Growing only millimeters a year, some lichens are thought to be the oldest living things on Earth. Lichens are not single organisms, but rather a partnership, or mutualistic symbiosis, between fungi and algae or cyanobacteria. The fungus builds the body of the lichen while the alga or cyanobacteria live within this structure and are photosynthetic, producing energy and nutrients for both organisms. This partnership allows lichens to colonize harsh environments such as the alpine areas of Grand Teton National Park. Acids secreted by lichens dissolve the surface of the substrate they grow on.
This, along with their action of infiltrating and wedging apart pieces of rock, are the beginnings of soil formation. Lichens serve as food in times of stress for many organisms including bighorn sheep, elk, and humans. Many birds also use lichens for building their nests. Many lichen species are highly sensitive to air quality; therefore, they are vulnerable to habitat alteration and serve as good bio-indicators. The presence or absence of lichens in an area is a good indication of the area's air quality. Lichens absorb air pollution and heavy metals from their surrounding environment. Analyzing the pollutants absorbed by lichens allows scientists to determine the amount and kinds of air pollutants and how far they have traveled.