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).