The Chinese Midden Crab - Eriocheir sinensis




Species Directory

















While many of the alien species seem to remain insignificant additions to the native flora and fauna, the international focus is on the few invasive species which threaten ecosystems, habitats or species. For German waters 26 species are recognized which have already altered the habitat or have sustain lasting effects on native biota (a.o. Nehring 2005; Reise et al. 2005). And, there is no indication that these alien species will ever leave German waters again.

It is highly probable that in the near future new alien species will arrive our waters. The current ‘tens rule’ of thumb of biological invasions in terrestrial habitats will be applicable to European aquatic systems, too: out of 100 established alien species, at least 10 percent will become invasive. However, this statistical approach says nothing about the next species and its impacts on biodiversity, ecosystem functioning, human health and economy. Every alien species has the potential of unwanted and uncontrollable consequences and their introduction as well as their spreading should be minimized wherever possible (Klingenstein 2004; Nehring & Klingenstein 2005).


Updated: 27-09-2009




Crassostrea gigas - Photo: S. Nehring

In the German Wadden Sea the invasive Pacific oyster Crassostrea gigas is about to transform

intertidal mussel beds of the native blue mussel Mytilus edulis into oyster reefs.



Many conventions, codes of conduct and other instruments reference the subset of alien species, including genetically modified organisms. These range from legally binding treaties to non-binding technical guidance focused on particular pathways to prevent the introduction of alien species. Most instruments are specific to a sector, taxonomic group, type of environment or type of harm (for review see SCBD 2001).

At global level the only instrument that covers all aspects of invasive alien species as they relate to biodiversity, is the Convention on Biological Diversity (CBD, which was adopted in 1992 and entered into force in 1993. Its aims are the conservation of biological diversity, the sustainable usage of biological resources, and the fair and equitable sharing of benefits arising from the use of genetic resources. Article 8h of the CBD requires all Contracting Parties “as far as possible and as appropriate, to prevent the introduction of, control or eradicate those alien species which threaten ecosystems, habitats or species”. This statement was specified by the decision VI/23 "Guiding Principles on Invasive Alien Species” by the 6th Conference of the parties to the CBD in 2002. Its adoption suggests comprehensive national strategies on the basis of a hierarchical approach (prevention, early detection, measures). The CBD becomes law in Germany in 2002 by the revised Federal Nature Conservation Act (BNatSchG), which includes a permission requirement for the release of alien species (§ 41.2). The permission is to be denied if a risk for the existing flora and fauna cannot be excluded.


In marine environments and inland water systems, alien species can be hard to detect and organisms disperse rapidly. Therefore several international instruments are being prepared for preventive measures against unwanted introductions to aquatic ecosystems. An important example is the guideline of the International Maritime Organization on minimizing current risks and side effect to the environment and human health arising from the transfer of species in ships’ ballast water and sediments, which was actually adopted as a convention by IMO member States in 2004. This convention will enter into force 12 months after ratification by 30 States, representing 35 per cent of world merchant shipping tonnage.

The EU Water Framework Directive (WFD), aiming to restore good ecological quality in all inland, transitional and coastal water bodies (i.e., estuaries, Wadden Sea), could be a potentially powerful legislative measure for all kinds of environmental pressures and impacts. However, in the course of meeting discussions about environmental quality standards, it was decided by administrative experts that alien species will not be taken into account as a specific quality criterion, because aliens are not directly involved in the degradation of water quality. During the last months first comments on this decision were announced by scientists to reassess the status of alien species within the WFD. The final summary - to which everyone can still contribute - is to be awaited full of suspense.



Nature conservation and measures

The main focus of international environmental policy is to achieve, as far as possible, a natural and sustainable ecosystem in which natural processes proceed in an undisturbed way. Especially invasive alien species pose a serious threat to such nature conservation interests (EC Council 2004). Up to now the introduction, establishment and spreading of alien species in German waters is perceived only on a descriptive level in some ways, a purposeful strategy in dealing with the phenomenom in regard to the protection and conservation of the German aquatic habitats is missing.

In a first approach Nehring & Klingenstein (2005) designate potential handling options for nature conservation management in relation to aquatic alien species. Depending on the species efforts should target on one of the four categories:

a. acceptence of established non-invasive species;

b. prevention of introductions through enlightenment and regulations;

c. monitoring of occurrence, impacts and spread by monitoring programmes;


d. minimization of impacts by eradication or control.



