Biological crusts (also known as cryptogamic, microbiotic, crytobiotic and microphytic crusts) are microscopic non-vascular assemblages composed of blue-green algae, diatoms, golden brown algae, lichens, mosses and a few xerophytic (adapted for growth with a limited water supply) liverworts on more mesic (moderate amount of moisture) sites. Blue-green algae contribute to the bulk of the algal tissue. Biological crusts grow on and within soils and other substrates, modifying these surfaces in significant ways. The blue-green algae fix atmospheric nitrogen and thus enhance the nutrient status of the soil. Cryptogamic crusts retard erosion by wind and water, help retain soil moisture during dry periods, slow evaporation rates, and enhance seedling establishment. These living crusts are extremely fragile and one footprint can set back development for decades.