The key role of vicariance for soil animal biogeography in a biodiversity hotspot region

Aim: The Indo-Australian Archipelago is known as a biodiversity hotspot with a high level of endemism typically ascribed to vicariance as reflected by the “Wallace’s line”. However, it is unknown how vicariance has affected belowground biodiversity, especially process-based beta diversity. Here, we relate beta diversity of soil oribatid mite (Oribatida, Acari) assemblages to geographic distance as well as climatic and soil factors to explore the factors shaping the diversity of oribatid mites across eleven regions of the Indo-Australian Archipelago.

Location: Indo-Australian Archipelago.

Time Period: Present.

Major taxa studied: Oribatida, Acari.

Methods: We compiled a list of 2,549 oribatid mite species in the Indo-Australian Archipelago and investigated the level of endemism and beta diversity of oribatid mites in the eleven regions at species, genus and family level. We then summarized the biogeographical dissimilarity patterns of oribatid mites using ordination and clustering methods, and compared the patterns with the zoological boundaries based on aboveground taxa such as the Wallace’s, Lydekker’s, Weber’s and Holt’s line. We integrated data on geography, climate and soil to reveal the key drivers of species compositional dissimilarity of oribatid mites among regions using Mantel tests.

Results: Generally, the level of endemism of oribatid mite assemblages in the eleven regions was high; they formed three groups (west of New Guinea, New Guinea and south of New Guinea) with dissimilarity changing from northwest to southeast. The patterns reflect and integrate the lines of Weber, Lydekker and Holt. Species turnover generally correlated with geographic distance reflecting the critical role of vicariance in dispersal-limited oribatid mites.

Main conclusions: Our results, for the first time, demonstrate contrasting patterns in below- and aboveground organisms in the Indo-Australian Archipelago, and elucidate how geographic distance-based vicariance has structured soil animal diversity in this biodiversity hotspot region.

Methods
We extracted the list of species of oribatid mites (including genus and family) of the Indo-Australian Archipelago from Colloff (2023), Corpuz-Raros and Ermilov (2019, 2020) and Subías (2022), and grouped the distribution of the species into eleven regions, i.e., Australia, Bali, Borneo, Indo-China Peninsula, Java, Lesser Sunda Islands (excluding Bali), New Guinea, New Zealand, Philippines, Sulawesi and Sumatra (Figure 1, Table S1). Species names of oribatid mites were standardized following Subías (2022) for the following reasons: (a) Subías’ (2022) includes oribatid mites across the world. By contrast, the classification used in other papers such as Corpuz-Raros and Ermilov (2019, 2020) and Niedbała and Liu (2023) only includes certain taxa, with most of the subdivisions of large families and genera also being adopted from Subías but from his 2004 catalogue (Subías, 2004) and not the more recent and updated one from 2022. (b) A recent study based on molecular data of 317 oribatid mite species/taxa revealed that a number of traits used for the taxonomic classification of oribatid mites evolved convergently and therefore the current classification is likely to change substantially in future (Cordes et al., 2024). (c) Subías’ (2022) classification was used in a recent biogeographic study (Lu et al., 2024) allowing to compare our results with this previous publication. Species lists of the eleven regions and distribution data are given in Tables S2–S12. Species occurrences across the eleven regions were transformed into a presence–absence (‘1’–‘0’) matrix (Table S13).