Habitats vary considerably in the level of invasion (LI), i.e. the number or
proportion of alien plant species. To determine typical LIs for different habitats,
we compiled a database of 52 480 relevés from three contrasting regions:
Catalonia, Czech Republic and Great Britain. We classified plants into neophytes,
archaeophytes and native, and calculated the proportion of each group in 33
EUNIS habitats. Only 8 of 545 aliens found in the relevés occurred in all three
regions. Despite this large difference in alien species composition, habitat LIs
were consistent among regions. Few aliens were found in nutrient-poor habitats,
e.g. mires, heaths and alpine grasslands. Many aliens were found in frequently
disturbed habitats with fluctuating nutrient availability, e.g. in man-made habitats.
Neophytes were also frequently found in coastal, littoral and riverine habitats.
Besides local habitat properties LI depends on propagule pressure of aliens. To
determine real susceptibility of habitats to invasions (invasibility), it is necessary
to factor out the effects of propagule pressure on the LI. We did it by statistical
modelling based on phytosociological data and digital maps. Using regression
trees, the proportion of aliens was related to variables representing habitat
properties, propagule pressure and climate. Propagule pressure was expressed
through urban, industrial or agricultural land cover and human population density
in the region. Urban and industrial land use positively affected the proportion of
both archaeophytes and neophytes. Agricultural land use and higher population
density positively affected the proportion of archaeophytes. After removing the
propagule pressure effect, some habitats with intermediate LI had very low
relative proportions of aliens. This indicates that these habitats, e.g., dry, wet and
saline grasslands, base-rich fens or deciduous forests are invasion-resistant.
Overall, habitats explained much more variation in the LI than propagule pressure.
Our findings that (1) habitat LIs are consistent across contrasting parts of
Europe, and (2) LI mainly depends on habitat type and less so on propagule
pressure, enabled us to extrapolate our data to wider Europe. By plotting the
quantitative information on the LIs from Catalonian, Czech and British data on the
CORINE land cover map of Europe, we produced the first European LI map
