Managing shipborne species introduction requires global coordination: Evidence from robustness thresholds in maritime networks

Global maritime shipping is a major pathway for the introduction of non-indigenous species via ballast water discharge and hull fouling. Understanding the structure of global shipping networks can help identify where interventions may most effectively reduce propagule pressure. In this study, U.S. related global shipping networks were constructed from terrestrial Automatic Identification System (AIS) records and decomposed into vessel-type subnetworks. Network robustness under progressive node and edge removal scenarios were assessed using connectivity- and efficiency-based indicators, and a log-binned network disintegration model was proposed to better capture uneven traffic intensity. Management thresholds were defined at which network connectivity or efficiency declined sharply, and the results revealed marked vessel-type differences in pathway robustness. Fragmentation tipping points, identified from changes in the giant connected component, occurred at node-removal fractions of 0.39 for bulkers, 0.23 for tankers, and 0.28 for containerships, whereas passenger networks fragmented almost immediately under targeted intervention. Generally, global efficiency declined earlier than visible structural fragmentation across subnetworks, indicating that shipping-mediated introduction pathways may lose transport efficiency before pronounced topological breakdown. Although different targeting strategies identified only partially overlapping sets of priority bioregions, they produced comparable reductions in robustness, suggesting flexibility in intervention design. Notably, many priority bioregions were located outside the United States, indicating that effective risk reduction cannot rely on domestic port management alone. These findings support vessel-type-specific and internationally coordinated management, with broader intervention required for bulker networks and more targeted control likely sufficient for passenger networks.