Delimiting Species—Prospects and Challenges for <scp>DNA</scp> Barcoding

Discovering, describing and cataloguing global species diversity remains a fundamental challenge both for biodiversity research and for the management and conservation of biodiversity. Among animals, the challenge is particularly acute within the arthropods, which comprise approximately 85% of all described animals, with approximately 1 million described species. The true number of arthropod species is estimated to be in excess of 10 million species. This estimate is likely to be revised upward in the light of global DNA barcode sequencing initiatives that are cataloguing unprecedented levels of cryptic or overlooked diversity. The scale of diversity that is being recovered with barcode sequencing places further strain on a taxonomic system confronted by ever-limited global taxonomic capacity to verify and describe new species. It is predicted that the number of novel operational taxonomic units delimited by barcode sequencing is likely to eclipse the number of species described by Linnean taxonomy by as early as 2029. Unless addressed, this may see an increasing proportion of arthropod species falling outside of protective legislative frameworks as a consequence of their lack of formal description. Confronted with this challenge, there is increasing, but controversial, acceptance of species delimitation and species description based on barcode sequence clustering thresholds. In response to the evolving controversy surrounding this issue, it is both timely and important to identify and clarify prospects and challenges for DNA barcoding, with a specific focus on species delimitation to address important shortfalls and impediments in biodiversity research.