Scale‐dependent assembly shapes taxon‐dependent responses of soil microbes to island isolation and area

The theory of island biogeography (TIB) remains contentious for microbes, as their immense dispersal potential and microscopic size may decouple diversity from classic isolation/area effects. We explicitly tested TIB scaling rules for soil bacteria and fungi across 20 oceanic islands using a rigorous multi‐scale framework that integrated high‐resolution ASV analysis, coverage‐based diversity estimation, and phylogenetic null models. We found that geographic isolation emerged as the predominant driver of microbial diversity, yielding consistent negative species–solation relationships (SIRs, where species richness decreases with isolation) across scales and most taxonomic groups, reflecting pervasive dispersal limitation. In contrast, species–area relationships (SARs, where species richness increases with area) at the island scale were largely attributable to sampling intensity. After standardizing sampling effort and integrating local‐scale (mean plot ASV richness) analyses, significant positive SARs disappeared for all groups except arbuscular mycorrhizal fungi. This pattern reflects the unique ecological dependence of arbuscular mycorrhizal fungi on host‐mediated environmental heterogeneity. Community assembly analyses using iCAMP and variation partitioning consistently revealed that while dispersal limitation dominated community assembly (62.1–81.5% of the variation), homogeneous selection intensified on more isolated islands for most microbes. As island area increased, environmental filtering was relaxed, yet spatial assembly processes diverged: dispersal limitation intensified for most fungi, whereas homogeneous selection became more prominent for most bacteria. These results demonstrate that geographic isolation drives microbial diversity, yet its effects are scale‐dependent and taxon‐specific. These findings highlight the need to incorporate scale‐ and lineage‐specific assembly processes into TIB for microbial community.