Handling options for nature conservation management in relation to former and future

 introductions of alien species (Nehring & Klingenstein 2005).


a. Acceptance

Many alien species, which are already introduced and established, are innocuous and have no relevant ecological or economic effects. These species should be accepted as new components of our native flora and fauna.


b. Prevention

Since it is well known that eradication of an introduced species, once it has become established in the marine environment, will be very expensive, or even impossible, the prevention of introductions is the first and most cost-effective option. During the last decade, first binding and non-binding instruments were adopted for preventive measures against un-intentionally as well as for controlled intentionally introductions to aquatic ecosystems (see above). However, there are gaps, overlaps and inconsistencies in these instruments and all of them have lacked efficiency up to now. Thus, once invasive species become established within one country, they pose a threat to an entire region by natural dispersal. Further on, climate warming favours the establishment of more cosmopolitan species across wider geographic areas. For these reasons, from the perspective of abating the risks of alien species, more effective actions and instruments with an internationally focus are warranted.

An important additional field in prevention, which shouldn’t be underestimated, is the enlightenment of politicians, authorities and business people, scientists as well as citizens about alien species, their risks and the possibilities to prevent further introductions. In addition to specific presentations and discussions in the press and scientific literature, in the radio and on TV, especially webbased information platforms offer a great chance to enhance persistently the awareness of the alien problem on a national (e.g. Handbook of Invasive Alien Plant Species in Germany, and international level (e.g. Nordic-Baltic Network on Invasive Species NOBANIS


c. Monitoring

The development of effective monitoring programmes is necessary to aid the early detection and determination of the status of newly introduced alien species. This is essential for taking rapid measures of eradication and control especially in the case of new observed invasive species, because these species can spread quickly and cause unwanted negative effects. However, aquatic environments are much more difficult to monitor than terrestric habitats and measures should be well thought-out and developed.

Additionally monitoring is an important basis for the assessment of impacts and the invasiveness of alien species as well as for the efficiency of eradication and control measures.


d. Eradication or Control

Where an alien species has become invasive, eradication is an effective action to prevent its spread and to minimize impacts. The best chance for a successful eradication of most unwanted species is during the early phase of invasion, while the target populations are small and/or limited to a small area.

However, in aquatic environments we are often faced with the impact at a very late stage when the species might have been there for several generations and already spread their offspring to other areas. Thus, in most cases it is  hopeless to find an efficient method to eradicate aquatic alien species once they have become established.

Once the establishment of an alien species is accepted as irreversible, unwanted species can be controlled by reducing density and abundance to keep their impact to an acceptable level. It is to note that probably such measures make sense only for specific species, at which definite criteria exist (e.g. occurrence restricted only to terrestric or intertidal areas). Control methods should be selected taking into consideration efficiency, selectivity and the undesired effects they may cause. This should go along in accordance with Community regulations and codes.

In principle, every measure should be based on an individual case decision. In order to evaluate the success or failure of a management programme, it will be necessary to monitor changes and impacts and evaluate to what extent the targets set at the beginning of the efforts have been met. This will provide an opportunity to change and adapt the programme to new perceptions and situations.




Even against the background that continuous climate change will probably influence the biocoenosis of Northern Europe much stronger (Nehring 1999, 2003), alien invasions are irreversible and should be avoided wherever possible. Alien species pose a serious impact to native biodiversity because they have the potential to alter the natural state of an ecosystem into which they were introduced and may enhance the trend of global unification of flora and fauna. Thus, the development of alien species management plans for German ecosystems are absolutely essential. However, because of the high potential for natural dispersal in introduced species, and many human vectors existing for secondary dispersal within Europe, national management plans are to discuss and agree on an European level.





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

EC Council (2004): European Strategy on Invasive Alien Species. - Nature and environment 137: 67pp.

EC Parliament and Council (2000): Directive 2000/60/EC of the European Parliament and of the Council establishing a framework for the Community action in the field of water policy.

IMO (2004): International convention for the control and management of ships’ ballast water and sediments. - International Maritime Organization, BWM/CONF/36:1-36

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.

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

Nehring, S. (2005): International shipping - A risk for aquatic biodiversity in Germany. - In: Nentwig, W., Bacher, S., Cock, M.J.W., Dietz, H., Gigon, A. & Wittenberg, R. (Eds.), Biological Invasions - From Ecology to Control. Neobiota 6: 125-143.

Nehring, S. & Klingenstein, F. (2005): Alien species in the Wadden Sea - A challenge to act. - Wadden Sea Newsletter 31: 13-16.

Reise, K., Dankers, N. & Essink, K. (2005): Introduced species. - In: Essink, K., Dettmann, C., Farke, H., Laursen, K., Lüerßen, G., Marencic, H. & Wiersinga, W. (Eds.), Wadden Sea Quality Status Report 2004. Wadden Sea Ecosystem No. 19: 155-161.

SCBD (2001): Review of the efficiency and efficacy of existing legal instruments applicable to invasive alien species. - Secretariat of the Convention on Biological Diversity Montreal, CBD Technical Series No. 2: 31 pp.



© 2005 by Stefan Nehring / / Disclaimer Impressum