Biological soil crusts are formed in open spaces between vascular plants. Frequently, single-celled organisms such as cyanobacteria or spores of free-living fungi colonize bare ground first. Once filaments have stabilized the soil, lichens and mosses can colonize. Appressed lichens are generally earlier colonizers or persist in more stressful conditions, while more three-dimensional lichens require long disturbance-free growth periods and more moderate conditions. Recovery following disturbance varies. Cyanobacteria cover can recover by propagules blowing in from adjacent undistrubed areas rapidly after disturbance. Total recovery of cover and composition occurs more rapidly in fine soil textured, moister environments (~20 years) and more slowly (>1000 years) in coarse soil textured, dry environments. Recovery times also depend on disturbance regime, site, and availability of propagules.