There are at least 13,000 species of lichens living throughout the world. Lichen species are so numerous and diverse that there are individual exceptions to most general statements about them. Scientific knowledge about lichens has expanded significantly during the past few decades, and new discoveries continue. Most lichen species grow best where there is sufficient light and moisture within a moderate temperature zone. However, some lichen species are very adaptable and hardy. When left undisturbed, lichens live in many varying climates and altitudes throughout the world. Some species can survive the most unfavorable climatic extremes of arctic, alpine and desert regions by reducing metabolic activity for extended periods of time. Yet individual species may only exist within a restricted habitat or geographic range. Most lichens are very sensitive to air pollution, and like canaries in coal mines, may serve as indicators of air quality.