Molecular delimitation of evolutionary significant units reveals hidden geographic drivers of extinction risk within island arthropods

Aim: Globally, arthropod biodiversity is under threat, with increased risk
of species-level extinctions, and this threat is particularly acute on
oceanic islands. A fundamental first step toward understanding extinction
risk is to understand genetic connectivity among the constituent
populations of a species. Our aim is to develop and implement a protocol
to characterise genetic connectivity among island populations within
arthropod species to reveal otherwise hidden range size drivers of
extinction. Location: Canary Islands, Spain. Methods: We implement a
protocol based on mtDNA sequence data for the delimitation of evolutionary
significant units (ESUs) to evaluate extinction risk among species of
beetle and spider distributed across multiple islands. Results: Our
results reveal that more than half of the species analysed are comprised
of two or more ESUs. We also find that low dispersal ability was a
significant predictor of ESUs within species of Coleoptera, but with no
significant difference for Araneae. Main conclusions: Most ESUs are
consistent with early stage differentiation, or incipient speciation, with
some exceeding a conservative interspecific threshold, thus indicative of
cryptic species. We suggest that extending our approach with the
integration of other species-level traits may provide for a more refined
predictive framework for understanding extinction risks across island
arthropod species.