The
number of alien species in the Wadden Sea continues to increase. An
important new addition to the range of alien species since QSR 2004
is the American comb jelly Mnemiopsis leidyi. It is suspected
to be able to cause major changes in marine ecosystems. The Pacific
oyster Crassostrea gigas has continued its invasion of the
Wadden Sea.
A major component
of global change in the biosphere is the introduction of species
across natural barriers. If this process continues at present rate,
the result will be a complete mix of biota from all climatically
similar biogeographical provinces around the world. The young,
relatively species-poor Wadden Sea will have to accommodate more and
more species, particularly from shores with higher biodiversity such
as the Pacific regions. Once introduced marine species have become
established, there is no way to eliminate or to control their
populations without harming other components of the ecosystem. The
only way to stem the advancing tide of non-native immigrants is to
prevent further introductions. Does the current status provide
sufficient evidence that swift action is required? To answer this
question it is necessary to evaluate the effects of introduced
species on native biota and human affairs.
At the North Sea
coast, introduced algae and invertebrates arrived via shipping or
via aquaculture. They most often became established within estuaries
and on hard substrates, with more than 80 known species of which
about 52 also occur within the Wadden Sea (see Table 1). While many
seem to remain insignificant additions to the native biota, the
focus of this status report is on the few species that have the
potential to attain high abundance, to alter the habitat, and to
displace residents. In addition, some introduced phytoplankton
species occasionally form conspicuous blooms in the coastal waters.
Such species include the toxic flagellates Gymnodinium mikimotoi
(syn. Gyrodinium aureolum) and Fibrocapsa japonica, and
the non-toxic diatoms Odontella sinensis, Thalassiosira
punctigera and Coscinodiscus wailesii.
An overview of
selected introduced species in the Wadden Sea is given the QSR 2004.
Table 1: Number
of introduced species which became established in the Wadden Sea,
and their origin and mode of transport. Where two alternatives are
assumed to be equally alike, a species is counted twice, if unknown
no entry is made (source: QSR 2004)
Invasion of the
Pacific Oyster
Since 2001, the Pacific oyster has
achieved a continuous distribution throughout the entire Wadden
Sea. Main sites of introduction were near the island of Texel
(The Netherlands) and near the island of Sylt (Germany) where an
oyster culture was started in 1986. The spreading of the Pacific
oyster is also a global phenomenon: aquacultures on all
continents have caused the establishment of wild populations on
almost all coasts.
Figure 1: Invasion of the Pacific Oyster in the Wadden Sea (from: QSR 2004)
Figure 2: Pacific oyster and blue
mussels in the Wadden Sea (Photo: H. Marencic)
