Fine-scale genetic structure of mainland invasive Rattus rattus populations: implications for restoration of forested conservation areas in New Zealand

The ship rat or black rat (Rattus rattus) is one of the most widespread invasive rodent species on earth, and is a known cause of extinction of several endemic species in invaded ecosystems. While some information is available for insular populations, very little is known about the genetic population structure of this species on mainland areas. In this study (the first to characterize genetic population structure of invasive R. rattus on the mainland), we focused on the population structure of rats located in Puketi Forest Conservation Reserve, Northland, New Zealand, to help conservation managers optimize control programs. We used eight microsatellite markers and classical population genetics tools (F-st, clustering methods) as well as individual-based descriptive methods using GPS coordinates for each sample (Genetic Landscape Shape, Bandwidth Mapping) in order to determine whether there was any undetected genetic structure over the 5-km(2) area. Very little genetic structure was detected. Nevertheless, a weak but significant isolation-by-distance pattern was inferred. No isolation with external sites (encompassing an area up to 20 km(2)) was found, suggesting the presence of a contiguous population at an even larger scale, presumably by exchanging genes mainly between neighbours. We discuss the implications of these findings in terms of management of ship rats to protect native biodiversity.