Lichen are composite organisms composed of fungi and certain species of algae in a symbiotic relationship. The fungus provides structural stability, nutrients absorbed from the substrate and a relatively stable microenvironment for the algae. In turn, the algae provides carbohydrates produced by photosynthesis to the fungi. Both the fungi and algae benefit from the association.
Map lichen belong to the crustose group. Crustose lichen have their entire thallus (the body of the lichen) attached to the substrate. Most crustose lichen occur on rocks. The yellow-green thallus of map lichen grows as a thin layer atop a black fungal layer (the hypothallus which does not contain algae). The fungal hyphae (threads) of the hypothallus anchor the lichen to the rock substrate. The thallus of map fungus cracks and forms little yellow-green islands (called areoles) allowing the black hypothallus to appear between the areoles. With imagination, the bright areoles separated by black fungal borders resemble a map, hence the common name. Geographic lichen is another common name for this lichen.
The term map lichen is applied to a number of species distinguished from each other by spore features and microchemical tests. Rhizocarpon geographicum is the most common of these map lichen.
Map lichen grow on rocks, mainly acidic, and are common in alpine environments and arctic tundra with low pollution throughout the world.
Map lichen is used as a bio-indicator of atmospheric pollution. Levels of heavy metals found in map lichen are correlated to the concentration of pollutants in the air.
Using lichenometry the age of exposed rock surfaces can also be estimated, especially in rocks exposed during the last 500 years where carbon dating is less accurate. Lichen of the genus Rhizocarpon are commonly used in lichenometry. The largest lichen on rocks of know age (gravestones, historic buildings, dated moraines, for example) are measured and the growth rate is determined. Then the lichen on the unknown rocks are measured and the approximate minimum age can be calculated. There are many pitfalls to this method, but it is still one tool in determining rock age.
These map lichen were growing on rocks along the Symbol Bridge Trail in the Lava Beds National Monument (Siskiyou County CA).