5. Biotic interactions of insular arthropods
Islands are providing important advances in the application of
ecological network approaches for the understanding of ecosystem
function, and vulnerability to disturbance (Traveset et al., 2016). The
time-consuming task of arthropod sorting and identification that is
needed to quantify plant-arthropod or arthropod-arthropod associations
is a bottleneck for addressing the Eltonian shortfall for arthropods,
defined as the lack of knowledge of interactions among species or among
groups of species (Hortal et al., 2015). However, progress toward
addressing this shortfall can be scaled up with the application of HTS
barcoding. WocDNA metabarcode data can be used to estimate
ecological/trophic networks through co-occurrence analysis (Bohan et
al., 2017). Sampling strategies that yield few arthropod individuals per
sampled plant would be most efficiently coupled with multiplex
barcoding. However, larger arthropod samples, such as aggregating
arthropod samples by plant species (e.g. Rego et al., 2019; Ribeiro et
al., 2005), can be coupled to wocDNA metabarcoding. Recently,
environmental DNA metabarcoding from plant material offers promise as an
additional tool to recover arthropod-plant interactions (Thomsen &
Sigsgaard, 2019). Barcode reference sequences for the taxonomic
assignment will be desirable, but even in their absence, ecological
networks can still be established with higher-level taxonomic
assignment. One largely unresolved challenge for understanding the
biotic interactions of many arthropod species is the different biology
of the life history stages. The problem is exacerbated by the fact that
most arthropod sampling methods favour adults, although the larvae of
many species may be important for biotic interactions. An in-depth
understanding of the interaction thus often requires multiplex barcoding
of adults and larvae in order to establish barcode matches between adult
and larval stages (Yeo, Puniamoorthy, Ngiam & Meier, 2018).
Moving from association data to trophic interactions can be integrated
within a multiplex barcoding framework, and may be particularly useful
to improve predictions within the trophic theory of island biogeography
(Gravel, Massol, Canard, Mouillot, & Mouquet, 2011; Holt, 2009). Gut or
digestive system metabarcoding from appropriate individual DNA
extractions (e.g. whole organism) can be used to characterise both
herbivore (e.g. Kitson et al., 2015) and predator diet (e.g. Cuff et
al., 2021; Kennedy, Lim, Clavel, Krehenwinkel, & Gillespie, 2019). This
can be more challenging in predators due to the issue of
co-amplification of predator DNA (see Kennedy et al., 2020 for a
review), although this obstacle can be overcome via careful primer
design (Krehenwinkel et al., 2019). Advances in the characterisation of
arthropod microbiomes offer new dimensions to investigate the dynamics
of both ecological success (e.g. invasive species) and vulnerability
(e.g. range-restricted endemic species), while also investigating the
temporal and spatial dynamics of microbiome evolution (e.g. Leo et al.,
2021). For example, independent but geographically coincident patterns
of island colonisation and speciation, such as those conforming to the
progression rule (Shaw & Gillespie, 2016), can be used to understand
potential generalities of microbiome evolution, associated with a
history of founder event speciation. As a proof of concept, Armstrong et
al. (2022) have explored how the associated microbial communities within
a lineage of spiders have changed with colonisation across a
chronosequence of volcanoes in Hawaii.