Effects of climate change on invasive fish species and the vulnerability of tropical freshwater ecoregions

Biological invasions and climate change are major drivers of biodiversity loss, yet their combined effects on freshwater ecosystems in tropical regions remain poorly understood. Here, we assess the current and future climatic suitability of Brazil's freshwater ecoregions for non-native fish species and identify regions most vulnerable to biological invasions under climate change. We compiled a comprehensive database of non-native freshwater fishes in Brazil and modeled the potential distributions of 138 species using ecological niche models (ENMs). Occurrence records from native and non-native ranges, bioclimatic variables, and three climatic scenarios (current, SSP2-4.5, SSP5-8.5). We overlaid model projections with Brazil's freshwater ecoregions to quantify changes in suitable areas and identify invasion hotspots. Model performance was good, AUC values ranging from 0.80 ± 0.06 to 1.00 ± 0.01 and TSS values from 0.61 ± 0.08 to 1.00 ± 0.02. Most non-native species (65-66 %) are projected to lose climatically suitable areas under future conditions, particularly in northern ecoregions such as the Amazon Basin. In contrast, several ecoregions in southern and southeastern Brazil are expected to remain or become increasingly suitable for non-native species, despite an overall reduction in suitability at large scale. These regions are also those most affected by urbanization, habitat fragmentation, aquaculture, and hydropower development, which may amplify invasion risks. Our findings demonstrate that climate change will shift invasion risk toward southern Brazil, reinforcing the need for targeted prevention, improved aquaculture management, and awareness programs to reduce the establishment and spread of non-native freshwater fishes.