Norway rats and house mice are two of the most widespread invasive species worldwide. But where did the Norway rats and house mice in New Zealand come from? Our geneticist Lara Shepherd and colleagues from Manaaki Whenuawhenua land Māori | Noun | Listen Landcare Research, University of Waikato, and Place Management NSW have shed some light on this question by sequencing DNA from rodent bones from a 19th-century archaeological site in Sydney.
Norway rats and house mice have been in New Zealand since the 18th century and 19th century, respectively, and they have been causing trouble for our native species ever since.
New Zealand rodents have been well studied genetically but to understand their origins we need more samples from overseas for comparison.
From across the ditch
One of the most important sites for understanding the origins of New Zealand rodents is Sydney, which was the primary port from which European ships sailed onto New Zealand. It has long been assumed that the rats and mice that reached New Zealand were stowaways on these ships from Sydney.
In the 1990s, an archaeological excavation at The Rocks, an early settlement site at the Sydney wharves, found a number of rodent bones dating to between 1788 and 1900. These have provided an opportunity to compare the DNA variants in these old bones to those found in New Zealand.
Our DNA sequencing showed that the most common Norway rat DNA variant in the Sydney bones was identical to the most common variant found in New Zealand’s North Island and offshore islands. This variant is also the most frequent variant in Britain.
However, a different DNA variant has been found in Norway rats from the South Island of New Zealand and this variant was not found in our sampling of Sydney Norway rats. This DNA variant has only otherwise been found in China.
The house mouse results showed a similar pattern. In New Zealand, there are genetic variants shared with Sydney and British Norway rats. But there are also genetic variants in New Zealand that can be traced to eastern Asia and these were not detected in our sampling from Sydney.
Our results are consistent with at least two independent introductions of both species. One introduction was from Britain, likely via Sydney. The other was from eastern Asia, probably through undocumented trade between China and New Zealand prior to 1820.
Veale AJ, King CM, Johnson W, Shepherd L (2022) The introduction and diversity of commensal rodents in 19 century Australasia. Biological Invasions.