Effects of Taxonomic Uncertainty on Range Size and Niche Estimation in a Southern Ocean Cryptic Species Complex

ABSTRACT Aim Understanding how biodiversity varies over space and time is critical for informing environmental policy and decision making. The application of molecular techniques for sequencing organisms has increased the ability to identify cryptic species, which provides a source of taxonomic uncertainty that can potentially affect such decisions. Our aim was to examine the effect of taxonomic uncertainty on (1) geographic range size estimates over space and time and (2) environmental niche quantification and comparison. Location Southern Ocean. Taxon Crinoidea: Promachocrinus . Methods We used an updated, taxonomically resolved dataset of the feather star Promachocrinus spp. abundance in the Southern Ocean. Measures of range size and niche overlap were estimated using down‐sampled Random Forest species distribution modelling (SDM). This approach accounts for the class imbalance that is prevalent in presence‐only SDMs. Identity tests were used to assess niche equivalency among species. Additionally, we used n ‐dimensional hypervolumes as a second, independent means of estimating the environmental niche for each species and assessed overlap using hypervolume overlap statistics. Results Range size estimates varied within and among Promachocrinus species. Furthermore, taxonomic uncertainty associated with cryptic species affected predicted species distributions under different emission scenarios, potentially leading to different range size trends over time. Promachocrinus species also may, by and large, have different environmental niches, determined by both SDM derived identity tests and hypervolume overlap tests. Main Conclusions We show that taxonomic uncertainty, as exemplified by cryptic species, can potentially affect the measurement and monitoring of biodiversity, with our results suggesting different environmental niches and range sizes for members of the Promachocrinus kerguelensis complex. Developments in biodiversity metadata could help account for taxonomic uncertainty in downstream analyses, thus leading to more accurate estimates of biodiversity change and better‐informed conservation assessments.