The combination of environmental factors prevailing in the dry valleys; desiccation, frequent freeze-thaw cycles, limited maximum temperature, salinity, short growth periods, high ultraviolet radiation levels and low organic matter severely limit the types of micro-organisms which can survive and multiply. The lack of available water is clearly the most critical factor. The low level of higher plant and animal life further simplifies the soil biology. The dry valleys represent a unique biological system which is almost exclusively microbiological, but the life in the system is well adapted to one of the world’s most hostile environments.