Where We're Going, We Won't Need Eyes to See: Distributional Range Size Drivers of European Subterranean Spiders

ABSTRACT Aim Understanding why closely related species can have drastically different distributional range sizes is a fundamental question in biology. However, answering this question is challenging due to the multiple inter‐related factors that shape species distributions, particularly in continental regions, where unclear ecological and geographic boundaries and long evolutionary histories obscure the main drivers of range size variation. Island‐like subterranean ecosystems such as caves provide ideal models for studying species' biogeography and evolution due to their well‐defined boundaries, reduced species pools, and often easily defined abiotic limiting factors. We aim to better understand the factors influencing distributional range size in subterranean species. Location Continental Europe. Taxon Spiders (Araneae). Methods We calculated the range size of all subterranean spiders in Europe. We used Variation Partitioning Analysis to determine the relative influence of phenotypic subterranean‐related traits, historical factors, and potential sampling biases on various range size metrics while controlling for phylogenetic effects via different approaches. Results Subterranean phenotypic related traits emerged as one of the most significant determinants of range size. In addition, climatic events and conditions during the Last Glacial Maximum also played an important role in shaping the species' range sizes. Conversely, sampling biases, approximated as the density of roads and research institutions across the study area, had limited influence. We also detected that closely related species typically exhibited more similar range sizes. Main Conclusions While subterranean related traits emerged as the main predictor of range size variation, the processes shaping species' range sizes are complex and multifactorial. In addition to morphological specialization, historical events, phylogenetic relationships, and other unaccounted factors have also played a significant role in determining the current distributions of European subterranean spider species. Despite these uncertainties, our study represents an initial step toward understanding range size variation in subterranean spiders, with implications for cave evolution, subterranean biogeographic patterns, and conservation priorities—particularly given the close link between range size and extinction risk.