Differential processes underlying the roadside distributions of native and alien plant assemblages

Abstract

Although biological invasion often alters ecosystem properties and community composition at different scales, considerable uncertainty still exists regarding the underlying mechanisms that regulate the spread of alien species into new habitats. An alien invasion is generally achieved through multiple processes from multiple sources; this type of invasion often prevents us from understanding of the dispersal mechanisms. Here, we aim to disentangle the processes of alien invasion by focusing on a single migration source. We surveyed the distribution of alien and native plant species in Shiretoko National Park, located in northern Japan. We measured the coverage of each species and the environmental and spatial factors in 362 quadrats established along roadsides. We found 101 native species and 35 alien species (γ-diversity) throughout the quadrats. The local species richness (α-diversity) was higher for the alien species (6.1 species) than for the native species (3.2 species). There was a significant negative correlation in α-diversity between native and alien species. Moreover, the α-diversity and distance from the nearest town (migration source) showed a negative relationship for alien assemblages while the native assemblages showed the opposite trend. These results suggest that the alien species are expanding their distribution outward from the town, resulting in a decrease in the α-diversity of native species in localities close to the migration source. Overall, our results emphasize that roadsides could unintentionally act as corridors for alien species, even in protected areas. Careful consideration is thus needed for utilizing these human-created habitats even though they were designed for conservation and management purposes.