| Summary: | Molara is a small island belonging to the Marine protected Area Tavolara—Punta Coda Cavallo, in Sardinia. During 2006–2007, a bio-monitoring program reported a strong presence of the black rat, Rattus rattus, on Molara island. Rat predation has detrimentally affected the unique biodiversity of this island, thus, in 2008 an eradication campaign was conducted. Our eradication protocol included a pre-eradication genetic investigation, using 8 microsatellite loci, on a rat population of Molara as well as on neighbour islands within the Marine Protected Area (MPA). The main goal of this genetic investigation was to establish the correct borders of the eradication unit of Molara island. As several recent eradication campaigns have been unsuccessful, due to incomplete and unstable eradication, we also aimed to assess possible hidden sources of reinvasion. Specimens were also collected during post- eradication monitoring on Molara for genetic screening to establish their origin, and thus validate the effectiveness of our eradication campaign. According to our genetic analysis, within the MPA there are four different eradication units, corresponding to the islands of Molara, Tavolara, Piana and to the Sardinia mainland. Gene flow among these four units is more or less absent. The assignment and clustering tests performed on pre and post-eradication samples seem to indicate that the population of Sardinia mainland is a possible source of re-invasion for the Piana and Molara populations. We are grateful to Massimo Putzu for help with rat sampling. We also thank Jenny Booth for the accurate linguistic revision of the manuscript. We thank two anonymous reviewers for their helpful comments. This research was partially supported by Fondi d’Ateneo to M. Vannini (ex 60 % University of Florence). published
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