Invasive alien species in Switzerland - Schweizer ...

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An inventory of alien species and their threat to biodiversity and economy in Switzerland FOEN 2006 136 and geophytic species that have become invasive compared to those that have become naturalized. Only few short-lived species have become invasive. In total, 85 % of the invasive flora in Switzerland consists of perennial species of which more than two-thirds are trees and geophytic plants. The proportion of alien species that are trees is three times the proportion of native species that are trees (Table 10.4), but the proportion of invasive species that are trees is about ten times the figure for native species. For comparison, the proportion of short-lived plants that are alien is about twice the proportion for native species, but the proportions of short-lived species that are invasive and native are similar. In contrast, the proportion of herbaceous perennials that are native is almost three times higher than the figure for alien species and nearly ten times higher than for invasive species (Table 10.4). In summary, the life form composition of the alien flora is different from the native one and it changes during the process of naturalization and invasion. While the highest number of introduced species are annual plants, they do not naturalize well. The relatively small number of introduced trees and geophytic plants are successful in the naturalization process and are aggressive invaders. The introduction, naturalization and invasion of alien plant species results not only in a change in the floristic composition, but also in plant life form changes with probable consequences on habitat structure and ecosystem functioning. 10.7 The habitats of alien plants in Switzerland The ecological plant groups used here have been taken from Lauber and Wagner (1998) to define plant habitats. Nine ecological groups have been defined: > F = forest plant > M = mountain plant > P = (lowland) pioneer plant > E = aquatic plant > H = marsh plant > S = dry grassland plant > G = grassland plant > R = ruderal plant Tab. 10.5 > The ecological groups of alien plants in Switzerland, excluding cultivated plants: number of species (%). Forest Mountain Pioneer Aquatic Marsh Dry grassland Grassland Ruderal Total Alien 34 (11.3) 4 (1.3) 19 (6.3) 12 (4.1) 26 (8.6) 14 (4.7) 4 (1.3) 189 (62.6) 302 Naturalized 23 (23.5) 3 (3.1) 11 (11.2) 8 (8.2) 11 (11.2) 4 (4.1) 0 38 (38.8) 98 Invasive 3 (15.0) 0 7 (35.0) 2 (10.0) 3 (15.5) 0 0 5 (25.0) 20 Native 443 (17.9) 644 (26.0) 126 (5.1) 96 (3.9) 308 (12.4) 347 (14.0) 74 (3.0) 438 (17.7) 2476
10 > Plants – Planta 137 The majority of alien species in Switzerland are ruderal plants (62.6 %), followed by forest plants (11.3 %) and marsh plants (8.6 %) (Table 10.5). However, ruderal plants represent only 38.8 % of all naturalized species. Forest species increase to 23.5 % of all naturalized species, followed by pioneer and marsh species (11.2 %), and aquatic species (8.2 %). There are very few mountain and grassland species naturalized in Switzerland. Thus, it appears that forest and wetland habitats are more suited to plant naturalization since 42.9 % of all naturalized species belong to these ecological groups (F, E, H). It should be noted, however, that three out of four mountain species have become naturalized. Only 20.2 % of ruderal species have become naturalized. Ruderal and pioneer species represent 60 % of all invasive species, indicating perhaps less resistance of these habitats to invasion. Wetlands and forests seem to be more resistant to invasion. However, 40 % of invasive species in Switzerland belong to either the forest, aquatic or marsh ecological plant groups (F, E, H) and thus are a potential threat to the most valuable ecosystems of Switzerland, i.e. forests and wetland habitats. Mountain and grassland habitats seem to be at less risk based on invasions to date. About 50 % of all naturalized and invasive species are ruderal and lowland pioneer species. These species are often not restricted to waste ground or urban areas, but are known to invade (sometimes after a long period) semi-natural habitats such as meadows, riverbanks, gravel shores, forest margins or clearings. Several of the worst invasive plants in Switzerland such as Reynoutria japonica, Heracleum manteggazianum, Impatiens glandulifera Royle, Buddleja davidii Franchet and Solidago canadensis L. are treated as ruderal or pioneer plants but are known to invade less disturbed places as well. Solidago spp. in particular have been recognized for over a decade as a threat to protected areas (Voser-Huber, 1992). Impatiens parviflora DC. was introduced from Asia into Central Europe in 1837 and was for many years a typical ruderal plant occurring only in towns, gardens, parks or cemeteries. A few decades ago, however, it started to penetrate into woods, at first only in badly degraded areas, but then becoming firmly established in relatively natural stands of deciduous forests in Central Europe (Kornas, 1990). Weber (1999a) gives the number of alien plant species recorded in various habitats (s.str.) in Switzerland: 16 % are known from forests and related habitats, 15 % from lakes and wetland areas, and 8 % from grassland habitats. Interestingly, 16 % of alien plants are found in rocky sites and on walls. As compared to the 61 % of ruderal species, some 35 % of alien plants grow only in ruderal sites. The number of alien plants recorded in various habitats is relatively similar to the numbers that can be extrapolated from the ecological groups. The data suggest that many ruderal alien plants are not restricted to ruderal sites but grow mainly in open habitats or less disturbed sites such as rocky areas, walls, or forests subjected to some disturbance. As shown above, the great majority of naturalized and invasive species in Switzerland are perennial species and most of them are forest, wetland and ruderal species. With very few exceptions (e.g. Impatiens glandulifera), naturalized and invasive short-lived species are uncommon in semi-natural or natural habitats.
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