Invasive alien species in Switzerland - Schweizer ...

2 > Vertebrates – Vertebrata 45
The mute swan (Cygnus olor (Gmelin)) (see Fact Sheet) was released on park ponds in
the 17 th century and has subsequently spread to all suitable habitats. It has a stable
population, so only local problems occur, and it is also a popular species with the
human population, so that control should be limited to public education concerning the
impact of feeding waterfowl (Schmid et al., 1998). Stopping the feeding of the swans
will eliminate the high densities of mute swans. The mute swans would then distribute
themselves more evenly, due to their territoriality based on intraspecific aggression.
A second member of the Anatidae, the greylag goose (Anser anser (L.)), the wild form
of the domestic goose, is not native in Switzerland, but has a growing breeding population
in several places, apparently originating from illegally released specimens (Kestenholz
and Heer, 2001), although the presence of wild birds that expanded their range
cannot be ruled out. Greylag geese are native in an area extending from north-western
to south-eastern Europe. They were probably released to enrich the local avifauna in
Switzerland. It can be assumed that the populations will grow, without hunting pressure.
No environmental impact is expected, since the Swiss population is not far from
the southern border of the natural distribution, but there might be damage to crops and
droppings on lawns close to lakes could mean the swans are considered a nuisance.
The ruddy shelduck (Tadorna ferruginea (Pallas)) (see Fact Sheet) is a Central Asian
and northern African species, which is a favourite asset of waterfowl collections
throughout Europe for its striking plumage of an almost orange-brown body and a
whitish head. In many cases the birds are not kept in cages and regularly escape. Some
isolated breeding pairs are recorded in many countries. However, Switzerland has the
only viable population of this species in Europe, and it is in the process of building up
numbers. Its spread to neighbouring countries should be prevented, since if it were to
cause subsequent damage and environmental impact, Switzerland would have some
responsibility. Therefore, it is recommended that this unnecessary population should be
eliminated while it is of manageable size.
The mandarin duck (Aix galericulata (L.)) (see Fact Sheet) is the only alien duck
species to have established in large numbers in Europe. As it is one of the most ornate
waterfowl species, it was and is frequently held on park ponds in Europe, from where it
escapes into the wild. Its origin is the eastern part of Asia, where populations have been
dramatically reduced by habitat changes (mainly logging) and over-hunting. Consequently,
the European populations might be of some importance for the species, especially
if there is no demonstrated impact. Thus, a strategy for this species in Europe
might recommend accepting it.
The native range of the common pheasant (Phasianus colchicus (L.)) covers a large
part of Asia with about 40 subspecies described. It has been kept in pheasant exhibitions
for at least a thousand years in Europe (Geiter et al., 2002). However, it is believed
that it established in the wild much later, perhaps only in the 18 th century, but its
history cannot be established with certainty, since the numerous releases obscure the
sustainability of wild populations. It is a favourite game bird all over Europe and
millions are released every year. Thus naturalized populations are supported by frequent
releases and are harvested by hunting, e.g. each year up to 20 million are re-