Island invasion and reinvasion: Informing invasive species management with genetic measures of connectivity

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Abstract Invasive species are the major threat to island biodiversity world‐wide. 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. 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. 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. 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. 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.