4. Arthropod species abundances, endemicity and rarity within islands
The Prestonian shortfall is defined as the lack of knowledge about the abundance of species and their population dynamics in space and time (Cardoso et al., 2011). This shortfall is extremely pronounced in the case of arthropods (Cardoso & Leather, 2019), likely associated with the high diversity and complexity of both individual population trends and species interactions of local arthropod communities. HTS barcoding, particularly multiplex barcoding, has much potential to address this shortfall. Generating abundance estimates through HTS barcoding for insular arthropod communities is a potentially rich source of information for empirical testing of island biogeographic theory. This is particularly relevant for questions regarding arthropod SADs within islands (e.g. Borda-De-Água et al., 2017), and questions at the intersection of species abundances patterns and the processes of speciation and extinction. Such abundance data can now be directly generated for complete arthropod assemblages, even in the absence of formal species description, using multiplex barcoding (e.g. see Srivathsan et al., 2021). PCR free metagenomic approaches can also provide abundance estimates (Ji et al. 2020). In addition to this, the integration of image analysis, together with either (i) multiplex barcoding, or (ii) wocDNA metabarcoding, can further remove limitations of scale. Beyond specific interest in island biogeographic process, it has also been pointed out that sampling across islands can provide for a more general understanding of how and why species abundances change through community assembly (Warren et al., 2015). The neutral spatially explicit model (NSIM, Rosindell & Harmon, 2013) predicts patterns as islands approach equilibrium conditions, in the classic sense of the equilibrium theory of island biogeography (ETIB, MacArthur & Wilson, 1967), reflected in immigration rates, extinction rates and species abundance distributions (SADs).
Within islands, HTS barcoding also harbours the potential to overcome the Wallacean shortfall for arthropod faunas, which is defined as the lack of knowledge regarding the geographical distributions of species (Hortal et al., 2015; Lomolino, 2004). Arjona et al. (2022) provide a clear example of this, revealing substantial improvements to beetle species distribution data from wocDNA metabarcoding. Geographical distribution data for insular arthropod faunas, together with genetic and abundance data at the community-level (also derivable with the HTS barcoding tools) offer an ideal setting to explore linkages between species rarity (geographical, habitat specificity and/or local population size) and endemicity (e.g. Fernández-Palacios et al., 2021; Ribeiro et al., 2005).