Spread of the ship rat (Rattus r. rattusL.) III New Zealand

Abstract

It has usually been assumed that both ship rats (R. rattus) and Norway rats (R. norvegicus) reached New Zealand at the time of Cook's voyages or soon afterwards and had become generally distributed throughout the country by 1800. Although this appears true for the Norway rat, the evidence assembled here shows that this assumption is not warranted for the ship rat. It appears more probable that ship rats did not spread through the North Island until after 1860, and in the South Island after 1890. It is likely that ship rats were present in isolated localities at least a few years prior to these dates. If this timing is correct it is possible that the spread of ship rats was directly responsible for major declines in the numbers of several species of native forest birds. The spread of ship rats may also have been responsible for the almost complete disappearance from the mainland of the Polynesian rat or kiore (R. exulans). A decline of kiore followed European contact with the country but kiore persisted in considerable numbers till the middle of the nineteenth century in the North Island and until at least 1888 in the South Island. Records of kiore on the mainland subsequent to these times have been restricted to a few isolated individuals. Ships visiting New Zealand in the late eighteenth and early nineteenth centuries appear likely to have carried R. norvegicus as the most common rat species. During the second half of the nineteenth century there appears to have been an undocumented change in rats aboard ships so that R. rattus became the common species, a position which is still the case today.