Alien species as a potential threat for Natura 2000 habitats: a national survey.

Invasion by alien species (AS) is one of the most serious threats to ecosystems. In Europe, the Natura 2000 habitats network was established to protect habitats vital for the conservation of biodiversity and function of ecosystems. Therefore, the appearance of AS in Natura 2000 habitats is a warning signal that the most valuable European habitats may be endangered. However, quantitative studies encompassing a wide spectrum of habitats are lacking, and there is no insight into the differences in the level of invasion among habitats. Our survey is based on the State Monitoring of Natura 2000 data and aimed at an assessment of the level of invasion in natural habitat types in Poland. The percentage of invaded locations, number and frequency of alien plant species was assessed in 79 Natura 2000 habitats, both terrestrial and water, investigated on 5,941 locations. The most invaded habitats (with the highest percentage of invaded plots) were dunes with (habitat code 2160), rivers with muddy banks (habitat code 3270), and alpine rivers and herbaceous and ligneous vegetation along their banks (habitat codes: 3220, 3230, 3240). Grassland, forest and most of the bog, mire and fen habitats and also some habitats on a rock were invaded by a relatively large number of AS, but their frequency was comparatively low. In contrast, a high frequency of AS was found in the majority of dune and costal habitats and calaminarian grasslands. Compared with the period 2000-2010, the number of AS in some riparian, grassland and forest habitats rose noticeably. The occurrence of AS showed a negative correlation with conservation status of the habitats. This study has demonstrated that standard monitoring of Natura 2000 habitats provides the basis for the detection of AS, including invasive ones, in all types of habitats, and can be used for development rapid and effective response programs.