Despite ongoing research on plant invasions, there is still a
lack of studies on intercontinental habitat patterns of
invasion and their underlying mechanisms. Invasions in habitats
have been so far described mainly at the regional or finer
scales, and studies differed in input data and methods used,
thus generalizations for larger scales based on previous
findings are limited. To compare rigorously the invasion
outcome at the intercontinental scale, analyses based on
extensive comparable data sets are needed. Previous
quantitative assessments at regional scales revealed that
habitats differ noticeably in their levels of invasion (i.e.
number or proportion of aliens). Although there was substantial
variation in invasion levels among habitats within regions,
analogous habitats in different regions showed similar extents
of invasion indicating habitat-dependent factors that can
operate even across large geographical distances. Habitat level
of invasion is generally supposed to be determined by habitat
invasibility and propagule pressure. However, another factor
can be also involved: the regional alien species pool as a set
of alien species having necessary traits to enter given habitat
and survive there. Traits of alien species are influenced by
adaptations to original (donor) habitats in their native range
and thus can give an advantage when species invade recipient
habitats with similar conditions. If habitats differ in their
donor role (i.e. amount of native species donated as aliens to
a region), regional species pools consisting of ecologically
compatible aliens can also differ and observed levels of
invasion of recipient habitats should depend on those
differences. The main aims of my Ph.D. thesis thus were (i) to
investigate intercontinental patterns of habitat levels of
invasion, with special emphasis on the differences in the
invasion outcome and alien exchange rates between habitats of
the New World and the Old World, (ii) to identify main donor
habitats of alien species for intercontinental invasions and
compare habitat affinities of aliens in the native and invaded
ranges, and finally, (iii) to test the relationship between
donor role of habitats and habitat levels of invasion to assess
the influence of regional alien species pools. Our analyses
revealed consistent intercontinental pattern of habitat levels
of invasion in analogous habitats of eastern North America and
central Europe. The most and the least invaded habitats were
the same between continents. On average, however, the mean
level of invasion was higher across habitats in North America
indicating higher invasion of the New World’s habitats compared
to the Old World. European habitats donated more alien species
to North America than vice versa, suggesting biased alien
exchange rates between the Old and the New World. When European
habitats were compared as donors of alien plant species
invading North America and the whole World, the most and the
least important donor habitats were the same for both invaded
ranges. Distinct similarities in habitat affinities of European
species were found in their native and invaded ranges. The
relationship between the donor role and levels of invasion was
found for donor and recipient habitats of alien species
exchanging between eastern North America and central Europe.
The more species a donor habitat on one continent provided to
the other continent, the higher level of invasion of analogous
habitat was found, indicating the direct species-pool effect.
The more species a donor habitat provided to the other
continent, the more alien species received the same habitat
itself from other continents, implying the reciprocal
species-pool effect.