This bulletin summarizes recent research on biological soil crusts, which are a complex of microscopic organisms growing on the soil surface in many arid and semi-arid ecosystems. These crusts perform the important role of stabilizing soil and reducing or eliminating water and wind
erosion. One of the largest threats to biological soil crusts in the arid and semi-arid areas of the western United States is mechanical disturbance from vehicle traffic and grazing. The spread of the annual invasive cheatgrass has increased the fuel load in areas that previously would not carry a fire, posing a potentially widespread and new threat to this resource.