RATS ecological anarchists

Forest and Bird, Volume 16, Issue 4, 1 November 1985, Page 8

RATS ecological anarchists

by

Tourist and Publicity, Department journalist,,working with Lands and

Survey Department.

David Gregorie,

R« are the weeds of the animal world — opportunistic species that will take advantage of any change made in the natural order of things by a natural disaster, or of the many radical alterations in the world ecology made by human beings. They are the camp-followers of the human race, living on our rubbish, surreptitiously sharing our homes, spreading. disease, and waxing prosperous on Our precious stores of food. They.are tough, adaptable, intelligent and prolifi¢= Where rats aré endemic; such as on the Eurasian land mass, theyare*kept partly | in check by predators}-but More hmportantly by fierce competition fer "the available food. In densely populated areas like China, India, and Europe, they take advantage of the enormous 1n* crease in available food — chiefly stores of food grains and edible rubbish produced by cities. Ecological chaos Rats are capable of creating ecological chaos, especially when they invade territories such as islands which have hitherto been rat-free. The oft-quoted example of the 1964 rat irruption on Big South Cape Island — off the south coast of Stewart Island — shows the catastrophic effect rats can have on wildlife. When Brian Bell of the Wildlife Service arrived on the island for a survey in 1964, he found no saddleback, wren, fernbird, robin or snipe, and there were few bellbird or parakeet. Birds were not the only sufferers. The Stewart Island short-tailed bat is apparently extinct and the once-abundant insect life has been all but eliminated. Plants were seriously damaged too, especially Pseudopanax species and the punui (Stzbocarpa lyallii), but these have recovered somewhat since the ship rat population has declined to a lower level. The tragedy of Big South Cape Island

could hayeibeen prevented if boatowners had been more aware/of the unportance of keéping ‘their boats’ ratfree,_,on of the dangers of; mooting/ thé’ boats’ with shore lines)to rat-free islands¢ New rat infestations of offshore 1slands have happened too’ recently for us tobe complacent/about them — Lord-Howe Island / (1918-ship’ rats)y Raoul Island (1921 + Norway fats)Big South Cape (1962 | /ship/ rats) Somes Island in Wellington Harbour(1968= ship rats); Lizard/Island, near Great Barrier Island (197/7/- kiore): Many’ people ‘take-the~attitude that since/ there are rats*already-On about 80 percent of the world’s oceanic.island, itas only/a matter of time*before-they»réach ther all: But New:Zealand’s.experience suggests that with~proper precautions and an aware population, it need not happen Suggested precautions include allowing no--wharves_orshoreline™mooring points, on rodentefree islands. Boats should moor at least 300 metresOffshorey stores and equipment should be patked in rodent-proof containers and ‘poison bait used on boats. On the mainland, rats must be deterred from boarding boats. They are known to get on to boats moored near the shore at night. Apparently, most leave by the morning or earlier if disturbed, but there is no guarantee that all will go, and a few might remain if the boat leaves its anchorage before daylight. Unfortunately, it seems that except on the smallest of islands, once rats have become established, they are there for keeps. The DSIR, Lands and Survey and the Wildlife Service have experimented with various rodent control methods to see if there is any feasible way of exterminating rats that have become established, but the answer has been far from encouraging. In this case, prevention is the only cure. People who use boats and visit offshore islands will have to be made to

réalise’ the respoxsibility they® faee-im Keeping islands rat-free. Aggressive. and. omnivorous New Zealand.does not have the-huge populations, of ~the»=longer-inhabited countries,-but-we do-have"a natural environment that,.is.extremely-vulmerable to _an-a@ereéssive, omnivorous animeal---the-rat. Rats? main.feod is.invertebratés par tietlarly insects, and_sééds=and™firuats. They will€at small birds;nestlings-and eggs, lizards, and-theé growing shoots of some plants?Ournativeanimals-arepar-ucularlyevulnerable. because-theyshave evolved-in@an senvirenment without mammalian predators.and because-they are not generally-very~prolific._Newly-arrived*rats, found themselves, presented with ajfood benanza without any. effec tive predators *to™keep™theirnumbers down. Mice are nota direet-threatto ourbird lifeybut they do’compete with ‘then? and with the larger invertebrates for food so wée™must do‘our utmost to stop their spread. here are three species of rat found in New Zéaland — the kiore or Polynesian rat (Rattus exttlans), the ship or black rat (R. rattus), and the Norway or brown rat (R. norvegicus) — and one species of mouse, the house mouse (Mus musculus). All were brought to New Zealand by human settlers or arrived as stowaways in their canoes or ships. The kiore is believed to have spread into the Pacific from South-East Asia with the first human settlers more than 3000 years ago, island-hopping with the Polynesian people and_ eventually reaching New Zealand with the Maori people about: 1000 years ago. The ship rat was the common rat in European ports and cities until about the beginning of the 18th century. It began its spread around the world with the voyages of Christopher Colombus in 1492. Most of the ships leaving Europe

