The (unknown) diversity of soil mesofauna within insular soils
The diversity of soil mesofauna within oceanic islands has been poorly explored. Literature on the topic is limited (i.e. Koh et al. , 2002; Maraun et al. , 2007; Fattorini, 2009; Cicconardi et al. , 2017), and even basic species inventory data are in general scarce for this ecologically important biodiversity fraction. Within the Canary Islands, the Biodiversity Databank of the Canary Islands (https://www.biodiversidadcanarias.es/biota/; from hereon referred to as BIOTA) is a constantly updated public database containing all species records for the archipelago published in the scientific literature. BIOTA currently reports 287 species of Acari, 88 of Collembola, and 1360 species of Coleoptera from the island of Tenerife. Our results demonstrate that current knowledge of species diversity within the island is greatly underestimated. By sampling only 52 soil communities (approximately a 2.6 x 10-8 of total island surface area) across the four dominant habitats of the island of Tenerife, we have recovered nearly 1800 mtDNA haplotypes from Acari, Collembola, and Coleoptera that clustered into 813 putative species (OTUs at 3%), 434 Acari, 129 Collembola and 250 Coleoptera (Table 2). Even with a substantially more conservative dissimilarity threshold of 15%, total lineage number remains above 500. For Acari and Collembola, sampled OTU numbers exceed the number of species recorded until the date for the island (Table 2). In the case of Coleoptera, it should be noted that many of the 1360 recorded species in BIOTA for Tenerife are not associated with soil, while our sampling is strictly focussed on soil lineages, so a direct comparison is difficult. Overall, our results reveal that the soils of Tenerife are much richer in mesofauna than previously reported, and highlight the generally appreciated problems of the Linnaean and Wallacean shortfalls (Cardoso, Erwin, Borges, & New, 2011; Hortal et al., 2015) for soil arthropod biodiversity.
By comparing obtained ASVs against public molecular repositories, we found that 135 (16.7%) of the 813 OTUs matched (97% similarity) non-Canarian records, and can thus be considered as non-endemic species, being either native or introduced (Table S7). However, attributing all remaining OTUs to endemic species is not possible, because of the incomplete nature of public molecular repositories. Comparison to public molecular repositories identifies 34 Acari (8%), 39 Coleoptera (16%), and 49 Collembola (38%) OTUs with high sequence similarity (≥99%) to individuals from other regions. It is plausible that most, if not all, are recent human-mediated introductions, rather than worldwide distributed species requiring unrealistic passive dispersal kernels to maintain species cohesion. These findings are in agreement with Cicconardi et al. (2017), who concluded from genome skimming data that 88% of the 25 Collembola species they sampled from laurel forests in Tenerife result from human-mediated introductions. Distinguishing between native and introduced origins for soil-adapted species is challenging, and focused studies are needed to elucidate the extent of species introductions within oceanic islands (Andersen et al., 2019).
Alpha, beta, and gamma diversity estimations at the OTU and haplotype levels point to lower diversity values in island soils compared to continental soils (Fig. S2). In contrast, high endemicity by sample and significant community differences among habitats are similar to patterns found in continental soils (see the section below). Using the same field, laboratory, and bioinformatic protocols, Arribas et al.(2020) sampled 12 sites within each of three forested and three grassland areas in Iberia. While sample sizes are comparable between both studies, spatial scale differs, with maximum distances between samples of 70 km within Tenerife habitats compared to only 15 km in Arribas et al. (2020). Within habitats, both α and β diversity (restricted to a comparable spatial scale of 15 km) were significantly lower in Tenerife, suggesting that insular soil mesofaunal diversity may be lower, compared to continental areas of a similar size, consistent with previous suggestions for aboveground plant and animal communities (Kier et al., 2009; Whittaker & Fernández-Palacios, 2007). While interesting, the generality of this pattern awaits further investigation. With appropriate measures to harmonise methodology and optimise data comparability, the generality of patterns observed here can feasibly be assessed across independent studies (Arribas, Andújar, Bidartondo, et al., 2021).