Invasive alien species in Switzerland - Schweizer ...

An inventory of alien species and their threat to biodiversity and economy in Switzerland FOEN 2006 66
D. villosus (Soyinski). The latter species is the largerst of the invaders and has considerable
impact on the ecosystem (see Fact Sheet).
Introduced freshwater crayfish species, comprising four species in two families,
Astacidae and Cambaridae, represent one of the greatest threats to Swiss biodiversity.
Astacus leptodactylus Eschscholtz (see Fact Sheet) introduced from parts of southeastern
Europe and south-western Asia is of least concern, although it can potentially
compete with native crayfish. The other three species (Pacifastacus leniusculus (Dana),
Orconectes limosus (Rafinesque) and Procambarus clarkii Girard) (see Fact Sheets)
are of North American origin. They are not susceptible to the crayfish plague (see the
chapter on Fungi), but they are vectors of the disease, carrying it into European crayfish
(Astacus astacus (L.)) populations. The severity of the disease in these populations
raises grave concerns about the survival of the native crayfish species.
The three Isopoda species (see Table 3.1) may not pose a threat to Swiss freshwater
ecosystems, despite the enormous densities of the tiny Ponto–Caspian invader Jaera
istri Veuille.
With respect to the taxonomic composition of the established species, two groups are
dominant, i.e. the amphipods with seven species and the crayfish with four species. The
amphipods are a successful group of about 6,000 species (Pöckl, 2002). They often
dominate ecosystems in numbers of individuals as well as in biomass owing to their
high fecundity under optimal conditions. Their omnivorous behaviour renders them
adaptive to changing species compositions. The rapid invasions and build up of tremendous
densities of alien amphipods are legendary. The interactions of native and
rapidly invading alien species are very complex and difficult to understand. The fluctuations
in densities are considerable and sometimes new invaders replace earlier
invaders. Haas et al. (2002) give an overview of changes in abundance of alien species
in the Rhine. The crayfish, as the second dominant group, belongs to the Decapoda,
whose major centre of distribution is in North America and there are only a few species
in Europe. The economic importance of crayfish is, of course, the major incentive for
introducing alien crayfish into Europe.
A comparison of alien crustaceans established in Switzerland and in neighbouring
countries is not without flaws, because of gaps in knowledge. However, the 17 species
listed here compare quite well with the 19 species recorded for Austria (Essl and
Rabitsch, 2002). Geiter et al. (2002) list 26 species for Germany, but this includes
species living in marine and brackish environments.
The origin of the alien crustaceans established in Switzerland is equally divided between
North America (including three crayfish species), the Ponto–Caspian region
(five of the seven amphipods) and Mediterranean parts of Europe. The Ponto–Caspian
region is essentially the area of the Black and Caspian Seas and their adjacent rivers.
Many species endemic to the Ponto–Caspian region have become established in Europe,
the Baltic Sea and, more recently, the Great Lakes in North America. This extraordinary
spread has been facilitated by the construction of numerous canals allowing
species to disperse by active migration and, to a greater extent, with ship traffic (in