Invasive alien species in Switzerland - Schweizer ...

6 > Molluscs – Mollusca 117
This leads to an evaluation of the pathways by which these 19 species arrived in Switzerland.
It is possible that at least some species could have arrived by several pathways,
and for some species the pathways by which they were introduced are speculative
rather than proven. The most likely pathways for each species are listed in Table
6.2. About 74 % (14 species) were accidentally introduced, while the others were
released for unknown reasons and from aquariums (often with the good intention of
‘freeing’ surplus animals). Five of the eight aquatic species were probably transported
by boats and ships, either in ballast water or attached to hulls. The accidental terrestrial
introductions were most likely made with imported vegetables and other plant material.
The impacts of the established alien molluscs are discussed in some detail and further
references are given in the Fact Sheets and in the text above. Only five out of 19 species
can be regarded as harmless, based on present knowledge. The terrestrial slugs
(and to a certain extent snails) are mainly economic pests of agriculture and in gardens.
However, research on environmental impacts is largely lacking, with the exception of
Arion lusitanicus displacing the native A. rufus (L.). The situation is different in freshwater
ecosystems, with demonstrated dramatic impacts of the introduced bivalves on
native biodiversity and ecosystem functioning. The introduced bivalves are a novel life
form in their new range, because of their densities and intense filter-feeding activities,
which alter the correlation between benthic and pelagic communities. They may also
have some economic impact because of their colonization of pipes and other artificial
structures.
In general, alien species should be treated separately from the native fauna and should
not appear on a Red List of endangered species, when they are beyond doubt introduced.
This is especially true for the intercontinental invader, since some European
species could also expand their range into Switzerland and distinguishing between the
two categories (aliens of European and extra-continental origin) is often challenging.
However, species such as Physella heterostropha, a Nearctic invader, should not
appear on the Red List as potentially vulnerable (‘potentiell gefährdet’), as it does.
Thus, the concept of the Red List of Switzerland should be re-addressed, with alien
species being excluded.
Prevention of new mollusc invasions is dependent on identification of the pathways of
introduction. Plant health inspection has improved recently and should be vigilant for
new arrivals of slugs and snails, as well as insects and other invertebrates. The ballast
water issue is currently being addressed for sea-going ships by the International Maritime
Organization (IMO) whereby methods are evolving to treat ballast water. Some of
these measures can also be used for ships on inland waterways. Another crucial topic is
public education and awareness; when using a boat, boots or fishing gear, people
should take care not to transport potential hitchhikers. Aquarium owners and pet store
merchants need to be made aware of potential problems arising from the release of
pets.
Several slug species are major pests in crops, and management strategies to reduce
harvest losses have been implemented. Slug pellets, a bait that contains slug attractants
and a molluscicide (e.g. metaldehyde), are frequently used. Alternative methods in-