Functional and taxonomic perspectives for understanding the underlying mechanisms of native and alien plant distributions

Land-use changes often facilitate alien species invasion, impacting on biodiversity and ecosystem processes at the regional scale. Transport infrastructure is a potential corridor for biological invasion, but processes underlying invasion of alien species from human settlements along transport networks are uncertain. We surveyed alien and native plant species and their functional traits in roadside vegetation in Shiretoko National Park, Japan. The site is unique because there is a single invasion source (i.e., town). Vegetation, environmental and spatial factors were measured in 362 quadrats on two transects established along roads. Mean species richness within each quadrat (taxonomic alpha-diversity) was higher for alien assemblages, whereas mean functional richness (functional alpha-diversity) was higher for native assemblages. Alien species shared similar traits as generalists, which show lower functional diversity than specialists. Functional alpha-diversity of alien assemblages decreased with increasing distance from the source town, and the nestedness-resultant component of functional beta-diversity of alien assemblages accounted for a relatively high proportion of their total functional beta-diversity. Alien species with high dispersal ability were widely distributed, whereas other alien species were still limited to areas adjacent to the human settlement. Alien species showed a greater probability of having seed floss, lower percentage of perennial species, and lower seed dry mass than native species. Thus, alien species showed superior dispersal and competitive abilities compared with native species, and exclusion of all alien species is likely impractical. Consideration of functional traits may aid selection of alien species of particular concern for implementation of control and extermination measures.