[Invasive-species] about naturalization

Roger Cammaerts rcammaer at ulb.ac.be
Mon Feb 13 19:02:57 CET 2006

Dear all: 

About the latest version (2.1) of the black and watch list of invasive non-native species.

How should we define the concept of naturalization ?

As I earlier told, to put items in categories can be somewhat arbitrary.

In the B2 category I read the crayfish Astacus leptodactylus, which appears (to me) not to be naturalized: it is located in ponds and piscicultures where it has been stocked. It may invade part of the downstream river outlet of such locations, but does not appear to be really invasive. Nevertheless, it should be monitorized. 

As for the Dikerogammarus species:
Are there naturalized ? They seem to be confined in waters of poor or moderate quality only, having invaded turbid waters, mainly the River Meuse and its connecting waters, but have not (yet ?) succeed in invading tributaries with clear water. In the River Meuse they have eliminated most if not all of the former endigenous Gammarid species. Of course, it is perhaps not the end of their story. Moreover, they are in our waters only since few: 1998 !
Thus we could put them in the A1 category if we take into account that they are hazardous for our fauna or in the B1 list if we think that they have a limited geographical impact (for the moment). Future will tell us ! 

May I draw your attention to the definition of naturalized species we used in our article on the native and exotic Amphipoda of the River Meuse (Josens, Bij de Vaate, Usseglio-Polatera, Cammaerts, Chérot, Grisez, Verboonen & Vanden Bossche: Hydrobiologia (2005) 542: 203-220): a naturalized species is a former exotic species, well established since a long period and in balance with its environment (p. 207). According to this definition Dikerogammarus is not at all naturalized.

Dr Roger Cammaerts, PhD.
Laboratoire de Systématique et d'Ecologie Animales,
Université Libre de Bruxelles,
CP 160/13
av. F.D. Roosevelt 50
B-1050 Bruxelles

Chargé de recherche au

Centre de Recherche de la Nature des Forêts et du Bois (CRNFB)
Direction des Ressources Naturelles et de l'Environnement (DGRNE),
Ministère de la Région wallonne,
av. Maréchal Juin 23
B-5030 Gembloux


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