Fig. 4. Conceptual figure displaying the relative influences of distinct ecological processes structuring the common and rare microbial biospheres.

This figure depicts a rank-abundance curve in which species abundance (purple line) is ordered from high (common biosphere) to low (rare biosphere). In this study, we demonstrated that the majority of the rare species constantly remain at low abundances and are structured by homogeneous selection (red arrow). In contrast, dispersal limitation (brown arrow) plays a reduced role. Due to variable selection, a substantial fraction of the common species and the rare species shift between them across space and time (green arrows). In contrast with the rare biosphere, the abundance of common species is also driven by homogenizing dispersal (brown arrow) and drift (blue arrow), and to a lesser extent, by homogeneous selection (red arrow).