Invasive alien species in Switzerland - Schweizer ...

10 > Plants – Planta 139
10.9 Discussion
About 362 alien species are established in Switzerland and almost one third of these
have become naturalized with certainty. With the exception of aquatic plants in the
Hydrocharitaceae, no one plant family appears to be particularly successful in naturalization.
This review stresses the importance of establishing the biological and ecological
characteristics of the naturalized flora for determining the potential invasiveness of
alien species in Switzerland. The life form composition of the alien flora is different
from that of the native one and changes during the process of naturalization from
dominance by short-lived to perennial species. The flora of naturalized plants consists
of almost 80 % perennial species. There is a further shift during the process of invasion
towards large woody perennial and geophytic plants. The invasion by herbaceous
perennials and short-lived plants has been negligible so far. Invasive species come
from geographically distant areas and no European or Mediterranean species is considered
to be invasive or potentially invasive in Switzerland.
More than 40 % of all naturalized and invasive species consist of either forest or wetland
species, thus it appears that these habitats are at a higher risk of invasion. In
Europe, over 50 % of the naturalized flora occurs in river border communities (Sykora,
1990). This is because rivers are an effective means of transportation for many species,
natural riverbank communities have been largely destroyed by human activities, and
riverbanks are regularly disturbed by water movement. Alluvial zones and mires are
therefore of primary concern with regard to invasive species. The typical vegetation of
lowland alluvial zones such as riverine floodplains consists of a mosaic of pioneer
communities, shrubs and alluvial forests. Invasion by pioneer or ruderal alien species is
likely to occur and expand in such sites due to human activities or colonization from
upstream or adjacent fields. Fenlands also are at risk because they are often used for
agricultural purposes. In contrast, raised bogs are at a lower risk since only highly
specialized plant species can flourish in such habitats, and there is less permanent
natural disturbance.
To date, the invasibility of mountain and grassland habitats has been low and few alien
species have become naturalized in these habitats. Because meadows and grasslands
owe their existence almost exclusively to human management, the threat from alien
species will increase with land-use changes and reduced grassland maintenance. In the
absence of any management, most meadows and grasslands below the timberline
degenerate to scrub and revert to woodland. Alien shrubs and trees may profit from
such a situation.
The restoration of biological diversity in intensively used agricultural land has been
supported for several years through various agri-environmental schemes. In many areas
this involves the conversion of intensively managed arable land to extensive pastures
or so-called areas of ecological compensation. The transition period from intensively to
extensively managed land or semi-natural habitats is highly favourable to alien species.