2. Island community assembly of arthropod biodiversity
Understanding how biological communities form, and why they differ spatially and temporally, is a key objective in ecology. The integration of phylogeny into the analysis of community ecology has provided new dimensions for comparing and contrasting communities, within which insular environments have proven to be useful sampling frameworks (Emerson & Gillespie, 2008; Shaw & Gillespie, 2016). Island systems can yield significant arthropod structuring and turnover over relatively limited spatial scales and across entire assemblages (e.g. Salces-Castellano et al., 2021). Such limited spatial dimensions allow for fine-scale but geographically representative community-level sampling to characterise the community assembly process. When executed across multiple islands and coupled with remote sensing data, there is much potential for an improved understanding of the factors that shape such patterns (Bush et al., 2017).
Community-scale investigation to describe patterns and infer processes for island biodiversity requires multiple site-based characterisations of communities, which is a clear bottleneck in the case of the arthropod biodiversity. Existing studies in this area have been limited to specific arthropod lineages, such as beetles or spiders, for which conventional taxonomical and molecular processing is time-consuming (e.g. Malumbres‐Olarte et al., 2021; Antonia Salces-Castellano et al., 2020). HTS barcoding provides an opportunity to increase both the taxonomic and geographic scale of arthropod community sampling. When combined with distribution data across gradients (e.g. elevation, disturbance, island age) or trait data (e.g. dispersal ability, body size), the opportunities for macroevolutionary and macroecological unification become tangible. As a proof of concept, Lim et al. (2021) applied wocDNA metabarcoding to characterise complete arthropod communities across elevational gradients within the island of Hawaii. They revealed that climatic niche conservatism is an important factor shaping ecological assembly across elevation, thus implicating topographic complexity as an important driver of diversification. Similarly, wocDNA metabarcoding approaches to characterise soil arthropod assemblages on the islands of Tenerife and Cyprus have revealed strong habitat filtering and dispersal limitations as drivers of community assembly within islands (Andújar et al., 2022; Noguerales et al., 2022 this issue).
Islands, particularly remote islands, offer much potential for integrating intraspecific-scale analyses together with phylogenetic sampling for the investigation of community assembly. Speciation represents an important contribution to both the origin and evolution of community structure on remote islands (Shaw & Gillespie, 2016), thus providing opportunities to link diversification patterns within species to patterns of speciation at higher levels. Community-level intraspecific sampling on islands has seen less implementation, and it is here that HTS barcoding can play an important role. As well as recording species presence, HTS barcoding provides a measure of haplotype variation within and across communities, thus addressing the traditional Darwinian shortfall (defined as the lack of knowledge regarding the evolution of lineages, species, and traits; Diniz-Filho, Loyola, Raia, Mooers, & Bini, 2013) for arthropod island faunas. Thus, alpha and beta diversity can be analysed for hyperdiverse arthropod communities from genetic to different levels of taxonomic hierarchy to understand how community-level processes drive macroecological and macroevolutionary patterns. Processes that can be characterised include the relative importance of stochasticity, isolation by distance, and habitat or host-associated differentiation (e.g., see Andújar et al., 2022; Noguerales et al., 2022; Arribas, Andújar, Salces-Castellano, Emerson, & Vogler, 2021). Additionally, the extent to which species diversity and genetic diversity covary can be derived from such data (Vellend et al., 2014; Vellend, 2010). Overcast, Emerson, & Hickerson, (2019) have recently described a mechanistic model of community assembly that can generate linked patterns of abundance and genetic diversity under an assumption of joint ecological and evolutionary neutrality, allowing for the estimation of community abundance structure using only intraspecific genetic variation. As proof of concept, this study demonstrated that the abundance structure of spiders on the island of Reunion could be accurately estimated from intraspecific variation from barcode data (Emerson et al., 2017). Further theoretical developments will be needed to fully exploit the potential of genetic community-level data for unifying macroevolution and microevolution, together with macroecology and microecology, and thus further advance island biogeography theory (see Overcast et al., 2022, for a review).