R ATT US RATTUS FROM RAKITU (ARID) ISLAND

by R. A. Hitchmough School of Biological Sciences, Victoria University of Wellington, PO Box 600, Wellington

SUMMARY

140 cage-trap nights and 9 snap-trap nights on Rakitu (Arid) Island in late December 1979-early January 1980 yielded 7 Rattus rattus, all from forested habitats. No other rodents were caught. A l l 7 rats were adults, of the cream-bellied agouti morph. Most females were pregnant and/or lactating, and had 2-14 uterine scars plus embryos, suggesting synchronous seasonal breeding.

INTRODUCTION

Rakitu (also known as Arid) Island lies to the east of Great Barrier Island at 36°06 'S 175°30'E. It is about 350 ha in area, and has a central valley which is farmed, with forested hills to the east and west. The island's physical features and history were described by Hayward (1982), and its vegetation by Cameron and Wright (1982).

Hutton and Kirk (1868) provided the only previously published reference to rats on Rakitu, stating "we were informed by the natives of Great Barrier that they [grey-faced petrels] used to be very numerous, but that latterly they have become scarce, having been killed off by rats."

Ewen Cameron (pers. comm.) visited the island in June 1980, and saw rats which his description indicated were probably Rattus rattus Linnaeus. The Offshore Islands Research Group visited Rakitu from 30 December 1980 until 8 January 1981, to survey the geology, flora, fauna, and archaeology of the island. I laid traps to confirm the presence of this species, and determine whether any other rat species were present.

METHODS

Twenty cage traps and three snap-traps were set each night, baited with pumpkin, cooked goose, or cheese. They were set in three habitats: open bush with the understorey heavily browsed by stock; a water catchment reserve, close to the first area and with a similar canopy, but with stock excluded, allowing vigorous regeneration of the bush understorey; and around farm buildings and a nearby pond (Table 1). The rats caught were weighed, and head plus body, and tail were measured. Their reproductive status was assessed from external

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features, and the presence of any fleas or mange noted. They were preserved in alcohol and later dissected, and their internal reproductive condition noted.

Table 1: Distribution of trapping effort on Rakitu Island.

Trap--nights Rats Habitat Cages Snaps Bait caught

Open bush 35 8 pumpkin 0 5 0 goose 0

20 0 cheese 2 R. rattus Catchment reserve 40 0 cheese 5 R. rattus Buildings and pond 40 0 cheese 0

R E S U L T S

Seven R. rattus were trapped, five in the fenced catchment reserve, and two in the more open, browsed bush area nearby. No other rodents were trapped or seen.

A l l seven R. rattus (five females and two males) were adults of the cream-bellied agouti morph. The males weighed 215 and 220 grams, and had head-body lengths of 200 and 180 mm and tail lengths of 220 and 230 mm respectively. The females all weighed about 195 grams, and had head-body lengths of 175-190 mm (mean 182 mm) and tail lengths of 220-230 mm (mean 225 mm). Two of the females were pregnant, one with 7 embryos plus 3 uterine scars visible, the other with 8 embryos plus 6 uterine scars. The other three females, which were not pregnant, had 2, 7, and 13 uterine scars visible. Four of the five females appeared to be lactating. Both males had scrotal testes, their right testes measuring 24 and 15 mm. Both males had visible fleas and bald patches presumably caused by mites; one female had fleas and another had bald patches.

The specimens have been deposited with the DSIR Ecology Division and Auckland Institute and Museum collections.

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DISCUSSION

R. rattus is probably the only rodent on Rakitu Island. Trapping around the farm buildings and pond, where R. norvegicus might be expected, was completely unsuccessful, and there was no sign of R. exulans. The island's owners said their cats sometimes caught mice, but I did not see or catch any mice, nor was there any interference with the traps suggestive of mouse activity. It is possible that the island's owners were mistaking small juvenile R. rattus for mice, or that mice periodically arrive on the island in stores but have not become established.

The numbers of uterine scars and embryos of the five females trapped are all consistent with their having borne one or two litters. Though the sample is small, the even size and similar reproductive status of the animals suggests seasonal breeding and annual population turnover, as in R. exulans on Tiritiri Matangi Island (Moller 1977) and Motukawanui Island in the Cavalli Group (Hitchmough 1980).

