Monitoring Biological Invasions in Freshwater Habitats

Biological invasions of freshwater habitats are due to the accidental or deliberate introduction of organisms associated to aquaculture, sports fishing, passive transport by vessels, ornamental uses and man-made canals and corridors. In European freshwater habitats, about 296 species of alien invertebrate, mostly crustaceans, and 136 species of alien fish have been introduced (Gherardi et al. 2009). Inland surface waters have also been invaded by 444 species of alien plant (Lambdon et al. 2008). Fish have attracted most attention and the majority of these have originated from Asia, North America or within Europe (e.g. from southern to northern countries). Many aquatic alien species are widely distributed in Europe: the brook trout Salvelinus fontinalis in 26 countries; the crayfish Pacifastacus leniusculus in 25; the zebra mussel Dreissena polymorpha in 22. Among plants, the Canadian pondweed Elodea canadensis is the most widespread, being present in 36 countries (http://www.europe-aliens.org/). Alien species invading freshwater systems are causing loss and degradation on all levels of biological organization from genes to populations and with cascading effects to entire ecosystems. One of the best-known European examples of genetic loss is the hybridisation between the native rare white-headed duck Oxyura leucocephala and the American ruddy-duck O. jamaicensis, the hybrids of which are very aggressive (Muñoz-Fuentes et al. 2007). Hybridization between native and alien species is occurring in other taxa such as plants (Spartina), crayfish (Orconectes), snails (Melanoides) and fish (Salmo). Invasions can also cause synergic