Primary and secondary invasion pathways: why the distinction matters

The pathways by which non-native species are introduced and spread shape the rate and geographic patterns of biological invasions. They can be classified as primary, where species cross jurisdictional or biogeographic boundaries, or secondary, where they move within those boundaries after introduction. Despite economic, political, social, and ecological differences between these pathway types, most classification schemes and regulatory frameworks do not explicitly distinguish between them. Primary pathways are relatively well-defined and subject to biosecurity regulation, while secondary pathways remain poorly characterized and largely overlooked in policy and practice. Secondary pathways create multiple nodes from which species spread, complicating containment. Here we refine the distinction between primary and secondary pathways and explore how failing to recognize this distinction can limit the effectiveness of biosecurity systems, particularly by leaving secondary pathways inadequately addressed. Distinguishing between primary and secondary pathways can improve control responses, strengthen cross-scale coordination, reduce economic damage, and support global biodiversity targets.