Mature biological cryptobiotic crust on desert soil surface, Coconino National Forest, Arizona. Biological soil crusts are important components of desert ecosystems. They are a mix of organisms and their metabolic byproducts, including cyanobacteria, bacteria, liverworts, green and brown algaes, lichens and fungi. They help plants by stabilizing sand and dirt, preventing erosion, retaining water, and fixing atmospheric nitrogen. Cryptobiotic soils are extremely fragile: a footprint or tire track can disturb the crust and harm the organisms. In arid regions, recovery can take a century or more.