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.