Genetic assessment of the introduced rats found on Lord Howe Island post rodent eradication, new invaders or remnant survivors?

The black rat ( Rattus rattus ) is one of the most prevalent invasive species globally, having significant impacts across ecosystems as a competitor, predator, and driver of disease for native biota. These impacts have been especially severe on islands. Rat eradication programmes as a conservation management intervention have been undertaken on many islands around the world, including on Australia’s Lord Howe Island (LHI), located in the southwest Pacific, 600 km east of the Australian mainland. In 1918, the black rat was accidentally introduced to LHI, now recognised as a World Heritage-listed biodiverse subtropical island. The impact on LHI’s unique biodiversity was catastrophic, with many endemic taxa becoming extinct or severely declining. A Rodent Eradication Programme (REP) for LHI was launched in early 2019, and by October 2019 detections of rodents had ceased. However, in April 2021, black rats were again detected in the settlement area of LHI. To determine if the rats detected on LHI post-REP were new invaders or remnant individuals, mitochondrial DNA and microsatellite analysis were carried out. Only a single identical CytB haplotype was detected in the pre- and post-REPLHI rats. The black rats sampled from New South Wales (NSW) on the east coast of the Australian mainland and southwest Pacific islands mostly had similar but consistently different CytB haplotypes to LHI rats, or highly divergent haplotypes. Microsatellite data demonstrated that the NSW mainland and LHI pre-REP rat populations were genetically differentiated, with little evidence of gene flow between them in either direction despite potential dispersal routes. The LHI post-REP rats had low genetic diversity, were highly interrelated, and appear to be descended from a small number of individuals. Genetic clustering analyses grouped the LHI post-REP rats with the LHI pre-REP population rather than the NSW mainland population. Thus, both the microsatellite and mtDNA results are consistent with the LHI post-REP rats being the descendants of a small number of the LHI pre-REP population and not new invaders from the NSW mainland population.