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.