Lichens are dual organisms composed of a symbiotic partnership between a fungus and an alga. Lichens are intimately connected to their environment. They lack roots and rely upon the atmosphere for their water and nutrients. Because they do not have an outer epidermal layer, they cannot discriminate between nutrients and pollutants, and, as a result, they absorb both. Lichen species react differently to pollutants, and the particular kinds of lichen species in a given area provides specific information about local air quality. Due to little seasonal variation, monitoring lichen community composition has become one of the best biological measures of nitrogen and sulfur-based pollution in forests. There are nearly 1,000 species of lichen documented in California, and a wide variety of habitat types make the greater San Francisco Bay Area particularly rich in lichen diversity.