Lord Howe Island declared its rats eradicated, then scientists found suspicious rodents; DNA is revealing whether they are survivors or new invaders |

Lord Howe Island was considered free of rats after its rodent eradication programme ended in 2019. The last rat was detected on 9 October that year. However, black rats were found again in the island’s settlement area in April 2021.Scientists needed to determine whether these rodents had survived the eradication programme or arrived later. According to research published in NeoBiota titled ‘Genetic assessment of the introduced rats found on Lord Howe Island post rodent eradication, new invaders or remnant survivors?‘, researchers used genetic evidence to investigate their origin. The study compared rats found after eradication with rats collected before the programme and populations from mainland New South Wales.

What did DNA analysis reveal about the returning rats

Researchers examined whether black rats found on Lord Howe Island in 2021 came from the island’s pre-eradication population or arrived from mainland New South Wales.The study analysed 130 black rats: 38 collected on Lord Howe Island in 2021, 27 representing pre-eradication island populations, 60 from mainland New South Wales and five from southwest Pacific islands. Not every rat was used in every analysis. Out of the 130 rats, 123 passed microsatellite quality checks, and because the 2021 rats were so closely related, the tests of their genetic affinity used only six adults.Scientists used mitochondrial DNA and microsatellite markers to compare the animals. These methods allowed researchers to examine maternal lineages and wider genetic relationships between populations. The analysis showed that the 2021 rats had genetic affinities with the earlier Lord Howe Island population. They were not genetically linked to the sampled mainland New South Wales rats.

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Black rats reappeared on Lord Howe Island after eradication

The Lord Howe Island Rodent Eradication Programme began in early 2019 after years of planning and community consultation. It targeted black rats and introduced house mice across the island. The eradication effort used brodifacoum delivered through ground-based and aerial methods.The programme was considered successful after no further rodents were detected on the island. The last rat was detected on 9 October 2019. In April 2021, however, black rats were found again in the settlement area. Their appearance raised two possibilities. The animals could have survived the eradication programme and remained undetected, or new rats could have reached the island afterwards. Researchers investigated these possibilities by comparing the genetic profiles of rats detected after eradication with those of rats collected before the programme and rats from mainland New South Wales.

Genetic differences separated island rats from mainland rats

The mitochondrial DNA analysis found that all sampled Lord Howe Island rats, both before and after eradication, shared the same CytB haplotype. This haplotype differed from those found in mainland New South Wales rats. Microsatellite data also showed genetic differentiation between the Lord Howe Island pre-eradication population and the mainland New South Wales population. Genetic clustering placed the island rats collected before and after eradication together, while mainland New South Wales rats formed a separate group.The post-eradication rats also showed low genetic diversity and were highly interrelated. Their genetic profiles were largely a subset of those found in the pre-eradication island population. These findings provided genetic evidence connecting the rats detected after eradication with the earlier Lord Howe Island population.

DNA evidence connected the rats to the island’s earlier population

The researchers noted that the black rats detected on Lord Howe Island after the eradication programme were consistent with being descendants of a small number of rats from the pre-eradication island population.Both mitochondrial DNA and microsatellite analyses supported this conclusion. The results showed that the sampled mainland New South Wales rat populations were not the source of the post-eradication rats. The researchers noted that the 18-month gap between the last rat detection in October 2019 and the return of rats in April 2021 remains difficult to reconcile with rat biology. Following the 2021 detections, a rapid response was launched to remove remaining rats. The study was funded by the Lord Howe Island Board, which also supplied the 2021 specimens.