Alien Plants Introduced by Different Pathways Differ in Invasion Success: Unintentional Introductions as a Threat to Natural Areas

BACKGROUND: Understanding the dimensions of pathways of introduction of alien plants is important for regulating species invasions, but how particular pathways differ in terms of post-invasion success of species they deliver has never been rigorously tested. We asked whether invasion status, distribution and habitat range of 1,007 alien plant species introduced after 1500 A.D. to the Czech Republic differ among four basic pathways of introduction recognized for plants. PRINCIPAL FINDINGS: Pathways introducing alien species deliberately as commodities (direct release into the wild; escape from cultivation) result in easier naturalization and invasion than pathways of unintentional introduction (contaminant of a commodity; stowaway arriving without association with it). The proportion of naturalized and invasive species among all introductions delivered by a particular pathway decreases with a decreasing level of direct assistance from humans associated with that pathway, from release and escape to contaminant and stowaway. However, those species that are introduced via unintentional pathways and become invasive are as widely distributed as deliberately introduced species, and those introduced as contaminants invade an even wider range of seminatural habitats. CONCLUSIONS: Pathways associated with deliberate species introductions with commodities and pathways whereby species are unintentionally introduced are contrasting modes of introductions in terms of invasion success. However, various measures of the outcome of the invasion process, in terms of species' invasion success, need to be considered to accurately evaluate the role of and threat imposed by individual pathways. By employing various measures we show that invasions by unintentionally introduced plant species need to be considered by management as seriously as those introduced by horticulture, because they invade a wide range of seminatural habitats, hence representing even a greater threat to natural areas

Alien plants invade more phylogenetically clustered community types and cause even stronger clustering

Aim Several hypotheses postulate that species invasion is affected by an interplay between the phylogenetic position of the invading species and the phylogenetic structure of the invaded community type. Some of them suggest that phylogenetic relatedness of invaders to native species promotes naturalization, because phylogenetically related alien species tend to have similar environmental adaptations as native species. Others predict that phylogenetic relatedness hampers naturalization because of stronger competition of aliens with native species and shared enemies. Here we ask how phylogenetic diversity of native species affects invasion across community types. Location Czech Republic. Methods All major plant community types at a national scale (n = 88) were characterized by their species pools, i.e. lists of species that can potentially occur there. Of the total number of 2306 species, 1785 were native, 246 were archaeophytes and 275 were neophytes. For each species pool, we related the number of alien species to the phylogenetic diversity of the native species pool, calculated as mean phylogenetic distance (MPD) and mean nearest taxon distance (MNTD), including null models. Results The number of alien species was related both to the phylogenetic structure of community types and to the phylogenetic position of alien species. Frequently disturbed herbaceous community types with strong phylogenetic clustering were more invaded than others, possibly due to disturbance acting as an environmental filter. Here, alien species increased the degree of phylogenetic clustering as they tended to be from the same lineages as native species. Such trends were not detected for phylogenetically more diverse community types such as forests. Main conclusions Our findings support the hypothesis that relatedness of invaders to native species promotes invasion because of their shared adaptations to the same environments. Alien species more strongly invade community types that are phylogenetically clustered, and because they tend to be related to native species, invaded community types become even more clustered