Fish Fauna in Iberian Mediterranean River Basins: Biodiversity, Introduced Species and Damming Impacts

1. We used a basin approximation to analyse distribution patterns of different components of biodiversity (taxonomic richness, endemicity, taxonomic singularity, rarity) and conservation status of freshwater fish fauna in 27 Mediterranean Iberian rivers. 2. Basin area alone explained more than 80% of variation in native species richness. Larger basins featured not only a higher number of native species, but also more endemic and rare species and less diversified genera than smaller ones. 3. In contrast, smaller basins scored higher community conservation values, due to their lower degree of invasion by introduced species. 4. The presence of dams was the most important factor determining the conservation status of fish communities, and it was also positively associated with the number of introduced species. 5. While the most important components of Iberian freshwater fish biodiversity are located in large basins, small unregulated ones feature better conserved fish communities.