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.