Assessment of rodenticide resistance, eradication units, and pathogen prevalence in black rat populations from a Mediterranean biodiversity hotspot (Pontine Archipelago)

Biological invasions are a growing threat to biodiversity. The black rat, one of the worst pest in the world, is responsible for extensive population decline of many autochthonous and endemic species, particularly in island ecosystems. A number of rat eradication campaigns have been conducted, however, such endeavors do not always result in a complete removal of the pest. This may be due to the occurrence of individuals resistant to common rodenticides and/or a re-invasion of the same environment from interconnected areas when appropriate eradication units are not defined before starting an eradication campaign. Our study is a multidisciplinary approach whereby genetic and epidemiological methods were used to provide background information for successful eradication of black rats. We investigated the occurrence of mutations in the VKORC1 gene known to confer resistance to rodenticides and evaluated the spread of zoonoses across three islands of the Pontine Archipelago, an Italian hotspot of endemic Mediterranean biodiversity and a possible mainland source of invasion. As part of an eradication campaign, we also assessed patterns of genetic diversity at 10 microsatellite loci in order to identify eradication units. We recorded a strong population structure and revealed at least two distinct eradication units. Some degree of admixture was recorded on Ponza, the largest island and likely the main source of rats invading the other two islands. We did not record the occurrence of rats resistant to anticoagulants, but we revealed transmission of vector-borne pathogens in commensal habitats of the Archipelago.