In arid and semiarid lands throughout the world, where the cover of
vegetation is sparse or absent, the open spaces between the higher
plants are generally not bare of autotrophic life, but covered by a
community of highly specialized organisms. This soil-surface community
consists of cyanob- teria, algae, lichens, mosses, microfungi, and other
bacteria in differing proportions. Cyanobacterial and microfungal
filaments, rhizinae and rhi- morphs of lichens, and the rhizinae and
protonemata of bryophytes weave throughout the top few millimeters of
soil, gluing loose soil particles together. This forms a crust up to a
few centimeters thick that stabilizes and protects soil surfaces from
erosive forces (Cameron and Blank 1966; Friedmann and Galun 1974; Belnap
and Gardner 1993).These crusts occur in all hot, cool, and cold-arid and
semiarid regions of the world, and may constitute up to or more than 70%
of the living cover. Biological soil crusts have only recently been
recognized as having a major influence on terrestrial ecosystems.