Introduction
One of the key goals of ecology is to understand the spatial scaling of species diversity. Spatial patterns of species diversity provide important clues about the underlying mechanisms that regulate biodiversity and are central in the development of biodiversity theory (MacArthur & Wilson, 1967; Rosenzweig, 1995; Brown, 1995; Gaston & Blackburn, 2000; Hubbell, 2001; Holyoak, Leibold & Holt, 2005). Assumptions regarding the spatial scaling of biodiversity are a fundamental component of conservation biology and are frequently used to identify local- and global-scale priority conservation areas (Ferrier et al., 2004; Desmet & Cowling, 2004) and to predict extinction risk due to climate change (Thomas et al., 2004) and habitat loss (Gaston, Blackburn & Goldewijk, 2003). Although scaling patterns have been documented in hundreds of studies of plant and animal diversity, such patterns in microbial species (i.e. bacteria, archaea, and single-celled eukarya) have not been well documented. This is a serious omission, given that microorganisms may comprise much of Earth's biodiversity (Whitman, Coleman & Wiebe, 1998; Torsvik, Ovreas & Thingstad, 2002) and play critical roles in biogeochemical cycling and ecosystem functioning (Balser, 2000; Wardle, 2002; Morin & McGrady-Steed, 2004). Furthermore, microbial biodiversity is a major source of novel pharmaceuticals and other compounds of industrial importance, and an understanding of the scaling of microbial biodiversity is crucial to the search for such compounds (Bull, 2004).