Invasive alien species in Switzerland - Schweizer ...

Résumé 29
expanding their populations from neighbouring countries. In conclusion, many serious
invaders were deliberately introduced, often without much incentive beyond the wish
to colour the landscape, e.g. with ornamental plants and waterfowl.
Prediction and prevention of new invaders is difficult since trade in species can be
changed by changing demands. A group not currently a problem may yet become
problematic in the near future. If, for example, the pet trade in reptiles shifts from
predominantly tropical and subtropical species to species originating in regions in the
same latitudes as Europe, such as North America and China, the situation could become
much worse.
1.5 Impacts of invasive alien species
The impacts of IAS are often considerable, in particular where ecosystem functioning
is being altered or species are being pushed to extinction, as has been shown for many
bird species. Most extinctions are likely to be caused by a combination of factors and
are not attributable to a single cause, but there is no doubt that IAS play an important
role. The environmental impacts can be divided into four major factors:
> competition,
> predation (including herbivory), and more subtle interactions such as
> hybridization and
> transmission of diseases.
All these factors alone or in concert with other factors can decrease biodiversity and
cause extinction. The most obvious examples for competition are between introduced
and native plants for nutrients and exposure to sunlight. Resource competition has also
led to the replacement of the native red squirrel (Sciurus vulgaris L.) by the introduced
American grey squirrel (S. carolinensis) in almost all of Great Britain and it is predicted
that this trend will continue on the continent. The latter forages more efficiently
for food and is stronger than the native species (Williamson, 1996). Impacts due to
predation and herbivory are very extensive on island fauna and flora, as mentioned
above. The brown tree snake eliminated most of the bird species on Guam, and feral
goats are a menace to native vegetation on islands, where they were often released as a
living food resource. A well-known example of hybridization from Europe is the ruddy
duck, which hybridizes with the native white-headed duck, as mentioned above. In
some cases IAS can harbour diseases and act as a vector for those diseases to native
species. This is the case with American introduced crayfish species to Europe, which
are almost asymptomatic carriers of the crayfish plague (Aphanomyces astaci Schikora),
but the native noble crayfish (Astacus astacus (L.)) is highly susceptible to the
disease, and thus is struggling to coexist with populations of the American crayfish
species. Introduced species can interact with natives in a variety of ways and indirect
effects can be very difficult to demonstrate. Direct and indirect effects can lead to very
complex interactions and a combination of effects can cause complex impacts.