Establishing new grid-size-dependent attributes to rank areas of endemism for conservation priorities.

Delineating Areas of Endemism (AEs) is crucial for identifying priority areas for biodiversity conservation in a spatial planning framework. Endemicity Analysis in the NDM/VNDM software is one of the primary methodologies for its delineation. Larger grid sizes have been employed to yield higher endemicity scores for AEs, recovering more endemic species and enhancing their conservation appeal. Compiling a robust geographic distribution dataset for 399 freshwater fish species from Southern Brazil, we identified AEs by conducting endemicity analyses with three different grid sizes. We also developed a spatial conservation Priority Index incorporating three grid-size-dependent attributes. We identified 153 AEs, each varying in endemicity scores, species richness, and threatened species. These variations were influenced by the analysed grid size and spatial overlap with specific freshwater ecoregions. The recovered AEs show freshwater fish distribution patterns corroborating significant vicariance events and faunal exchanges between river basins in distinct bordering ecoregions. We found an almost 50% reduction in spatial area when ranking Endemicity and Priority Index scores. This finding demonstrates the effectiveness of the Priority Index in highlighting similar sets of endemic, sympatric, and threatened species within smaller areas. This approach can effectively reconcile attributes easily extracted from the NDM/VNDM program when prioritizing spatial conservation.