Lichens are composed of two different species, but they function as one. They consist of a fungus and algae, living together in a symbiotic relationship where the fungus is the dominant organism. The algae are either green algae or blue-green algae, known as cyanobacteria. The algae produce carbohydrates through photosynthesis which serve as food for the fungus, while the fungus physically protects the algae and provides it with moisture. Lichens can live in a wide range of locations and climates -- from the polar regions to the tropics. They form on undisturbed surfaces such as rocks and tree bark. Lichens come in various forms, but all need clean air and most have beneficial effects on the ecosystem.
Major Types of Lichens
The major types of lichens are crustose, foliose and fruticose. Crustose lichens form crusts over rocks, soil, tree trunks or roof shingles. They are usually grayish-green, but may also be yellow or red. Crustose lichens attach firmly to their surface, forming rough patches. Foliose lichens are flat, but have convoluted, bumpy or leafy forms. They grow in layers with distinct upper and lower surfaces. Fruticose lichens are hair-like or shrubby and often are found hanging from trees. They are pendant or upright with no distinguishable upper and lower surfaces.
Lichens Contribute to Soil Formation
Lichens thrive in undisturbed sites where nothing else will grow. They grow on rocks, barren soil and the bark of dead or live trees. Lichens are not parasitic when they grow on trees, they just use the tree bark as a home. They enrich the soil by trapping water, dust and silt. When lichens die they contribute organic matter to the soil, improving the soil so that other plants can grow there.
Lichens Fix Nitrogen
Due to their association with algae, lichens are able to convert nitrogen in the air into nitrates, which they need for their growth. Conversion of atmospheric nitrogen impacts the ecosystem, because when it rains, nitrates are leached from lichens for use by nearby soil-based plants.
Lichens Need Clean Air
Although lichens are tough and can survive in extreme climates, including extreme heat, cold and drought, they are sensitive to air pollution. Because lichens are so pollution-sensitive, some scientists use them to assess the air pollution coming from industrial plants and urban areas. Lichens absorb everything from the air, including carbon dioxide and heavy metals. Scientists can extract the toxic compounds from lichens and determine the level of air pollution in a given area. The dying of lichens at a site is an early warning sign of harmful pollution.