Environmental Problems, Their Causes, and Sustainability 1

20 AirPollutionAirPopulationPollutionControlControlCASE STUDYWhen Is a LichenLike a Canary?Nineteenth-century coal miners took canaries withthem into the mines—not for their songs but for themoment when they stopped singing. Then the minersknew it was time to get out of the mine because the aircontained methane, which could ignite and explode.Today we use sophisticated equipment to monitorair quality, but living things such as lichens (Figure20-1) can also warn us of bad air. Lichens, whichare not plants, consist of a fungus and an alga livingtogether, usually in a mutually beneficial (mutualistic)partnership.You have probably seen lichens growing as crustsor leafy growths on rocks (Figure 20-1, right), walls,tombstones, and tree trunks or as beards hangingdown from twigs and branches (Figure 20-1, left).These hardy pioneer species are good biologicalindicators of air pollution because they are alwaysabsorbing air as a source of nourishment. A highlypolluted area around an industrial plant may haveno lichens or only gray-green crusty lichen. An areawith moderate air pollution may have orange crustylichens on walls. Walls and trees in areas with fairlyclean air may have leafy lichens.Some lichen species are sensitive to specific airpollutingchemicals. Old man’s beard (Usnea trichodea)and yellow Evernia lichens, for example, sicken or diein the presence of too much sulfur dioxide.Because lichens are widespread, long lived, andanchored in place, they can also help track pollutionto its source. The scientist who discovered sulfur dioxidepollution on Isle Royale in Lake Superior, whereno car or smokestack had ever intruded, used Evernialichens to point the finger northward to coal-burningfacilities at Thunder Bay, Canada.In 1986, the Chernobyl nuclear power plant inUkraine (Figure 17-1, p. 350) exploded and spewed radioactiveparticles into the atmosphere. Some of theseparticles fell to the ground over northern Scandinaviaand were absorbed by lichens that carpet much ofLapland. The area’s Saami people depend on reindeermeat for food, and the reindeer feed on lichens. AfterChernobyl, more than 70,000 reindeer had to be killedand the meat discarded because it was too radioactiveto eat. Scientists helped the Saami identify where tofind the remaining uncontaminated reindeer by analyzinglichens to pinpoint the most contaminated areas.We all must breathe air from a global atmosphericcommons in which air currents and winds can transportsome pollutants long distances. Lichens can alertus to the danger, but as with all forms of pollution, thebest solution is prevention.Kenneth W. Fink/Ardea, LondonFigure 20-1 Red and yellow crustose lichens growing on slate rock inthe foothills of the Sierra Nevada near Merced, California (right), andUsnea trichodea lichen growing on a branch of a larch tree in GiffordPinchot National Park, Washington (left). The vulnerability of variouslichen species to specific air pollutants can help researchers detectlevels of these pollutants and track down their sources.