Habitat and the diversity and structure of insular soil mesofauna
Across all communities, the greatest differentiation was among communities from the different habitats (Fig. 3A), and we find up to 30% of OTUs and lineages that are consistent with high habitat specialisation (Fig. 5). Habitat specificity in soil mesofauna has been previously reported, with strong evidence for specialisation between open versus forested vegetation types (Arribas et al. , 2020, Caruso et al. 2012) and different forest types (Noguerales et al., 2021). Our results extend the generality of these patterns to the soils of an oceanic island.
Islands have been suggested to favour generalist species, either by colonization and persistence filters that select for species with wide niche breadth (ecological tolerance) (Gaston, 2003; Reaka, 1980) or through lower levels of competition favouring ecological release following colonisation (Olesen et al., 2002). However, our results are not consistent with these proposals, revealing that for much of the soil mesofauna, habitat features could be driving a scenario of species sorting (Leibold et al., 2004), with the existence of largely separate (still overlapping) metacommunities inhabiting the different habitats within the island. Two contrasting but not mutually exclusive models can be evoked to explain these patterns of habitat specificity. The first involves niche conservatism, with colonising species establishing into habitats to which they are preadapted and with intraisland diversification primarily constrained within the same habitats (Lim et al., 2021; Salces-Castellano et al., 2020). The second involves niche lability, and it has been described as an essential process generating diversity within oceanic island biotas through selection gradients across different habitats (Gillespie et al., 2001). Our results reveal that among the 533 lineages that are assumed to be independent colonisations to Tenerife, 312 are restricted to a single habitat. Furthermore, among the 128 15% lineages where genetic differentiation associated with habitat type was tested for, 21 presented a significant association (Fig. 6). Thus, our data provides only limited evidence for habitat shifts promoting diversification, suggesting an important role for climatic niche conservatism driving ecological assembly of soil mesofauna within the island.
Despite contrasting biotic and abiotic features among the sampled habitats (del Arco Aguilar, González-González, Garzón-Machado, & Pizarro-Hernández, 2010), ɣ and mean α diversities were similar within each, albeit with some differences between dry scrublands and the remaining three habitats (Fig. 2). Dry scrubland soils have significantly lower species richness by sample, whereas lineage accumulation across multiple sites resulted in similar values of ɣ diversity. This pattern is mediated by significantly higher local endemicity within dry scrubland soils, and thus higher turnover not spatially structured (Fig. 3). Habitat specific differences related current and past habitat patchiness and connectivity could be driving such differences. Under the habitat stability hypothesis (Ribera & Vogler, 2000; Southwood, 1977), lineages with high dispersal potential are expected to be primarily selected within more ephemeral habitats. Within this framework, it can be hypothesised that a lower habitat stability for scrublands, due to higher exposure to sea-level changes in geological times, could be contributing to their observed lower local richness and more limited spatial structuring of their soil mesofaunal communities. However, the number of spatially structured OTUs and 15% lineages was very similar among habitats, and habitat specificity at different hierarchical levels of analysis was comparable among them (data not shown). Other factors, such as fine-scale habitat heterogeneity, may have eroded the signature of geography into the assembly of mesofaunal communities within the dry scrublands. Further studies are needed to explore the contrasting ecological and evolutionary processes that drive the community assembly within different habitat types, such as those described here.
Conclusions
Our results reveal an important role for niche conservatism as a driver of insular soil mesofaunal community assembly, with limited evidence for habitat shifts promoting diversification. These results also support a fundamental role of habitat features in the assembly of soil mesofauna, in agreement with previous studies (Arribas et al., 2021; Noguerales et al., 2021), with much habitat specialism being explained as the result of independent colonisation and establishment of preadapted species. Hierarchical patterns of distance decay at the community level and metaphylogeographical analyses reveal geographic structuring over limited spatial scales from the level of haplotypes through to species and lineages, as expected for taxa under strong dispersal limitations. We also reveal broadly consistent patterns of local-scale species richness across different insular habitats and find that local insular richness is lower than in broadly comparable continental settings. These results demonstrate the potential for wocDNA metabarcoding to advance our understanding of biodiversity, particularly for the so called dark taxa – important fractions of biodiversity that have traditionally been difficult to work with.