The Chinese Midden Crab - Eriocheir sinensis




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The global scale of alien species is becoming more and more evident. As many examples prove, aquatic invasions are irreversible and alien species may be associated with unforeseeable ecological as well as economical risks (e.g. Carlton 1985; Bartley & Minchin 1996; Reise et al. 1998, 2002). Even against the background that continuous climate change will probably influence the biocoenosis of European aquatic systems much stronger (Nehring 1998, 2003), the introduction of alien species enhances the trend of global unification of flora and fauna associated with an irretrievable loss in biodiversity. Today the spread of alien species is now recognised as one of the greatest threats to the ecological and economical well being of the planet.

However, until today scientific interest in alien species is often only descriptive (e.g., documentation of invasion history, studies on distribution pattern, and abundance assessment; Nehring 2000, 2002; Reise et al. 1998, 2002; Tittizer et al. 2000). The functional role of these species in aquatic ecosystems and their realized niches in the invaded communities remains to be quantified.


Updated: 27-09-2009

The invasive cord-grass Spartina anglica (at the back) displaces

i.a. the native glass-wort Salicornia stricta (in the foreground):

Wadden Sea of Schleswig-Holstein, Germany, August 2004.



Ecological impacts

In spite of the insufficient scientific analyses, approximately 20% of the established aquatic alien species in Germany have clearly negative effects on the natural balance, which means that they endanger biological diversity on one or more levels (ecosystems, habitats, species or genes) (Nehring & Klingenstein 2007). In contrast to definitions used in science (e.g. Kowarik 2003), but in accordance with nature conservation definitions (e.g. CBD 1992, 2000), these species are seen as invasive species and therefore demand appropriate measures.

‘Strict’ definitions which clearly relate to ecological damage are of special importance for nature protection, as they help to separate damage relevant to nature from economic, health or other damage, which may be seen as a sphere of activity for other relevant stakeholders (Klingenstein 2004). It should also be clarified that the measures applied in these areas, for example antibiotic, pesticide, hormone, or drug applications in aquaculture to protect cultivated fishes from invasive parasites and pathogens, cannot be considered conservation measures.

A simple classification, modified after Jansson (1994), can be used to document different ecological impacts of alien species on German waters in general, viz:

  • Disruption of existing interactions between species or food web links (e.g. predators, prey, grazers, and competition) (e.g. Crassostrea gigas, Dikerogammarus villosus, Dreissena polymorpha, Rana catesbeiana, Spartina anglica);

  • Hybridisation with native and other alien species, resulting in changes of biological and genetic diversity (candidates in German waters: Acipenser spp., Crassostrea gigas, Spartina anglica);

  • Introduction of parasites and disease agents. The introduced species may function as a host for pathogens or parasites which affect indigenous species (e.g. Anguillicola crassus, Orconectes limosus);

  • Habitat modification (e.g. Chelicorophium curvispinum, Crassostrea gigas, Sargassum muticum, Spartina anglica).


These alterations may be widespread or regional in particularly valuable habitats, which are usually protected habitats. Such “ecological costs” are usually difficult or impossible to quantify (Reinhardt et al. 2003).



Schematic representation of the zonation of saltmarshes in the European Wadden Sea.

(a) Native saltmarsh without the invasive cord-grass Spartina anglica.

(b) Native saltmarsh invaded by the invasive cord-grass Spartina anglica.

(modified after Nehring & Hesse 2008)



Economic impacts

Most intentional introductions into aquatic environments aim to achieve some positive economic or socio-economic effects, often by improving angling opportunities or water quality, etc. In some cases the desired positive effect is realised whereas in others, the introduction has serious negative economic effects, often associated with negative consequences for the environmental or biodiversity (Weidema 2000). The documentation of economic impacts from introduced species in German waters is still insufficient to determine the precise extent. Numerous economic sectors may be negatively affected by aquatic alien species, viz:

  • Damage to waterways, watercourses and hydraulic structures (e.g. Dreissena polymorpha, Teredo navalis);

  • Impact on species used in fisheries and aquaculture, resulting in a decrease of outputs (e.g. Anguillicola crassus, Eriocheir sinensis, Crassostrea gigas);

  • Impact on resource users may result in harmful consequences for human health and well-being, recreation, and socio-economics (e.g. Crassostrea gigas, Elodea canadensis, Spartina anglica).



