Microbiome spatial scaling varies among members, hosts, and environments across model island ecosystems

The species area relationship is a classic ecological law describing the relationship between habitat increase and the number of species. Species area relationships are resoundingly positive across macrobes such as plants and animals, and emerge through nonexclusive stochastic and deterministic processes including changes in immigration and extinction, drift, and environmental heterogeneity. 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 species area relationships than free-living microbiomes due to strong host filtering, and that the species area relationships of bacteria will be shallower than fungi due primarily to differences in dispersal ability. We test these predictions in a relatively simple field system where bromeliad phytotelmata comprise aquatic ecosystems that support invertebrates and environmental substrates such as detritus. Larger phytotelmata generate larger habitat islands for microbiomes allowing us to explicitly examine their species area relationships. We find that the species area relationships of free-living and host-associated microbiomes 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 species area relationships 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 represent microbiomes, and that the influence of area on diversity appears to be more important for some microbiomes and microbes than others.