The river Rhine: a global highway for dispersal of aquatic invasive species

The river Rhine is heavily influenced by human activities and suffers from a series of environmental constraints which hamper a complete recovery of biodiversity. These constraints comprise intensive navigation and habitat modification by hydraulic engineering. Improving water quality while these constraints remain in place has led to increased colonization by aquatic invasive species. This tendency has been accelerated by the construction of canals connecting river basins. Over the last two centuries, the total surface area of river catchments connected to the river Rhine via inland waterways has been increased by a factor 21.6. Six principal invasion corridors for aquatic species to the river Rhine are discerned. The extensive network of inland waterways has allowed macroinvertebrate species from different bio-geographical regions to mix, changing communities, affecting the food webs and forming new constraints on the recovery of the native biodiversity. From the eighteenth century onward, in the freshwater sections of the river Rhine, a total of 45 non-indigenous macroinvertebrate species have been recorded. The average number of invasions per decade shows a sharp increase from < 1 to 13 species. Currently, the contribution of non-indigenous species to the total species richness of macroinvertebrates in the river Rhine is 11.3%. The Delta Rhine and Upper Rhine exhibit higher numbers of non-indigenous species than other river sections, because the sea ports in the Delta Rhine and the Main-Danube canal function as invasion gateways. Important donor areas are the Ponto-Caspian area and North America (44.4 and 26.7% of the non-indigenous macroinvertebrate species, respectively). Transport via shipping and dispersal via man made waterways are the most important dispersal vectors. Intentional and unintentional introductions are highest for the period 1950-1992. The cumulative number of non-indigenous species in time is significantly correlated with the increase in total surface area of other river catchments connected to the river Rhine by means of networks of canals. The species richness of non-indigenous macroinvertebrates is strongly dominated by crustaceans and molluscs. Invasive species often tolerate higher salt content, temperature, organic pollution and current flow than native species. Spatiotemporal analyses of distribution patterns reveal that average and maximum dispersal rates of six invasive species vary between 44-112 and 137-461 km year(-1), respectively. Species arriving in upstream sections first show a shorter time lag between colonisation of the Delta and Upper Rhine than species initially arriving in downstream areas. Temporal analyses of macroinvertebrate assemblages in the littoral zones indicate that native species are displaced by non-indigenous species. However, established non-indigenous species are also displaced by more recent mass invaders.