Monitoring the occurrence of invasive plants in different types of natural habitats

Protected areas and the Natura 2000 network are keystones of the EU nature and biodiversity policy. However, alien plants do not stop their spread at the border of protected areas and invasive plants are reported to threaten many ecosystems, from aquatic and riparian areas to dry and xeric sites.
The presentation summarizes three large-scale quantitative assessments of the occurrence of exotic plants in Wallonia, i.e. the southern part of Belgium. Three types of natural habitats were the focus of the assessments: i) ponds and lakes (400 sites); ii) river banks (187 sites); and iii) xeric ecosystems such dry grasslands, rocky habitats and screes (86 sites). In the three studies, sites were selected through a stratified sampling then visited. Exotic plants were recorded and their abundance assessed. Additional information about population dynamics, environmental conditions and visible impacts was recorded.
Elodea spp. were the most common species in water bodies, with occurrence rates reaching 2.7%. Other aquatic alien species were found, but with an occurrence rate below 1%. Along rivers, 51 alien species were observed. Some were widespread (e.g. Impatiens glandulifera, with 17 % of linear banks invaded) whereas others were either rare or considered emergent alien species. Analyses showed that typical riparian species’ occurrence increased with the size of the watershed, indicating propagule pressure within protected areas through hydrochory. In xeric sites, the most common species were either cultivated or ornamental ones such as Juglans regia, Cotoneaster horizontalis, Prunus serotina, Robinia pseudoacacia and Buddleja davidii. The former was found in 15.1% of the visited sites.
The implications of the different results, notably about emergent species, are related to the need for an effective early detection system.