How traffic facilitates population expansion of invasive species along roads: the case of common ragweed in Germany

1. Because common ragweed (Ambrosia artemisiifolia L., henceforth Ambrosia) has negative effects on human health, it is a common focus for management, which would benefit from a better understanding of the underlying mechanisms by which the species spreads. Road systems are known to be invasion corridors, but the conduit function of vehicles for the rapid spread of Ambrosia along roads and for population extension along roadside verges has not yet been demonstrated convincingly. 2. To quantify the effect of different traffic volumes on the dispersal and population extension of Ambrosia we used two approaches: First, by combining field experiments along roads with records of the seed rain around single plants, we simulated a combined dispersal kernel that revealed the interactions between primary dispersal and traffic-mediated secondary dispersal. Second, we recorded seedling recruitment around isolated roadside populations over two years to determine how traffic-related parameters affec...