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The number of biological introductions has increased since the 1970's and is now considered as the second major cause of the biodiversity erosion, after fragmentation or disappearance of habitat. Beyond the threat they represent for the ecosystem equilibrium, introduced species are interesting models to study fundamental issues in ecology and evolution like the processes of dispersal and adaptation to novel environments. In this context, species introduced over a large geographic range and spectrum of habitats provide an excellent opportunity for comparing the mechanisms that promote introduction and settlement between different environments. In this paper, based on a case study, the worldwide introduction of the brown alga Undaria pinnatifida, and on the use of molecular tools, we aim at examining several processes promoting or occurring during biological introductions. Our results showed that i) multiple processes can account for the success of the pandemic introduction of this alga, highlighting the necessity to study introduced species in relation with the ecosystem they invaded, ii) the recurrence of introductions is a critical component in the dynamics of settlement and iii) human activities can play a major role not only during the primary introduction but also for the sustainable settlement of introduced species in natural environments by providing reservoir of migrants. Taken together, these results demonstrate that the complexity of mechanisms occurring in biological invasion require spatial but also long-term analysis.