Comments on increasing number and abundance of non-indigenous aquatic macrophyte species in Germany.
Non-indigenous aquatic plants are a major cause of biodiversity loss in many countries. In this study, our own field data and a literature review have been used to get an overview of the history and the present distribution of non-indigenous aquatic plant species in Germany. Results show that the number of non-indigenous aquatic plant species in Germany rose from 1 in 1860 to 12 in 1980, but doubled to 24 during the following 29 years. Thirteen of these species are naturalised in at least one federal state, 11 are only ephemeral. The number of non-indigenous aquatic plant species in the German federal states is significantly correlated with the population size and area. The increase in species number and abundance is probably caused by enhanced trading and increased invasibility of waters by eutrophication/re-oligotrophication and climate change. We propose a trading ban for some highly invasive non-indigenous aquatic species. This will not stop their natural spread, but should reduce the risk of further unintended entry and thus can be a major control factor.