Biodiversity in heavily modified waterbodies: native and introduced fish in Iberian reservoirs
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1. Introduced species often thrive in modified systems, although it is not clear whether they have negative effects on native biota that might also be present or, alternatively, occupy niches that have been left empty due to the original habitat modification. 2. We used 247 surveys of fish assemblages (48 species in all) in riverine reservoirs across the Iberian Peninsula to analyse the relationships between the species richness of introduced and native fish. Most of the samples (195) were from areas with a Mediterranean climate, while the reminder (52) had a temperate climate. 3. Mediterranean reservoirs had more introduced than native species, with an opposite pattern in the temperate areas. The richness of non-natives was positively related to the area of reservoir and human pressures and negatively related to altitude. Native richness was higher in larger and deeper reservoirs and lower in older and more impacted reservoirs, as well as in those with more introduced piscivores. 4. There was a negative relationship between the richness of native fish and that of non-native piscivores in Mediterranean reservoirs, confirmed through multiple regression and structural equation modelling. Contrastingly, native fish richness was mainly controlled by variables related to water quality in temperate areas, implying that the negative impacts of introduced fishes in reservoirs can be context dependent. 5. The presence of introduced piscivores thus seems an important factor excluding many native fish from reservoirs in Mediterranean environments in the Iberian Peninsula.