Biogeographical origin effects on exotic plants colonization in the insular flora of Japan

Understanding the mechanisms of biological invasion is fundamental for biodiversity conservation in the Anthropocene. This study focused on a large-scale colonization pattern of exotic seed plants, which include 1094 species characterized by different geographical origins, into the insular flora of Japan. We investigated a nation-wide pattern of species richness and phylogenetic structure (clustering/over-dispersion) of exotic and its recipient native species (4664 species). We tested the invasion hypotheses associated with environmental filtering, biological resistance of recipient assemblages, human disturbance and biogeographical origins of exotics. The exotics originated from the same (Palearctic and Indo-Malay) and adjacent (Nearctic and Oceanic) biogeographical regions were widely distributed across the country under temperate climate condition, whereas tropical exotic plants from remote regions (Afrotropic, Australasian, and Neotropical) colonized mainly the south-western parts of Japan. Exotic species richness and phylogenetic structure, especially those from the same/adjacent regions, were well explained by climatic, edaphic, and topographic factors, supporting the environmental filtering hypothesis. For all the biogeographical origins, exotic richness was positively associated with native richness, opposing the biological resistance hypothesis. Human disturbance was positively associated with exotic richness, while its relationships with the exotics’ phylogenetic structure varied according to their biogeographical origins. These findings indicate that site’s invasibility was determined by the combination of exotic’s biogeographical origins and abiotic/biotic conditions of its recipient native flora. Our results suggest that global warming may accelerate the northward expansion of tropical exotic plants while future land-use changes can promote biological invasion regardless of exotics’ origins.