Bacteria are fantastically diverse. Understanding the differentiation of bacterial communities -- their beta-diversity -- has played a central role in elucidating the processes underpinning microbial ecology and evolutionary biology and has advanced theory in applied microbiology. Beta-diversity can be broken down into two components: turnover -- the replacement of taxa between communities -- and nestedness -- the tendency of communities to be subsets of each other. These components can indicate dispersal limitation, selection, ordered extinctions, and source-sink dynamics in the assembly of communities. Thus, understanding the levels of turnover and nestedness in microbial communities could unlock greater understanding of microbial community assembly. Here, as part of the Earth Microbiome Project (EMP), we investigate the worldwide importance of turnover and nestedness in microbial communities across planet Earth. The EMP affords a unique opportunity for this investigation, having data from >25,000 samples broadly distributed across geographic regions and environments, collected under standardized protocols. In this talk, we assess the degree to which the bacterial communities sequenced in the EMP are nested, and whether there is evidence that communities in certain environments have seeded communities in other environments.
Surprisingly, at global scales, across a broad array of environments -- soils, marine and saline water and sediment, non-saline water and sediment, aerosols, animals, and plants -- bacterial communities are highly nested. Regardless of where they were from, less diverse (rich) communities are subsets of more diverse communities. This nestedness is found both within environments and between most environments. It is strongest for higher taxonomic levels (phyla, classes, and orders), but exists to a lesser degree at finer taxonomic resolutions. Comparison to distributions of nestedness generated using null models indicated that the observed nestedness is highly statistically significant. Communities from plant- and animal-associated environments are nested within communities from saline environments (e.g., marine), which were in turn are nested within non-saline environments (e.g., lakes). Almost all communities are nested within soil communities. Taken together, these results suggest that certain environments may act as global cauldrons of microbial diversity, from which other environments are colonized, and they may point to the widespread role of island biogeographic processes in microbial community assembly.