Norway rats in rural and urban New Zealand
The vision of a predator-free New Zealand by 2050 is usually about possums, stoats and rats. While possums and stoats are single species, there are three species of rat in New Zealand. In order of arrival – and of increasing distribution around New Zealand – they are the kiore or Polynesian rat (Rattus exulans), the Norway or brown rat (Rattus norvegicus), and the ship, black or roof rat (Rattus rattus). Kiore are believed to be extinct on the North Island mainland, but persist on some islands and in parts of Fiordland, Southland and South Westland, while Norway and ship rats are both widespread. However, while ship rats are the common rat in New Zealand forests and shrublands, and therefore are virtually ubiquitous, Norway rats are distributed much more patchily, usually living near water (including the sea), and are common in urban areas and on farms.
Telling rats apart
Kiore average 100 g and are much smaller than ship rats, and have a small area of dark fur on their outer, rear ‘ankles’.
Ship rats come in three coat colours, or morphs, which confuses people (see photograph). Most ship rats in the North Island are the frugivorus morph, whereas in the South Island they tend to be the rattus morph, but any of the three morphs may occur anywhere. Ship rats have slender tails longer than their body length (unless shortened by injury), and have big ears which when bent forward cover their eyes.
Norway rats are always brownish (hence their other common name, brown rat); their tail is shorter than their body length and stout at the base, and their ears are small compared to those of ship rats (when bent forward they do not cover their eyes).
Size, behaviour and abundance
Besides appearance, there are other important differences between Norway and ship rats, especially in their size and behaviour. Norway rats are much bigger (average 210 g, maximum 420 g) than ship rats (average 145 g, maximum 295 g), which should be considered when trapping them. Norway rats can climb trees but rarely do, unlike ship rats which are truly amazing climbers and thus threaten tree-nesting birds. However, Norway rats are better swimmers than ship rats and so are much more likely to invade nearby islands, where they are key pests of ground-nesting birds and other fauna. Being near water is a major feature in the distribution of Norway rats on the New Zealand mainland, probably because they can dive readily to find food and to escape predation. They are known predators of crabs on rocky coasts, and dive to prey on freshwater mussels in North Island lakes. They also prey on the eggs and nestlings of New Zealand dotterels and shore plover, and probably numerous other wetland, braided river and coastal birds.
From studies so far, the proportion of trapped rats that are Norways varies from 2% in Waikato forest fragments to 100% in Waitaki Basin braided rivers. Outside forests, especially in urban and rural sites, there always seem to be a few Norway rats present. In urban and farmland sites, current data indicate that Norway rats comprise 5–50% of all trapped rats, mainly depending on whether the traps are placed near water. Norway rats are therefore typically very patchy across the landscape, and probably have long, thin distributions along waterways.
It is no surprise that Norway rats are found throughout New Zealand towns, cities and farms, because they (and not the ship rat) are the usual species in these habitats around the world. Contrary to some recent assertions, there is no city in the world that has actually been made rat-free, and so this remains a challenging milestone for New Zealanders taking on the vision of making New Zealand predator-free.
Neil Fitzgerald, John Innes and a student placement, Nicolas Sandoval (Waikato Institute of Technology), radio-tracked Norway and ship rats in September/October 2016 in a vegetated gully in urban Hamilton to see how far apart control devices could be placed and still be effective. In this study, Norway rats had a mean home range length of 269 m (see Figure) and moved further than ship rats, which had a mean home range length of 196 m (not shown). Both species stayed largely confined to the gullies and did not venture into nearby backyards.
This research also suggested that Norway rats are more likely than ship rats to be seen in the daytime, and some literature suggests that Norway rats are more likely to travel on defined trails than ship rats.
John and his colleagues’ results indicate that there are likely to be benefits from controlling only the more widespread rat species, rather than just lumping them all as ‘rats’.
Kiore are rare in all mainland habitats occupied by them and are not targeted in any control operations, other than on islands such as Aotea (Great Barrier Island). The two common rat species – ship and Norway – coexist all over New Zealand but have different habits and impacts. For example, to protect tree-nesting birds or freshwater mussels you would target ship rats or Norway rats respectively. Choosing what outcomes to measure to determine operation success should also be determined by the rat species targeted. Ship rats will be the only species killed if devices are set up trees, but both species will be killed by devices set on the ground. Traps that may catch Norway rats should perhaps be bigger and stronger, because Norway rats are much larger than ship rats. Humaneness approvals of control devices should be sought separately for the two species, again because Norway rats are much bigger than ship rats.
The researchers do not yet know if behavioural differences between ship and Norway rats mean that one will be harder than the other to eradicate in urban or rural settings.
This research was supported by core funding from New Zealand’s Ministry of Business, Innovation and Employment, and by Hamilton City and Waikato Regional Councils.