Variability among taxonomists in helminth species discrimination decisions: a noise audit

Determining whether or not superficially similar helminth specimens belong to the same species can be challenging, even for expert taxonomists. The possibility of cryptic species and host-induced morphological variation, combined with the lack of universally accepted thresholds for what can be considered intraspecific genetic variation, are largely to blame. In the end, decisions come down to the judgment of taxonomists. As with other domains of human judgment, however, taxonomic decisions are subject to noise, i.e., differences of opinions among taxonomists when presented with the same evidence. Here, we quantify this noise and test the role of past experience in taxonomic decision-making. We presented morphological, genetic and host data on 15 sets of hypothetical but realistic trematode specimens, each split into two groups, and asked many of the world's top trematode taxonomists to decide whether the two groups belonged to the same species, to different species, or they were not sure. Working independently on the exact same information, the taxonomists rendered species delimitation decisions that were largely inconsistent with each other, and unrelated to their past experience (measured as years of experience or numbers of published species descriptions). The inevitable conclusion is that whether two sets of trematode specimens are considered to represent the same species or two different species depends entirely on the particular taxonomist who examines them. We propose three strategies to reduce noise and achieve greater consistency and repeatability in species delimitation among different taxonomists: establishment of clear species discrimination guidelines, decomposition of the evidence into its separate components prior to a final decision, and aggregation of independent judgements from two or more experienced taxonomists. Limiting subjectivity in species delimitation decisions is essential if taxonomy is to continue underpinning other disciplines, from biodiversity and ecological research to conservation biology and wildlife management.