Invasive alien species in Switzerland - Schweizer ...

Résumé 27
Feral mammals introduced to islands brought many bird species to the brink of extinction
or beyond by feeding on their eggs and chicks (e.g. Long, 2003).
However, some taxonomic groups seem to include more invasive species than others
do. Mammals are a major threat to island faunas and floras. Whereas rats, mongooses,
mustelids and feral cats devastate the local bird and reptile fauna of islands, feral goats
(Capra hircus L.) can diminish the native flora drastically. Weeds alter the vegetation
on many archipelagos to the detriment of the entire ecosystem. Island ecosystems,
including isolated lakes, are particularly vulnerable to these invaders. However, all
continents and habitats seem to be vulnerable to invasions, although there appear to be
some differences between continents. In the highly populated area of Central Europe,
IAS seem to be of less importance to biodiversity than on other continents with large
tracts of more natural habitats. The smaller reserves in Central Europe are easier to
manage and control of alien species in these areas is often more practical. The long
association between introduced species and the human population in Europe is a very
different situation compared to other continents, as all habitats are highly altered and
human-made habitats dominate. These human-made habitats are often regarded as
valuable heritage in Central Europe and some of them are dominated by alien species
introduced centuries ago.
Comparing the numbers of species introduced to species that have established and
species that have become invasive, one has to bear in mind the long lag phases which
often occur. Most introduced species take some time before they become invasive, i.e.
enter the log phase. Kowarik (2003) shows that for woody alien plants in one part of
Germany, the average time lag between first introduction and expansion is about 147
years. The occurrence of time lags makes predictions on invasiveness of alien species
very difficult. A species doing no harm today can still become an invasive of tomorrow,
especially in combination with other global changes. There are three major factors
that determine the ability of a species to become invasive:
1. Intrinsic factors or species traits, such as the ability to adapt to different conditions, a
wide tolerance of abiotic factors, pre-adaptations to different climatic zones, and a
high reproductive rate.
2. Extrinsic factors or relationships between the species and abiotic and biotic factors,
such as the number of natural enemies, the number of competing species (native and
alien), other interactions with native and alien species (pollination, dispersal, food
source, ecosystem engineers), climatic conditions, soil conditions, degree of disturbance
(natural and human-induced), global climate change, change in land-use patterns,
and control and eradication of other IAS.
3. Human dimension. The attractiveness and importance of species to humans influence
introduction pathways, vectors, the number of specimens introduced, the number
of introductions, and the potential for eradication or control.
Predictions about the invasive potential of a species prior to introduction, as now made
in Australia and New Zealand, remain difficult, despite recent progress in science. The
best indicator is still whether a species has become invasive in a similar area elsewhere
(i.e. its invasion history).