Biogeographical patterns of the genus Plectostoma H. Adams, 1865 (Mollusca, Gastropoda) in Sundaland’s limestone ecosystems

Sundaland’s limestone ecosystems are biodiversity hotspots facing increasing threats from habitat fragmentation and human activities. Our study examines the biogeographical patterns of the limestone-obligate land snail genus Plectostoma across Sundaland to identify the drivers of species richness and endemism. Using 771 georeferenced records of 84 species, we delineated 33 bioregion (biogeographical region) areas based on species composition through Infomap Bioregions analysis. We then examined the effects of integrated island biogeography variables (limestone area, number of hills, isolation), MaxEnt-derived climate suitability, and landscape features such as topography and drainage networks on biogeography patterns and species diversity. Bioregion boundaries were best explained by elevation barriers, drainage basins, and river network connectivity, highlighting the importance of geomorphological and hydrological structure over geographic distance. Generalised linear mixed models revealed that larger limestone areas significantly predicted greater species richness than did higher climate suitability. Canonical Correspondence Analysis showed that endemism composition was primarily driven by the number and total area of limestone hills, while hill isolation and broad-scale climate variables played lesser roles. Most species (87%) were restricted to a single bioregion, with over half qualifying as short-range endemics (<10 km distribution range), underscoring strong spatial turnover. These patterns suggest that ancient geological connectivity, followed by progressive karst fragmentation, shaped current diversity. Future phylogenetic and multi-taxon studies will enhance the understanding of evolutionary processes and conservation strategies in Sundaland’s limestone habitats. We delineated 33 distinct bioregions for Plectostoma land snails across Sundaland. Precipitation defines the broad habitable envelope determining the occurrence of the genus, while drainage basins and elevation barriers (>250 m) serve as the primary determinants of bioregional boundaries. We established that the species richness and endemism of Plectostoma are primarily driven by the total area and total number of limestone hills within a bioregion, rather than broad-scale climatic suitability. Our results support a landscape-evolution model for diversity patterns, linking biological distribution to karstification stages where species dispersed across continuous surfaces during the early (Fengcong) stage of karstification before becoming isolated on “island” towers during the remnant (Fenglin) stage.