Multiple human-mediated introductions shape the disjunct distribution of an invasive weed Amaranthus palmeri in China1

1. Trade-mediated propagules serve as direct genetic proxies, overcoming the limitation of unsampled native sources in invasion pathway reconstruction. 2. Multiple human-mediated introductions, not secondary spread from the initial introduction, dominate the genetically diverse and disjunct distribution of Amaranthus palmeri in China. 3. Soybean import volume predicts local genetic richness, highlighting the role of trade-driven propagule pressure in shaping invasion hotspots. A central challenge in invasion biology is to determine whether disjunct distributions of invasive species stem from secondary spread from an initial introduction bridgehead or from recurrent, human-mediated introductions. The devastating alien weed Amaranthus palmeri , with its large-scale disjunct distribution across China, provides an ideal system to address this question. We tested the competing hypotheses of bridgehead-mediated expansion (originating from the initial introduction in Beijing, 1985) versus multiple independent introductions. By integrating genetic analyses with stable isotope geolocation, we treated propagules from imported soybean shipments as direct, traceable links to potential source populations. Newly field-collected populations in China harbored significantly higher nucleotide diversity (π=(0.78 ± 0.18) × 10 -3 ) and haplotype diversity (Hd = 0.47 ± 0.04) than both the initial introduced population and the pooled propagules from the primary source, the United States (US). Significant genetic differentiation (F ST > 0.20) was observed both among newly field-established populations and between them and the initial introduction. Non-significant neutrality tests, coupled with multimodal mismatch distributions (Raggedness index = 0.0946, P > 0.05), indicated that these populations did not undergo a recent demographic expansion or selection. Genetic diversity and structure correlated with regional soybean import volume ( r = 0.59, P < 0.05) but not with environmental distance (Mantel r = 0.24, P > 0.05). Our findings demonstrate that recurrent transcontinental introductions, mediated by global grain trade, are the dominant force shaping the genetic pattern and invasion process. This study provides a framework for reconstructing invasion pathways and highlights the need for proactive, source-targeted biosecurity strategies to manage invasions in the Anthropocene.