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1. Invasive species are the major threat to island biodiversity worldwide. Genetic analyses assist in identifying invasion routes as well as revealing population connectivity, which both represent crucial information for conservation management strategies and risk evaluation. Such information is critical to safeguarding vulnerable species on nearshore islands, which often serve as wildlife sanctuaries. 2. The house mouse Mus musculus has invaded islands around the world and is a model species for showcasing how genetic tools can be used to inform biosecurity management. The genetic population structure of 316 mice from 12 locations in the upper South Island of New Zealand was determined, using novel GenePlots and traditional population genetic tools based on 10 microsatellite loci, to identify gene flow and reinvasion pathways among mainland and island populations over a decade. 3. On the mainland, populations remained genetically homogeneous over landscape scales of many tens of kilometres. In contrast, historically established island populations only kilometres offshore had low genetic diversity from prolonged isolation. Two islands were potentially colonized from both the mainland and each other in a hybrid swarm. 4. Islands that had recently been invaded or reinvaded in the past decade had genetic profiles consistent with the adjacent mainland, suggesting failure of biosecurity procedures to prevent reinvasion, rather than eradication survivors. Although two islands were invaded by only a few individuals, on a third island many invaders simultaneously arrived. 5. Synthesis and applications. Assessing the genetic structure and connectivity of mainland and island populations of an invasive species, using a combination of traditional and novel visualization tools, has uncovered a spectrum of invasion mechanisms and pathways. These results have informed ongoing biosecurity measures by revealing the locations and intensities of biosecurity threats, allowing targeted management actions to reduce the likelihood of island reinvasion.

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Seaports are introduction hotspots for invasive alien species (IAS). This is especially true for rodents, which have accompanied humans around the globe since the earliest days of ocean-going voyages. The rapid spread of IAS soon after arrival in a new environment is facilitated by further human-mediated transport or landscape features, like roads. By measuring genetic diversity and structure to investigate dispersal pathways, we gained insight into the transport, spread and establishment stages of a biological invasion, leveraging the most common rodent species (R. norvegicus) in this setting. We characterized the genetic structure of three Norway rat populations along a busy industrial road used by trucks to access the Port area in Paranaguá city (Brazil). A total of 71 rats were genotyped using 11 microsatellite markers. The results revealed a pattern of gene flow contrary to the expected stepping-stone model along the linear transect, with the two furthest apart populations being clustered together. We hypothesize that the observed outcome is explained by natural dispersal along the corridor being lower than human-mediated transport. The sampled area furthest from the port is a gas station frequented by trucks which are considered the most likely mode of transportation. In terms of management strategies, we suggest more emphasis should be put on cargo surveillance to lower the risk of Norway rat dispersal, not only for biosecurity, but also for sanitary reasons, as this port is a major grain trading point.