Modeling potential fish species richness within the Upper Paraguay River Basin in Brazil

Understanding spatial patterns of fish richness in large tropical basins is essential for conservation under accelerating environmental change. This study explored absolute count and Ecological Niche Modeling (ENM) using the MaxEnt algorithm to estimate the potential distribution and species richness of native fish in the Upper Paraguay River Basin (UPRB), which includes the Pantanal, the world's largest tropical wetland. We integrated hydrological, geomorphological, and climatic variables to model potential richness across the basin. Contrary to our initial hypothesis, the models revealed higher potential fish richness in the plateau headwaters than in the Pantanal. Elevation emerged as the main environmental filter, acting as a proxy for temperature, habitat heterogeneity, and geomorphological transitions, while flow velocity highlighted the influence of local hydrodynamics on assemblage structure. The most species-rich rivers coincide with areas under severe and expanding anthropogenic pressures, including agribusiness expansion, deforestation, hydroelectric planning, and ongoing and future land-use intensification driven by agricultural expansion and climate variability. These threats are already causing local extinctions and habitat fragmentation. Without effective mitigation measures, they could lead to the complete loss of endemic species and the collapse of downstream biodiversity integrity. These findings underscore the need to incorporate hydrological predictors into ENMs and promote basin-wide conservation strategies supported by continuous scientific collaboration among Brazil, Bolivia, and Paraguay. Such initiatives should integrate biological data with hydrological, geomorphological, and land-use information to enable a comprehensive assessment of ecosystem integrity and guide long-term management across the entire Upper Paraguay Basin.