimental effects will limit the reproduction of lichens. Thus, one way to detect how stressed lichens are by air pollution is to look at their reproductive structures. 
• Pollutants can also cause discoloration, abnormal branching, and/or changes in thickness.
• Lichens are effected by metals, gasses, and acids. Specifically, they are effected by:
• Photochemical toxins
• Acid rain
• Heavy metals
• Radiation
• SO2 from coal and oil combustion can inhibit the Nitrogen fixing ability of some lichens
• More complex lichens are usually less tolerant to pollutants. Of the three types of lichens:
• Crustose are very tolerant to changes in air chemistry
• Foliose are the next most complex and are somewhat tolerant
• Fruticose are most complex and generally the least tolerant
• Change in lichen from pollution can affect lots of other plants and animals that use it for food, habitat or camouflage:
• One example of this is the decline of the light-colored peppered moths in England during the Industrial Revolution. The moths had evolved to blend in with the light-colored lichen that lived on the trees. Many of these lichen died out due to new, widespread pollution. Consequently, the lighter-colored peppered moths were no longer able to camouflage themselves, and soon died out. 
• Another example shows how pollution can move through lichen up the food chain: In Northern Canada, the lichen-caribou-human food chain has been identified as "the most critical food chain in the world for concentrating airborne radionuclides" (4). Lichen absorb radionuclides from pollution, and caribous, a staple of the Northern Canadian diet, eat these lichen.

Sources: (1), (2), (3), (4)