Air pollution is decimating lichens.

The scenario is familiar science fiction: Plants once ubiquitous in nature are choked and decimated by poisons that rain from the sky. The scene is very real, though, and the setting is not some distant planet or post-apocalyptic world, but modern North American cities such as Seattle, Wash.; Cedar Rapids, Iowa; and Montreal, Canada. The drama even is playing out in some national parks.

The plant is the lowly lichen--a weather-hardy and widely distributed life form that grows on rocks, stumps, shingles, and tree branches. The villain, according to James P. Bennett, a research ecologist with the National Biological Service and an adjunct professor of environmental studies at the University of Wisconsin-Madison, is chronic air pollution. Small plantlike organisms, lichens once were found in every corner of the world. Now, at least a dozen areas in North America--from Los Angeles to tracts of rural Pennsylvania--are classified as "lichen deserts."

The problem is that lichens--unique life forms that actually are two separate organisms, a fungus and an alga, living in symbiosis--are especially susceptible to air pollution. Because they lack a cuticle or skin, they are exposed directly to the atmosphere and, along with nutrients and water, they soak up lead, sulfur, zinc, cadmium, and an array of other chemical and heavy metal air pollutants. In fact, lichens are so sensitive to air pollution...