1. Global patterns and drivers of island arthropod
biodiversity
Studies of global-scale patterns and drivers of island biodiversity rely
on the existence of island species inventories. Arthropod species lists
for islands have been leveraged for comparative analyses to infer the
processes that shape variation in species richness (e.g. Chown, Gremmen,
& Gaston, 1998; Santos, Fontaine, Quicke, Borges, & Hortal, 2011;
Triantis, Economo, Guilhaumon, & Ricklefs, 2015). However, it is
increasingly recognised that many arthropod species are still not
formally described (commonly described as the Linnean shortfall,
Lomolino, 2004), strongly hampering inferences of richness, diversity
and endemism (e.g. Cicconardi, Fanciulli, & Emerson, 2013; Emerson,
Cicconardi, Fanciulli, & Shaw, 2011; Legros, Rochat, Reynaud, &
Strasberg, 2020), and hence, conservation efforts (Cardoso et al.,
2011). A structured HTS inventory of arthropod communities across
islands holds great potential to overcome this shortfall by providing a
way forward to overcome traditional challenges of identification. HTS
barcoding allows for communities to be inventoried independently of the
degree of undescribed species present, while simultaneously highlighting
species that are either described and absent from a local reference
library, or undescribed. This opens the door for inclusive inventories
that extend to the typically small and cryptic taxa fractions of
arthropod diversity that are associated with high levels of undescribed
species, the so called “dark taxa” (Hartop, Srivathsan, Ronquist, &
Meier, 2021; Hausmann et al., 2020). As proof of concept, Yeo et al.
(2021) implemented multiplex barcoding to inventory the tropical
arthropod fauna of Singapore across six different forests and a
freshwater swamp. They generated 140,000 barcoded specimens belonging to
ca. 8,500 species and identified an overlooked hotspot of insect
diversity within the mangrove habitat. Similarly, Arjona et al. (2022,
this issue) applied wocDNA metabarcoding to characterise the coleopteran
fauna sampled from soils within an island cloud forest, highlighting the
power of HTS for both detecting unrecorded species and species
discovery. They additionally demonstrated the value of barcode reference
libraries, even if incomplete, for efficient bioinformatic processing to
achieve reliable haplotype data (Andújar et al., 2021), and in doing so,
provide strong evidence for taxonomic inflation in the absence of such
processing. Complete island arthropod inventories (i.e. from exhaustive
sampling and encompassing undescribed species) that are comparable
across different insular systems (by direct comparison of DNA barcodes)
can be used to address fundamental topics within island biology, for
which understanding is still limited. These include: (i) the
relationships among island area, geological age, elevation and isolation
and arthropod species richness; (ii) the biogeographic processes driving
island species-area relationships (SARs), and (iii) how arthropod SARs
vary among different arthropod fractions and among different island
contexts, including oceanic, continental-shelf, continental fragment and
habitat islands.
WocDNA metabarcoding can be applied to generate vast amounts of
site-based data, and if combined with multiplex barcoding, barcoded
vouchers can be retained for specific taxonomic reference. Such HTS
barcoding data opens the door for the phylogenetic placement of
potentially all sampled species when combined with appropriate backbone
trees (e.g. Balaban, Sarmashghi, & Mirarab, 2018). With the development
of mitochondrial metagenomics (Crampton-Platt, Yu, Zhou, & Vogler,
2016), backbone trees can now be generated with good resolution for
major arthropod lineages (e.g. Arribas et al., 2020; Tang et al., 2019).
Thus, by returning to multiplex barcode DNA extracts or strategically
sampling vouchers from wocDNA samples prior to bulk DNA extraction, such
backbone trees for an island fauna can be obtained. Assemblage level
phylogenetic trees then provide the needed framework to ask, for
example, how taxonomic and phylogenetic diversity vary across gradients
of interest, within islands, among islands, and between islands and
ecologically comparable continental areas.