Invasion of non-native species is considered a major threat to global biodiversity. Among the vertebrates, fishes are the most numerous taxon and their invasion biology is relatively well studied. For freshwater fishes, continental and global records indicate that several bioregions are threatened by taxonomic homogenisation, i.e. an increased similarity of fish faunas caused by invasion of the same fish species into many regional species pools. The Palearctic realm has experienced drastic changes in faunal composition relative to former times. Here we analysed the occurrence, richness and biomass contribution of non-native fish species in 1943 standing waterbodies from 14 countries of the Western Palearctic, based on standardised fish catches by multi-mesh gillnetting. We ask if 1) non-native fish occurrence in single lakes would be lower, due to its isolated nature, than that suggested from catchment-scale data (c. 60%), 2) strong geographical gradients are linked to the occurrence of non-natives, 3) the contribution by non-natives to the local fish community biomass is correlated with local richness and the trophic level of native and non-native species.
Non-native fish species occurred in 304 of 1943 waterbodies (16%). If the average number of occupied waterbodies per country was weighted by number of waterbodies per country, the grand mean occurrence of non-natives in Western Palearctic waterbodies was 10%. Exotic (non-native to the Palearctic) and translocated (non-native only to parts of the Palearctic) species were found in 164 (8.4%) or 235 (12.1%) of the waterbodies, respectively. The occurrence and local richness of non-native fish species increased with temperature, precipitation and lake area and were substantially higher in reservoirs than in natural lakes. High local biomass contributions of non-native species were strongly correlated with low richness of native species and high richness of non-native species, whereas the trophic level of the fish species had only a weak effect. We conclude that lakes are less vulnerable to invasions by non-native fish than rivers, and hence the average occurrence is relatively low. While large-scale climatic drivers are primary determinants of fish species richness and community composition, local species richness of natives and non-natives may determine the local biomass contribution of non-natives and hence predict the strength of community and ecosystem effects exerted by non-native species.