Relative importance of biological and human-associated factors for alien plant invasions in Hokkaido, Japan.
Aims: The invasion success of alien plants is strongly affected by both biological and human-associated factors. Evaluation of the relative contribution of each factor is important not only for the further understanding of invasion processes but also for the better management of invasion risk, particularly in protected areas of high conservation priority. Here, we quantified the relative importance of species biological traits and association with a human activity, i.e. agriculture, in explaining the invasion success of alien plants across the entire region and in protected areas in Hokkaido, Japan. Methods: As a quantitative measure of invasion success, the distribution extent of naturalized populations across the entire prefecture and in protected areas was calculated for 63 alien species with equal residence time based on species occurrence records at a spatial resolution of 5-km mesh grid units. For each species, we identified seven biological traits (seed mass, dispersal mode, maximum plant height, capability of vegetative reproduction, flowering start time and period and life span) and two human-associated factors (introduction purpose and cultivation frequency for agricultural use). Cultivation frequency was determined based on the frequency of seed-sowing in pastures: (1) not sown, (2) accidentally sown as a seed contaminant and (3) intentionally sown for commercial cultivation. The importance of biological traits and human-associated factors in explaining the distribution extent was determined using an information-theoretic approach. Important Findings: In explaining the distribution extent across the entire prefecture, species biological traits and human-associated factors showed comparable importance; cultivation frequency exhibited the highest importance value closely followed by seed mass, maximum height and flowering period. In contrast, when focusing on protected areas, human association was more important than biological traits, as indicated by the greatest importance of cultivation frequency and much lower values for most biological traits. The results demonstrated that species biological traits and human association almost equally contributed to invasion success across the entire region, while invasions into protected areas were more attributable to human association than to biological traits. We highlight that the control of propagule pressure associated with artificial cultivation may be key to preventing further invasions into protected areas.