Rats and Ecology

Rats and Ecology on Great Barrier Island

There are about 60 species of rats in the World, and the three most wide-spread kinds arrived in New Zealand with people. In order of decreasing size they are: Norway rat (Rattus norvegicus), ship rat (Rattus rattus) and kiore or Pacific rat (Rattus exulans). Broadly speaking, this is also the order of decreasing impact on the native biota(1) .

Although kiore now seem to be relatively benign, they were brought to New Zealand by the first Maori visitors, possibly 2000 years ago, so they have been here much longer than the other species. The initial impact of these rats, the first terrestrial mammals to invade New Zealand eco-systems, must have been devastating: many large invertebrates, lizards, some of the tiny flightless birds (wrens) and ground nesting petrels probably became extinct at this time.(2) Kiore probably reached Great Barrier during the main Maori settlement of Aotearoa c. 1300 AD. By comparison with results from elsewhere we can assume they preyed extensively on the large colonies of sea-birds which must have been here, as well as impacting the vegetation. Recent studies on islands with and without kiore, and using rat-proof exclosures, demonstrate the effect they are still having on the vegetation: of the 17 species of coastal tree included in the study, 11 showed reduced seedling recruitment, some to the point of local extinction. Nikau palm, karo and puriri were amongst the trees seriously endangered due to kiore predation on their seeds(3) . While these effects might appear small compared to the ravages of fire as Maori cleared land for hunting, horticulture and bracken growth, they would have inhibited the forest re-establishment from the surviving bush patches.

Ship rats (rattus rattus) came to Aotea with Europeans, probably in the Nineteenth century. They apparently displaced kiore from the habitats richest in food supplies, especially the remaining lowland forest. Being bigger, more fecund, more aggressive and better tree-climbers than kiore, they largely took over, so that kiore now comprise only c. 11% of the total rat population on G.B.I., fewer in dense forest and greater in wet grass or rushy areas which ship rats seem to avoid. Ship rats continued the devastation started by kiore, and probably played a role in the loss of stitchbird, saddleback, kokako, whitehead, robin, quail and shore plover, all of which are now absent, although they were recorded here in 1868 by Hutton(4). The forthcoming release of robins in the rat controlled areas at Little Windy Hill and Glenfern Sanctuary represents the first homecoming for any of these species. Other species, some recorded as common by Hutton, (parakeets, cooks petrel, diving petrel, long-tailed cuckoo, tomtit and fern-bird) have hung on in the presence of rats, but their continued survival here is still in question.

Fortunately, Norway rats have not reached Great Barrier. This, along with the absence of possums, red deer and mustelids (stoats, weasels and ferrets) is the main reason we still have populations of brown teal, fernbird, black petrel and kaka. Even kiore would be at risk if Norway rats arrived. Thus biosecurity on ferries and air transport and at our wharves and airports is a crucial factor in keeping this one out, and maintaining our biodiversity.

If we are to eradicate rats from Great Barrier we need as much knowledge as we can get about their ecology. This has been one of the main research aims of the Awana Catchment Trust. The results from here, from Little Windy Hill, Benthorn Farm and D.o.C at Okiwi, clearly establish the seasonal cycle: there are about five times as many rats present in March and April as in September–November most years. There are also big differences between vegetation types, with maximum numbers in coastal flax, river-side rushes and tall forest. The manuka-kanuka scrub (Land Units 8 and 9) which covers so much of GBI, is poor rat habitat, but then, its not got much else in it either!

Another aspect of rat ecology which cannot be ignored is their place in the food chain, and the interactions involved in ‘trophic webs’. Rats and rabbits are probably the main food for feral cats on Great Barrier. Removing either (or both) could cause cats to shift their hunting to native birds, some of which are ‘sitting ducks’! Studies elsewhere show that mice would also increase if rats were eliminated. Likewise, removing cats could allow rats to increase. Striking a balance is probably impossible; the only answer seems to be to hit them all simultaneously. The technology is getting better all the time, and the prospect of a pest free Great Barrier Island is no longer the dream it might have been ten years ago.

John Ogden

References: (1) King, C. M. (Ed.). 1990. The handbook of New Zealand mammals. Oxford University Press. (2) Worthy, T. H. & Holdaway, R. N. 2002. The lost world of theMoa. Canterbury University Press. (3) Campbell, D. J. & Atkinson, I. A. E. 2002. Depression of tree recruitment by the Pacific rat (Rattus exulans Peale) on New Zealand’s northern offshore islands. Biological Conservation 107: 19-35. (4) Hutton, F. W. 1868. Notes on birds of Great Barrier Island. Transactions and Proceedings of the New Zealand Institute 1: 104-106.