The ecological parasitology of the Polynesian rat (Rattus exulans) on Tiritiri Matangi Island

This thesis presents a descriptive and a functional analysis of the ecology of an island host-parasite system consisting of the Polynesian rat, Rattus exulans (Peale) and its gut helminths. The results, which include an historical perspective, are presented in the form of 7 papers or sections. Each of these examines a particular aspect of this host-parasite relationship. A review of the origin and an update of the theorised dispersal of this rat from Southeast Asia to New Zealand is given in the first section of chapter one. Previous theories have derived the New Zealand populations from a line which passed through Micronesia. In accordance with new information from the Lapita cultural assemblage, this rat is now theorised to have accompanied these "Lapita" peoples through the Bismarck Archipelago and Solomon Islands, arriving in the Tonga- Samoa region about 3600-3000 Before Present (B.P.). From here, the Proto- Polynesians then dispersed further east, taking with them the commensal R. exulans, pig, dog, and chicken. This rat is thus thought to have arrived in New Zealand, the most southern and last-settled landmass in Polynesia, in the canoes of the Maori about 1000 years ago. Information on the ecto- and endoparasites of the Polynesian rat from throughout its geographical range is collated and presented in section two. This includes the results of the two surveys (one being part of this thesis) done on the parasites of this rat in New Zealand. All populations of R. exulans sampled in these two surveys came from offshore islands, to which this rat is almost totally confined, and where, on many, it is the only rodent species present. In contrast, most of the populations sampled beyond New Zealand are now sympatric with other rodent species. For the New Zealand populations only, it was also possible to identify those parasites only accidentally associated with this host; these are listed as "transients". In section three, an attempt is made to determine the probable biogeographical origins of parasites recorded from populations of this rat on "exulans only" offshore islands of New Zealand. Such a study was possible only because of the archaeologically documented commensal relationship between rat and Polynesian man. This information, detailed in section one, together with the parasitological data base assembled in section two, provided the material for this analysis. Several "heirloom" species are identified, theorised to have been inherited by this rat during speciation somewhere in Island Southeast Asia. Parasites acquired during dispersal are divided into "old" and "new souvenirs"; the former are thought to have been acquired from sympatric rodent species in Near Oceania sometime prior to 3000 B.P., and the latter from R. rattus, R. norvegicus or Mus musculus introduced in the last 200 years during European settlement in New Zealand. The conclusions further suggest that some at least of the "new souvenir" species have been acquired by R. exulans on "exulans only" offshore islands of New Zealand by cross-transfer from other rodent species which have temporarily gained access to these islands. This theory is examined in more detail in the fourth section, and reports of such accidental colonizations of offshore islands are presented as supporting evidence. In Chapter two, the influence of habitat on the population demography of the host is investigated. Nearly 1000 rats were trapped and necropsied over a 17 month period in three different habitats on Tiritiri Matangi, an "exulans only" island at the entrance to the Auckland harbour. Rank grassland which covers most of this island formed one habitat; a second consisted of forest remnants confined to gullies, and the third consisted of the small, inhabitated, lighthouse station and farmed area at one end of the island. Between-habitat differences were observed in diet, adult longevity, mean weight of immatures, the time of onset of sexual maturation, and annual reproductive output. These results suggested several modifications to existing models of this host's demography in New Zealand. Shelter in particular appears to play an important role in the demography of this species in temperate latitudes. The effects of parasitism on potential fitness parameters e.g. reproduction, and adult mortality/survival, are examined in chapter three. Based on the results obtained in chapter two, a number of hypotheses were developed, and the predictions arising from these were tested. Few significant results were obtained; these revealed habitat and some sexual interactions with the level of infection, at certain times of the year. However, no causal relationship could be established between these effects and host reproduction or mortality. It is concluded that the helminth parasites of this rat on this island have little or no effect on these host parameters, and support the suggestion that these species constitute a depauperate and well-adapted rodent parasite fauna. The last chapter presents the results of an analysis of the effects of habitat, season, host age, and sex on the distribution and abundance of the helminths of this rat on this island. Together, the graphs and the statistical analyses demonstrate that habitat has the most important influence, significantly affecting all 7 species; this effect is of greater magnitude than the other 3 variables on 5/7 of these species. Season and age also have important effects, while sex had no apparent influence. Explanations for the observed patterns are sought in known aspects of the biology of the host in the three habitats described in chapter two, and in the life cycles of the parasites. In total, this thesis provides a comprehensive account of the ecology of the Polynesian rat and its helminth parasites on Tiritiri Matangi Island. It also identifies gaps in the existing data base, formulates certain hypotheses, and makes a number of predictions all of which will hopefully stimulate further interest in this rat and its parasites.