Repeated Introductions and Anthropogenic Dispersal Fuel the Regional Expansion of Ambrosia trifida

Understanding the processes that shape biological invasions is essential for predicting the spread and persistence of introduced species, yet these processes remain poorly resolved for many unintentionally introduced plants. We combined genome-wide SNP data, demographic modeling, and climatic niche analyses to investigate the invasion of giant ragweed in South Korea. A total of 419 individuals from 35 introduced populations and 59 individuals from four native North American populations were analyzed. Introduced populations retained high genetic diversity and exhibited weak spatial structure, consistent with extensive human-mediated dispersal. Approximate Bayesian Computation supported a recent invasion within the last ~70 years involving multiple introductions from genetically divergent sources, followed by rapid divergence and severe but transient demographic bottlenecks. Climatic niche analyses revealed substantial overlap between native and introduced ranges but pronounced niche unfilling, indicating that Ambrosia trifida has yet to occupy its full potential climatic niche in South Korea. Together, these results show that repeated introductions, admixture, and anthropogenic dispersal, rather than major climatic niche shifts, have shaped the contemporary invasion. Our findings highlight how integrating population genomics with niche analyses can resolve invasion processes and inform management of rapidly spreading invasive species.