Invasions by Alien Species in Inland Freshwater Bodies in Western Europe: The Rhine Delta

Abstract

Invasions by alien species in freshwater habitats are discussed for the Dutch Rhine delta, as a model of a large Western European river strongly influenced by anthropogenic activities. The barriers to dispersal were lifted in this area by introductions and escapes from aquariums, aquaculture etc. (65% of the alien species), as well as by canals connecting rivers (25%) and invasions from the seaports caused by seagoing ships (10%). Habitat change and deterioration of water quality has led to changes in and weakening of indigenous flora and fauna. Periods of pollution and chemical spills followed by partial restoration of the water quality have led to increasingly successful invasions suppressing the population development of returned indigenous species. This fits in well with the hypothesis that anthropogenic disturbance paves the way for new invasions, as disturbance leads to unused resources from which invaders can profit.