Sundaland snail diversity linked to limestone extent, drainage and landscape history

An observational biogeographical study found that the diversity of limestone-dwelling Plectostoma snails across Sundaland was associated with the extent and arrangement of limestone habitats, climate, drainage, topography and landscape history. The study delineated 33 bioregions across Southeast Asia from 771 georeferenced species-level records and examined species richness, endemism and habitat characteristics. Its results indicate that climate alone did not explain the genus’s distribution, while total limestone area was the most consistent statistical predictor of richness at the bioregion scale.

Researchers used Infomap Bioregions v2.7.1 to group 116 grid cells, each approximately 15.625 km across, by Plectostoma species composition. The cells were aggregated into 33 bioregion areas representing clusters of limestone hills with similar assemblages. This approach did not automatically treat every hill as an isolated island, because areas that are now separate limestone outcrops may once have been connected before karstification and subsequent incision and alluvial deposition created barriers.

The spatial analysis found that bioregion boundaries did not consistently follow geographical distance alone. Instead, they were associated with uplands above 250 m, drainage divides, river networks and catchment areas. In the Centre of the Malay Peninsula, the Lipis-Gua Musang-Kuala Krai limestone cluster contained three bioregions: BA2 and BA15 in the Kelantan River basin and BA3 in the upper Pahang River basin. Elsewhere, limestone clusters separated by at least 20 km and not connected by drainage commonly formed distinct areas, whereas clusters linked by adjacent river networks sometimes formed one area.

These spatial associations supported the authors’ interpretation that rivers may act as corridors for passive dispersal, potentially through floodwaters, while topography can restrict movement between drainage systems. The study did not directly measure snail dispersal, and occasional transport by wind or mobile animals could not be excluded. The authors also proposed that some wide-ranging species may have dispersed across formerly continuous limestone during earlier stages of karstification. This remains a historical explanation inferred from distribution patterns rather than a directly demonstrated mechanism.

The distribution data showed substantial endemism among the 84 Plectostoma species represented in the results. Seventy-three species occurred in only one bioregion, and 47 were classified as short-range endemics, with maximum distribution ranges below 10 km or records confined to a single outcrop. Ten species occurred in multiple bioregions. Borneo contained 52 species, compared with 28 on the Malay Peninsula, while three occurred in southern Thailand, one in Sumatra and three in southern Vietnam. The highest richness was in BA1 in the Bau-Padawan-Serian limestone cluster of western Sarawak, where 17 species were recorded.

A MaxEnt model estimated climatic suitability from nine selected bioclimatic variables using 100 bootstrap replicates. Its mean area under the curve was 0.906, with a standard deviation of 0.007. Precipitation of the coldest quarter had the highest estimated importance at 29.3%, followed by isothermality at 14.7% and precipitation seasonality at 12.8%. The model predicted relatively high suitability in parts of the central and eastern Malay Peninsula, western Borneo and Sabah. It also predicted low suitability in some occupied areas, including parts of northern Sarawak and eastern Sabah, showing that broad-scale climatic estimates did not fully account for local occurrences. The study did not establish that any apparently suitable unoccupied limestone landscape supported the genus.

Generalised linear mixed models produced different results at two spatial scales. For 24 Malaysian bioregion areas with detailed limestone maps, climate suitability and climate variability were significant in a climate-only model, while total limestone area was significant in an island-biogeography model. In the integrated model, total limestone area was the only statistically significant predictor of species richness, with a coefficient of 0.251 and p = 0.0493. Hill number and hill isolation were not significant in the thematic island-biogeography model. At the cell scale, none of the three habitat variables was independently significant, while the random effect of bioregion identity accounted for much of the explained variation, indicating an important role for spatial autocorrelation or unmeasured area-level characteristics.

Canonical correspondence analysis also associated endemism composition with habitat characteristics. At the bioregion-area scale, the ordination explained 44.1% of total variance, and the number of limestone hills and total limestone area were individually significant predictors, with p = 0.002 and p = 0.004, respectively. Hill isolation, mean climate suitability and climate variability were not individually significant at that scale. The cell-scale ordination explained 25.8% of variance; isolation, total limestone area and mean climate suitability were significant in that analysis. These results are associations, not experimental evidence that the measured variables caused the observed diversity patterns.

The study had several limitations. It relied on compiled occurrence records and uneven sampling, including unpublished inventories from 52 Malay Peninsula limestone hills collected in 2018 and 2019. Detailed habitat metrics were available only for Malaysia, and the area-scale statistical analysis included 24 of 33 bioregions. Climate suitability was estimated from interpolated environmental data that may not represent the microclimates of topographically complex limestone outcrops. The study also did not include phylogenetic or phylogeographic analyses, so the proposed links among geological history, past limestone connectivity, dispersal and present-day endemism remain hypotheses requiring further testing.

Subject of Research: Biogeographical patterns and endemism of limestone-dwelling Plectostoma snails in Sundaland

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

Article References: Liew, T.-S., Nurinsiyah, A. S., Marzuki, M. E., Fletcher, W. K., Foon, J.-K., Bian, N. A., Chen, Y., Khalik, M. Z., Vermeulen, J. J., & Schilthuizen, M. (2026). Biogeographical patterns of the genus Plectostoma H. Adams, 1865 (Mollusca, Gastropoda) in Sundaland’s limestone ecosystems. Frontiers of Biogeography, 19, Article e155940. https://doi.org/10.21425/fob.19.155940

Image Credits: AI Generated

DOI: 10.21425/fob.19.155940

Keywords: Plectostoma snails, Sundaland, limestone ecosystems, karst biogeography, species endemism, species richness, landscape connectivity, climate suitability, drainage networks, island biogeography, Biogeographical, patterns