This relationship also differs among different lichens. In general, lichens take their shape from the fungus component, within which are zones containing algae. The lichen fungi get nutrition from the algae, which use sunlight and simple chemicals to produce food. How much this relationship is mutually beneficial and how much is parasitic is uncertain. It is interesting, however, that lichens flourish in places where both algae and fungi are rare.
Most lichens grow slowly and many grow only in a narrow range of habitat conditions.
This makes them especially susceptible to habitat disruption. Many species are sensitive to air pollution, and lichens are used throughout the world to assess and monitor air quality. Despite these sensitivities, lichens are capable of growing in the harshest of natural environments. They live in fresh and salt water, deserts, tropical forests, alpine summits and even in Antarctica. Because of the slow, uniform growth rates of lichens, scientists use some species of them in polar regions to establish the time and speed of glacial retreat.