There is no indication that not one established alien species will ever leave Germany again. So the net effect of alien invasions is generally a regional increase in species richness. And, it is highly probable that in the near future new alien species will arrive in our waters. However, the ecological consequences which arise for the biocoenoses as well as the scale on which the biodiversity is modified is not analyzed, understood or evaluated in detail yet. As species introductions are irreversible the extension of their distribution area and increase in abundance is an ongoing process. Therefore, further changes in the ecosystems of German waters and a growing biotic similarity with other regions can be expected.

Mainly by larval and postlarval drifting as well as by transport on ships’ hulls many alien species rapidly extended their distribution, often more than 100 km per year. On basis of this rate of spread studies on local effects often require knowledge of population dynamics on a scale of several hundred kilometers to differentiate the local phenomena from general trends.

It looks very much that the unique character of German aquatic habitats would still be manifest in the physical environment but not any more in their living components if the problem of alien invasions cannot be solved.

To protect the ecological integrity of our waters, a purposeful management strategy need to be supported.





Bartley, D.M. & Minchin, D. (1996): Precautionary approach to the introduced and transfer of aquatic species. – F.A.O. Fisheries Technical Paper 350/2: 159-188.

CBD (1992): The Convention on Biological Diversity. - UN Conference on Environment and Development, Rio de Janeiro,

Carlton, J.T. (1985): Transoceanic and interoceanic dispersal of coastal marine organisms: the biology of ballast water. - Oceanography and Marine Biology, Annual Review 23: 313-317.

Klingenstein, F. (2004): Invasive gebietsfremde Arten aus Sicht des Naturschutzes auf Bundesebene. - In: Bundesamt für Naturschutz (Ed.), Neophyten - Ergebnisse eines Erfahrungsaustausches zur Vernetzung von Bund, Ländern und Kreisen. BfN-Skripten 108: 21-30.

Kowarik, I. (2003): Biologische Invasionen - Neophyten und Neozoen in Mitteleuropa. – Ulmer Stuttgart: 380 pp.

Jansson, K. (1994): Alien species in the marine environment: Introductions to the Baltic Sea and the Swedish west coast. – Swedish Environmental Protection Agency Report No. 4357: 68 pp.

Nehring, S. (1999): Biocoenotic signals in the pelagial of the Wadden Sea: The possible biological effects of climate change. - Senckenbergiana maritima 29, Suppl.: 101-106.

Nehring, S. (2000): Neozoen im Makrozoobenthos der deutschen Ostseeküste. - Lauterbornia 39: 117-126.

Nehring, S. (2002): Biological Invasions into German waters: an evaluation of the importance of different human-mediated vectors for nonindigenous macrozoobenthic species. – In: Leppäkoski, E., Olenin, S. & Gollasch, S. (eds.), Alien species in European waters. Kluwer, Dordrecht: 373-383.

Nehring, S. (2003): Alien species in the North Sea - Invasion success and climate warming. - Ocean Challenge 13(3): 12-16.

Nehring, S. & Hesse, K.-J. (2008): Invasive alien plants in marine protected areas: the Spartina anglica affair in the European Wadden Sea. – Biological Invasions 10.

Nehring, S. & Klingenstein, F. (2007): Aquatic alien species in Germany - Listing system and options for action. – Neobiota 7: 19-33.

Reise, K., Gollasch, S. & Wolff, W.J.(1998): Introduced marine species of the North Sea coasts. – Helgoländer Meeresunters. 52, 219-234.

Reise, K., Gollasch, S. & Wolff, W.J. (2002) Introduced marine species of the North Sea coasts. – In: Leppäkoski, E., Gollasch, S. & Olenin, S. (eds.), Invasive aquatic species of Europe - distribution, impacts and management. Kluwer, Dordrecht: 260-266.

Tittizer, T., Schöll, F., Banning, M., Haybach, A. & Schleuter, M. (2000): Aquatische Neozoen im Makrozoobenthos der Binnenwasserstraßen Deutschlands. – Lauterbornia 39: 1-72.






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