Multiple source locations and long-distance dispersal explain the rapid spread of a recent amphibian invasion.

Rapid range expansions are characteristic for non-native invasive species when introduced outside their native range. Understanding the dynamics and mechanisms of expanding non-native invasive species is key for regional management. While population genetics and long-term occurrence records are often used in this context, each provides only partial insights, highlighting the need for a combined approach. We demonstrate this synergy using the American bullfrog (Lithobates catesbeianus) invasion in the Grote Nete river valley (Belgium) as a case study. It is commonly believed that this invasion constitutes a single metapopulation established by one primary introduction followed by downstream dispersal. However, recent evidence suggests a more complex scenario, involving introduction at multiple locations and bidirectional dispersal. To differentiate between both scenarios, we analysed nearly three decades of occurrence records and 8592 single nucleotide polymorphisms across 372 individuals from 31 localities, and determined the number of source locations, the range expansion rate, the population genetic structure, and the magnitude and direction of gene flow. We found that invasive spread originated from up to six source locations followed by bidirectional dispersal and downstream long-distance dispersal (LDD) events. Our results suggest that at least two source locations were founded by primary introductions, two from LDD events, while the remaining resulted from secondary introductions. A canal crossing the river was identified as a dispersal barrier, leading to different invasion dynamics on both sides. Our study shows how asynchronous introductions at multiple locations, dispersal barriers, and environmental heterogeneity can lead to distinct spread dynamics within a seemingly continuous and interconnected metapopulation.