A spatio‐temporal model for the invasion of the New Zealand archipelago by the Pacific ratRattus exulans

Abstract

New Zealand is at the southern limits of the human-extended geographic range of the Pacific rat Rattus exulans On the two main islands, radiocarbon dates on rat bones from natural sites show that rat populations were established more than 1000 years before permanent human settlement, presumably during transient visits by Polynesian voyagers Both main islands were colonised after these first contacts, but offshore and outlying islands were not reached by rats until after Polynesian settlement about 700 years B P Chatham Island was not colonised by Pacific rats until about 650 years B P I present a model that relates the time of first appearance of rats in the fossil record and the exploitation of native fauna to the pattern of spread of the rat through the archipelago I hypothesise that the stepwise spread of the rat through the archipelago is mirrored by the pattern of reduction and extinction of indigenous fauna vulnerable to rat predation The 1000 year delay between the arrival of rats and permanent human settlement suggests that the New Zealand biota was already stressed by an introduced predator before humans added habitat destruction and over-hunting Keywords Rattus exulans. Pacific rat, colonisation, New Zealand, radiocarbon dating, Polynesian voyaging INTRODUCTIONThe geographical range of the Pacific rat Rattus exulans has been vastly extended by human vectoring From its onginal distribution in South East Asia, the rat has spread into the north, east, and southern Pacific (Wodzicki & Taylor 1984, Roberts 1991) The present southern limit of its range is on a few of the small islets off the southwest coast of Stewart Island, New Zealand (Atkinson & Moller 1990) The source or sources of the New Zealand populations of Pacific rats are not known with certainty, but mitochondnal DNA (mtDNA) studies are providing important new data on possible source populations (Matisoo-Smith 1994) Roberts (1991) summansed the earliest radiocarbon dates for human presence m the Pacific as a proxy for the earliest dates for Rattus exulans Roberts's model is based on the assumptions that (1) rats and people travelled together, so first dates of colonisation by the two species should coincide, and (2) the island groups were settled, and Rattus exulans populations established, m order of accessibility of island groups to human colonisation, as proposed by Irwin (1989) Rats reached possible source areas at least 2000 years B P For New Zealand, Roberts's model implies first colonisation by people and rats about 1000 years B P , following Irwin et al (1990) Nonarchaeological occurrences of the rat were not presented by Roberts (1991), so the data she presented for human arrivals could not show whether rats were already present in each island group It is axiomatic that dates from an archaeological site cannot provide information on

Diet-Derived Variations in Radiocarbon and Stable Isotopes: A Case Study from Shag River Mouth, New Zealand

“…Crucially, attention must be turned to stable and radioisotope variations in the environs of the natural sites that provided the initial suite of old rat bone results (Holdaway 1996(Holdaway , 1999) upon which the model of early rat colonization has been advanced.…”

A requiem for North American overkill

“…This is because in all known cases, human colonization was associated with multiple possible impacts on the species that were lost. In New Zealand, for instance, people not only hunted moas, but they also set fires that quickly destroyed massive expanses of forest [62][63][64] and introduced competitors and predators in the form of rats and dogs [6,44,45]. Some combination of hunting, introduced species (including pathogens), and anthropogenic vegetational change caused the losses that are so well documented there.…”

Establishment and extinction of a population of South Georgian diving petrel(Pelecanoides georgicus)at Mason Bay, Stewart Island, New Zealand, during the late Holocene

“…The AMS 14 C ages on five R. exulans bones, four of which were presented in Holdaway (1999b), are listed in Table 2, together with a single determination on a black rat (Rattus rattus) bone collected at Site 10. The R. exulans samples all yielded ages of less than 460 14 C years BP.…”

“…In addition, North and South Island taxa are presently of limited use in determining any relationship between arrival time ofR. exulans and the extinction of any taxon because the time of arrival of the rat on the main islands is controversial (Anderson 1996a(Anderson , 2000Holdaway 1996Holdaway , 1999bBeavan & Sparks 1998;Beavan-Athfield et al 1999Holdaway & Beavan 1999;Hedges 2000;Higham & Petchey 2000;Lowe et al 2000;Beavan-Athfield & Sparks 2001a,b;Holdaway et al 2002c;Yaldwyn 2002).…”

“…In contrast, an arrival post-AD 1300 for R. exulans on Stewart Island is less problematical (Holdaway 1999b). Therefore, the population of P. georgicus that formerly bred at Mason Bay, Stewart Island (Worthy 1998b) was chosen to determine whether the extinction of a small petrel could be related, at least chronologically, to the arrival of the predator hypothesised to have exterminated it.…”