Unraveling dark diversity patterns of freshwater fish to inform conservation planning

Conservation assessments based solely on observed diversity can miss suitable but absent species, leaving biodiversity deficits and restoration opportunities underestimated. By quantifying the unrealized component of species pools, dark diversity complements observed diversity and, when interpreted with community completeness, helps identify areas of conservation or restoration importance. Yet freshwater applications remain limited and often rely on site-scale frameworks that overlook hydrological connectivity and catchment boundaries, potentially biasing species pool estimates and undermining applications. To address this gap, we used a subbasin-scale framework to investigate the drivers and conservation implications of freshwater fish dark diversity across species and subbasin levels in Yunnan, China, a global biodiversity hotspot. We found mismatches between dark and observed diversity in spatial patterns and drivers. Species-level dark diversity probability (DDPspecies) increased with maximum body length and relative eye size, but decreased with body lateral shape, body elongation, and oral gape position. At the subbasin level, drivers were metric-specific. Waterbody variability increased observed diversity and community completeness index (CCI), whereas subbasin area increased DDPsubbasin and species pool size but reduced observed diversity and CCI. Historical climate effects were limited and negative for DDPsubbasin or species pool size, while temperature and precipitation generally promoted observed diversity, CCI, and species pool size. Integrating observed and dark diversity identified distinct biodiversity states insufficiently covered by protected areas, including candidate conservation hotspots (16.48%), restoration-priority areas (24.92%), and biodiversity refugia (13.97%). Our findings show that dark diversity, when interpreted with observed diversity, can reveal hidden biodiversity deficits and support more effective freshwater conservation planning.