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).