Background: Although biological communities harbor many rare species, i.e., the rare biosphere, relatively little is known about how distinct ecological processes structure their existence. Here, we combined null modelling with spatiotemporal turnover in taxa rarity to model assembly processes of the rare biosphere.
Results: We found that an overall influence of homogeneous selection (i.e., spatiotemporally constant variables) in driving the assembly of the rare biosphere whereas the common biosphere is governed by a broader array of processes, from which variable selection (i.e., spatiotemporally fluctuating variables) plays the primary role. By partitioning rarity types, we further found that homogeneous selection explained the prevalence of permanently rare taxa, suggesting their persistent low abundances to be restrained by physiological traits. Variable selection explained the dynamics of conditionally rare taxa, indicating their ability to switch between rarity and commonness by responding to spatiotemporal variation.
Conclusion: This study contributes to the understanding of ecological mechanisms that maintain microbial diversity from the rare biosphere perspective. It also provides a new perspective on how to predict dynamic changes in the rare biosphere through space and time.