Spatial scaling varies among microbiome members, hosts and environments across model island ecosystems

The species area relationship (SAR), is a classic ecological law describing the relationship between area and the number of species it contains. SARs are resoundingly positive across macrobes such as plants and animals and emerge through non-exclusive stochastic and deterministic processes including changes in patterns of immigration and extinction, drift, and environmental heterogeneity as area increases. However, due to unique attributes of the microbial lifestyle, they may not abide by similar rules as macrobes, especially when it comes to spatial scaling. We predict that host-associated microbiomes will exhibit shallower SARs than free-living environmental microbiomes due to strong host filtering, and that the SARs of bacteria will be shallower than fungi due primarily to differences in dispersal ability. We test these predictions in a relatively simple model field system where bromeliad phytotelmata comprise aquatic ecosystems that support invertebrate hosts and environmental substrates such as detritus. Larger phytotelmata generate larger habitat islands for free-living and host-associated microbiomes allowing us to explicitly examine their SARs. We find that the SARs of free-living and host-associated microbiomes significantly differ, as do those of microbiome members. By assessing the relationship between environmental conditions and richness, and measuring diversity across scales, we posit that these observed differences in SARs are owed to differences in realized niches and dispersal abilities among microbes. These findings highlight that the classic laws of biological spatial scaling do not necessarily accurately serve microbiomes, and that the influence of area on diversity appears to be heavier for some microbiomes and microbes than others.