**2. Biogeography of microbial diversity**

The diversity of microbial communities varies within habitats as much as between habitats [2]. This variation can even occur within a few millimetres, suggesting that microbial diversity encompasses more than the documented evidence available. Hence, biogeography is gaining importance as a field of study from microbial diversity point of interest. Many reasons have been postulated to explain this phenomenon. Due to the innately small size of the microor‐ ganisms, environmental complexity plays a major role in determining diversity. Spatial heterogeneity is likely to lead to the formation of many niches within a habitat [3]. Recent tools like metagenomics aid in biogeography studies by providing information on nucleic acid sequence data, thereby directly identifying microorganisms (see Section 9). Therefore the phylogenetic information can be used to compare microbial diversity profile across habitats [2].

Generally, diversity within a particular location and in a community is called alpha diversity. Beta diversity measures the community composition between two or more locations while gamma diversity applies to a region, across continents and biomes and is larger in size than that used for measuring alpha diversity [4].
