Lord Howe Island is a 1455-hectare crescent of volcanic land in the Tasman Sea, roughly 600 kilometres east of the Australian mainland, and a World Heritage-listed hotspot of endemic biodiversity. Black rats arrived there in June 1918, when the steamboat SS Makambo ran aground for nine days and rats escaped aboard the vessel onto an island that had never known rodents. Within two years, rats reached plague proportions. They are implicated in the extinction of five endemic bird taxa, along with the decline or disappearance of reptiles, land snails, insects, and plants found nowhere else on Earth. Predation by black rats on the island was eventually listed as a Key Threatening Process under both New South Wales and Australian federal law, making a response unavoidable for anyone hoping to preserve the island’s ecological character.

The Rodent Eradication Programme that followed was, by any measure, a logistical feat. Because Lord Howe Island is permanently settled, with around 350 residents, the campaign could not simply blanket the island from the air without careful community negotiation. Beginning in early 2019, the team deployed the rodenticide brodifacoum through a combination of ground-based bait stations and hand-broadcast applications across 300 hectares of the settlement area, alongside aerial baiting of a further 1200 hectares of forested terrain. The last rat was detected on 9 October 2019, and the programme was declared successful. Then, in April 2021, monitoring devices in the settlement area registered rat activity once again, and the central mystery of this study was born.

The two possible explanations carried very different management implications. Remnant survivors would suggest weaknesses in the eradication method itself, particularly in the complex human settlement zone, and would argue for intensified local surveillance. A new incursion, by contrast, would point instead to biosecurity failures along the transport routes connecting the island to the mainland, including regular flights from Sydney and Port Macquarie and a fortnightly barge service from the NSW coast. Telling those scenarios apart required genetics, and fortunately the researchers had a crucial resource: archived rat specimens from the Australian Museum, collected on Lord Howe Island in 2003 and 2017 as part of unrelated research, which preserved the genetic fingerprint of the pre-eradication population.

The team assembled a dataset of 130 rats: 38 collected on the island in 2021 after the eradication, 27 representing the pre-eradication island population, 60 from mainland New South Wales, and 5 from southwest Pacific islands including Norfolk Island, Papua New Guinea, and Vanuatu. They amplified a 307-base-pair region of the cytochrome B mitochondrial gene in 125 individuals, and a longer 927-base-pair fragment in a subset of 66, then genotyped 12 polymorphic microsatellite loci across the genome. Mitochondrial DNA, inherited only through the maternal line, placed each population in a global context, while microsatellites, scattered across the nuclear genome, provided the fine resolution needed to distinguish closely related populations.

The mitochondrial results were unambiguous. Every rat sampled on Lord Howe Island, both before and after the eradication, carried the exact same cytochrome B haplotype, a Lineage I sequence that has otherwise been recorded only in widely scattered locations such as Hokkaido in Japan and Texas in the United States. This haplotype has never been detected in mainland Australia, Melanesia, or the southwest Pacific. The mainland NSW rats, by contrast, mostly carried different but similar Lineage I haplotypes, and ten individuals from the Port Macquarie area carried a divergent Lineage II haplotype associated with southeast Asia. The presence of both lineages in eastern Australia points to multiple historical introductions of black rats since European settlement, but the single island haplotype stood apart from all of them.

The nuclear DNA told a consistent and even more detailed story. Microsatellite diversity was highest in the large mainland NSW population, moderate in the pre-eradication island population, and dramatically lower in the 2021 rats, whose allelic richness had fallen by roughly half relative to their pre-eradication predecessors. Five of the twelve loci were monomorphic in the post-eradication sample, and the animals were highly interrelated, with mean relatedness values indicating that most individuals were first-degree relatives. That combination of low diversity and high relatedness is the classic genetic signature of a population founded by a very small number of individuals, possibly as few as two, and held at low numbers for multiple generations. Bayesian clustering and principal coordinate analyses both grouped the 2021 rats firmly with the pre-eradication island population rather than with the mainland.

Just as importantly, the study confirmed that the island and mainland populations were clearly genetically distinct in the first place, which is what made the forensic assignment possible. The fixation index between them was 0.10, and each population carried a substantial share of unique alleles. Despite concentrated sampling at the very ports involved in island resupply, the researchers found little evidence of gene flow in either direction, a pattern consistent with the incumbent advantage enjoyed by a long-established population that repels or fails to accommodate new migrants. Intriguingly, the island’s mitochondrial haplotype does not match the Australian east coast, suggesting the 1918 founders may have arrived from somewhere else entirely, a question that only more comprehensive global sampling of black rat diversity can resolve.

The eighteen-month gap between the last official detection and the reappearance of rats remains the hardest part of the story to explain biologically. Rats are famously fecund, with short generation times and explosive reproductive potential under favourable conditions, so a remnant population persisting undetected for that long, even under intensive monitoring, implies that a handful of animals endured at extremely low density for multiple generations, likely within the shelter and resources of the settlement area. A strikingly similar lag was documented on the Italian island of Ventotene, another inhabited island where remnant rats resurfaced long after eradication was declared complete. Both cases reinforce a sobering lesson: the presence of people, buildings, and infrastructure creates refugia that poison campaigns struggle to reach, and inhabited islands demand extraordinary vigilance even after apparent success.

The good news is that Lord Howe Island responded decisively. Once the 2021 rats were detected, a rapid and comprehensive response eliminated the remnants, with the last rat detected in August 2021, and an upgraded eradication and biosecurity programme was installed using chew cards, wax tags, tracking tunnels, kill traps, detection dogs, and camera traps at high-risk reinvasion points. Five years on, the island’s native fauna and flora are showing significant signs of recovery, from rebounding birds to recovering invertebrates and plants. The broader message for conservation science is procedural as much as biological: genetic monitoring, including the collection and archiving of samples before eradication begins, should be a standard component of rodent eradication programmes. In this case, museum specimens collected years earlier for unrelated research proved decisive, transforming an unsettling reappearance into a solvable puzzle and, ultimately, a vindication of the eradication itself.

Subject of Research: Genetic assessment of whether post-eradication black rats on Lord Howe Island were remnant survivors or new invaders

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

Article References: Frankham, G. J., Helgen, K. M., & Eldridge, M. D. B. (2026). Genetic assessment of the introduced rats found on Lord Howe Island post rodent eradication, new invaders or remnant survivors?. NeoBiota, 105, 25-45. https://doi.org/10.3897/neobiota.105.163728

Image Credits: AI Generated

DOI: 10.3897/neobiota.105.163728

Keywords: Lord Howe Island, black rat, Rattus rattus, invasive species, rodent eradication, mitochondrial DNA, microsatellites, population genetics, island conservation, biosecurity, bottleneck, NeoBiota