The faunal Ponto-Caspianization of European waterways

Abstract As alien invasive species are a key driver of biodiversity loss, understanding patterns of rapidly changing global species compositions depends upon knowledge of biological invasion dynamics and trends. The Ponto-Caspian region is among the most notable donor regions for aquatic invasive species in Europe. Using macroinvertebrate time series collected over 52 years (1968–2020) at 265 sites across 11 European countries, we examined the occurrences, invasion rates and interspecific interactions of freshwater Ponto-Caspian fauna, as well as biotic homogenization trends in European waterways. According to our data, Ponto-Caspian macroinvertebrates increased from two species in 1972 to 29 species in 2012 and we found a greater richness of invading Ponto-Caspian macroinvertebrates in datasets collected closer to the source region. This may reflect dispersal lag times, indicating that more distant sites will be increasingly invaded in the future. The pioneering Ponto-Caspian species that arrived first were often bivalves (46.5% of cases), particularly Dreissena polymorpha , followed secondarily by amphipods (83.8%; primarily Chelicorophium curvispinum and Dikerogammarus villosus ). With increasing invasions, the time between consecutively-appearing invaders decreased six-fold, indicating potential facilitations that improved invasion success of subsequent taxa. We detected a negative relationship between the abundance trends of the first and second Ponto-Caspian macroinvertebrate species, suggesting that interactions between these species are asymmetrical, with the initial invader facilitating the subsequent invader at the potential cost of its own population growth rate. Lastly, we found that macroinvertebrate communities became increasingly similar over time. European invasion rates from the Ponto-Caspian region suggest a high potential for future invasions from this region, with more hotspots and donor regions likely emerging.