Expansion and Environmental Drivers of Invasive Freshwater Molluscs in Catalonia

Freshwater ecosystems in the Mediterranean region are highly vulnerable to biological invasions, yet invasive macroinvertebrates remain less studied than other taxonomic groups. In Catalonia (NE Spain), non-native freshwater molluscs are increasingly widespread and may substantially alter ecosystem functioning. This study analyses the spatiotemporal expansion and environmental drivers of four invasive freshwater molluscs (Physella acuta, Potamopyrgus antipodarum, Corbicula fluminea and Dreissena polymorpha) across the Catalan River Basin District. Species occurrence data were compiled from regional and global biodiversity databases and long-term bio-monitoring programmes, harmonised and aggregated into a 10 x 10 km grid. Temporal trends in cumulative occupancy were used to quantify expansion rates, while Generalized Linear Models were applied to identify key environmental and anthropogenic correlates of species occurrence. Gastropods (P. acuta and P. antipodarum) exhibited rapid and extensive range expansion since the mid-2000s, whereas bivalves showed limited or negligible spread. Expansion patterns were partly influenced by increased sampling effort following the implementation of the EU Water Framework Directive. Species distributions were consistently associated with low-altitude, low-slope river reaches subject to high anthropogenic pressure. Physella acuta was strongly linked to degraded lowland habitats, C. fluminea to large, hydrologically stable river sections, and P. antipodarum was primarily constrained by channel slope. These results reveal a longitudinal zonation of invasion risk in Mediterranean rivers and highlight the combined role of hydromorphology and human pressure in shaping invasion dynamics. Maintaining long-term monitoring and protecting headwater and mid-mountain reaches are essential to limit further spread and safeguard native freshwater biodiversity.