Range size is a fundamental element of biogeography. For plant and animal species, range size distributions span over ten orders of magnitude. This variation is explained by factors including body size, population density and local abundance, dispersal mechanisms, and habitat traits. For microbial taxa, range size distributions and the factors determining range size remain poorly understood. To address this knowledge gap, we determined the range size and shape of 74,134 bacterial and archaeal taxa found in settled dust from 1,065 locations across the United States. We calculated range size using both the area of occupancy (AOO) and the extent of occurrence (EOO) approximations. We determined if the occupancy-abundance relationship, widely observed for plants and animals, also applies to these microbial taxa. Finally, we assessed if there was a phylogenetic signal to range size and to what extent phenotypic or genomic traits can explain the measured variability in range size.
The range size frequency distribution of dust-associated microbial taxa is best described with a log-normal distribution; many taxa have small ranges and few have very large ones. With regards to shape, ranges tend to be elongated east-west, and variation in north-south dimensions is greater than east-west dimensions, a pattern that is likely driven by climatic constraints. While we find a positive correlation between mean local abundance and range size, the occupancy-abundance relationship fails to explain much of the variance in range size, contrary to expectations. Range size differs significantly between phyla, and phylogeny is a significant predictor of range size. For the dominant phyla, some phenotypic and genomic traits correlate with differences in range size. For example, taxa that produce pigmentation are more likely to have large ranges, and anaerobic taxa are more likely to have small ranges. Additionally, taxa found in widespread source habitats (e.g. soil) tend to have larger ranges. Together these data suggest that factors including phylogeny, physiology, population dynamics, and habitat preferences determine microbial range size and shape. These factors likely influence both dispersal capabilities and habitat filtering, which are important processes that ultimately drive patterns of microbial biogeography.