Since there now appear to be no other rats on Rakitu, R. rattus is probably the species that Hutton and Kirk (1868) indicated had recently colonised the island and reduced petrel numbers in the early 1860s. R. rattus reached New Zealand between 1858 and 1865 (Atkinson 1973); it must therefore have been introduced to Rakitu at the very beginning of its invasion of the New Zealand region.

R. rattus invasions can have dramatic effects on island avifaunas, e.g., on Big South Cape (Atkinson and Bell 1973). A similar invasion probably caused the early extinctions on Rakitu, such as the loss of the petrel colonies (Bellingham et al. 1982). However some extinctions cannot be blamed entirely on R. rattus, because some bird species now absent coexisted with rats for 100 years (Bell and Braithwaite 1964). Lizards are uncommon on the island, although five species are present (McCallum and Hitchmough 1982). Rats (perhaps with wekas) are probably responsible for this rarity; there is strong circumstantial evidence that rats have greatly reduced lizard numbers on many New Zealand offshore islands (Whitaker 1973).

At present rats are the only introduced wild mammals on Rakitu, although farm stock, dogs and cats are kept by the island's owners. I recommend that any cats taken to the island be desexed; with rats for food, it would be very easy for a feral cat population to become established, and thereby greatly compromise the island's conservation values.

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Other papers presenting results of the OIRG visit to Rakitu have urged that stock be excluded from the remaining bush areas on the island (Bellingham et al. 1982; Cameron and Wright 1982). This would benefit the native flora and fauna, but the greater success of trapping in the catchment reserve than in neighbouring browsed bush indicates that rat numbers would also increase. Eradication of rats from islands the size of Rakitu is now feasible, though expensive (Taylor & Thomas 1989); this would greatly increase the conservation value of the island, and make it suitable for the reintroduction of threatened native species. However the presence of cats on the island would be incompatible with these uses.

ACKNOWLEDGEMENTS

I am grateful to Ewen Cameron for originally drawing my attention to the presence of rats on the island; to John McCallum for helping to organise the loan of traps; to Ecology Division, DSIR, and the Department of Zoology, University of Auckland loaning traps; to the Rope family (owners of Rakitu) for their cooperation and interest; to members of the OIRG party for help and companionship during the trip; and to Graeme Elliott and Charles Daugherty for comments on drafts of this manuscript.

REFERENCES

Atkinson, I. A. E. 1985: The spread of commensal species of Rattus to oceanic islands and their effects on island avifaunas. In: Moors, P. J. (ed.) Conservation of island birds. ICBP Technical publication No. 3: 35-81.

Atkinson, I. A. E. & Bell, B. D. 1973: Offshore and outlying islands. Pp 372-392 in: Williams, G. R. (ed.) The natural history of New Zealand. A. H. & A. W. Reed, Wellington.

Bell, B. D. & Braithewaite, D. H. 1964: The birds of Great Barrier and Arid Islands. Notornis 10: 363-382.

Bellingham, P. J. , Hay, J. R., Hitchmough, R. A. & McCallum, J. 1982: Birds of Rakitu (Arid) Island. Tane 28: 141-147.

Cameron, E. K. & Wright, A. E. 1982: The vegetation and flora of Rakitu (Arid) Island, northern New Zealand. Tane 28: 85-124.

Hayward, Bruce W. 1982: Offshore Islands Research Group trip to Rakitu (Arid) Island, north-east New Zealand, new year 1980-81. Tane 28: 79-84.

Hitchmough, R. A. 1980: Kiore (Rattus exulans) on Motukawanui Island, Cavalli Group, northern New Zealand. Tane 26: 161-168.

Hutton, F. W. & Kirk, T. 1868: Description of Arid Island, Hauraki Gulf. Transactions and proceedings of the New Zealand Institute 1: 108-112.

McCallum, J. & Hitchmough, R. A. 1982: Lizards of Rakitu (Arid) Island. Tane 28: 135-136. Moller, H 1977; Ecology of Rattus exulans on Tiritiri Matangi Island. Unpublished M.Sc. thesis,

University of Auckland. 319 pp. Taylor, R. H. & Thomas, B. W. 1989: Eradication of Norway rats (Rattus norvegicus) from Hawea

Island, Fiordland, using brodifacoum. New Zealand journal of ecology 12: 23-32. Whitaker, A. H. 1973: Lizard populations on islands with and without Polynesian rats, Rattus

exulans (Peale). Proceedings of the New Zealand ecological society 20: 121-130.

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