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.
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.
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:
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)
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:
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.
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