A rich lichen flora in a region indicates a lack of disturbance in the area for two reasons. First, lichens can only appear in an area if both the fungus and alga are propagated there and coincide. Second, lichens grow slowly, usually only a few millimeters a year. Thus, colonization of an area by lichen species typically does not occur even over the span of one human generation. Status The best estimates of the number of U.S. lichen species are between 3,500 and 4,000, grouped in about 400 genera.
Some species are cosmopolitan and are found from coast to coast.
Most species, however, are more limited in their geographic distributions. Thirty-eight percent of the lichen flora of Hawaii is considered endemic. No state has a complete lichen flora published. Incomplete floras or checklists are known for Alaska, California, Colorado, Connecticut, Florida, Hawaii, Louisiana, Michigan, Minnesota, North Carolina, New Mexico, New York, South Dakota, Tennessee, Texas, and Washington. Most of the rest of the country's lichen flora remains unexplored.