Microbiotic crusts resulting from the colonization of soil surfaces by communities of micro-organisms dominated by cyanobacteria, eucaryotic algae or lichens are widespread in arid and semi-arid regions. In the ‘tiger bush' of Niger (Sahel) microbiotic crusts occur in bare bands between densely vegetated bands. The tiger bush soils are sandy loams with pH 4.7–6.5. Three different microbiotic crusts are recognised, forming strips parallel to the vegetation banding. They are all formed mainly by filamentous cyanobacteria dominated by Schizothrix. Other important genera are Scytonema, Lyngbya, Microcoleus, Nostoc and Phormidium. The filamentous micro-organisms trap sand particles and finer particles stick on the filament surfaces. These effects enhance soil cohesion and resistance to erosion. Soil porosity is also increased by the microbial cover, with additional pores delineated by filaments on the surfaces of crusts and porous organic bodies derived from microbial remains at depth. The microbiotic crusts are essential components of the tiger bush ecosystem. Water leaving the lower sides of vegetated bands results in plant decay, and on the upslope sides vegetation grows because of increased water supply. Microbial covers help resist erosion, and increase water and nutrient retention, thus providing substrates for the expansion of plants from adjacent vegetated bands.