on voyages of trade or discovery during the next 200 years were infested with ship rats and these were able to establish themselves easily on any islands free of ship rats. Early in the 18th century the Norway rat spread from Eastern Europe into Western Europe where it largely.displaced the.shipwatincthe-Cities andsports-It too/began.to spread _around,the world.as an"Uuninvited -passénger-on.the sealers, whalers, .«traders~ andedimmigrant ships bourid-for.the-néewly-discovered lands. The Norway rat isthe largest.of-the thrée_-speetes, withadult weights-of 350=450¢-It excavates and-nestsan*bur= rows far more-frequently.thansthe ther two~species, but appears"to climb trees less-readily. Birdsnésting_on the ground are-mor€ likely-to be-preyed upon*than treé nesting birds. Twenty seven of more than50-bird species known to beeaten.by the=rat-are-seabirds. The-ship rat-uSually.weighs 100-180g. Itas-an_ excellent.climber and commonly nésts in tréésTwenty-five-of-some 40 récorded.-prey-speeies*are-perching birds, Attacks-adult-albatrosses Theekioré-is.the smallest-of the-three-rat speciesy=sseldom.-exceeding... 130g.=1n weight. Itcommonly,.nestsm-vegetation on,or-near-the~ground-orsin..shortebur-rows:Althoughitepreyston=fewer bird species, itjwillsometimes_tackleMarger birds"than-the-other-two-rats!"On. some Pacific islands} for example;-it-has been known to. attack adult and nesting albatrossessand frigate,birds. TLhe-effect. 6f*kidre..on, the: birdehfe of early New Zealandus not known withany. degreé-of cértaintysitree the major_impact was "prehistoric-and-many of, the birds that could-have-been affected are now extinct. However;~a ‘great deal can be inferred fronmthe"présent-distribution of smal! birds»lizards, andyinvertebrates and from comparison between rat-free and rat-infested offshore islands, The disappearance or_ reduced numbers/of small_perching birds during the Polymésian periods unlikely to have been caused. by eitherhunting or habitat destruction. Any small birds nesting near the ground may have been vulnerable to the kiore and larger birds may also have been at risk at the nestling stage. Ian Atkinson, of the DSIR Botany Division, pointed out in a paper for Symposium on the Ecology and Control of Rodents in New Zealand Nature Reserves, that the difference between kiore-free and kiore-inhabited islands was most marked. The abundance of certain plants, diving petrels, tuataras, lizards and some invertebrates on those islands without kiore contrasted with their absence or very low numbers on kiore-inhabited islands. He and other investigators have pointed out that many of the smaller petrels and all sub-species_ofethe New Zealand snipe are restricted to rat-free islands. It is now,known that’ the snipé did occur on the mainland_aswell as the offshore island:before»the arrival of the kiore.

Some rat invasions of oceanic or offshore islands in modern times are well documented. In 1918 ship rats came ashore on Lord Howe Island from a wrecked steamship and within a few years five species of endemic forest birds had become extinct. Glimbers and swimmers Ship-rats areseasily the most widespread rat spécieéssin N@w Zealand. They are abundant. on both@main-"islands, on Stewart-Island, @hathamsj"Gréat.Barrier, Arapawa, "Big. South @ape; Motuwhakakewa we stérim Cor: omandél)-and-Solomon=Asthem fame implies, they_arethoreughly. athomeyon ships and boats anduttakes only niomen= tary inattention on the "part-of"boat-owners and cargo_shippers formthém\to slip-en_board from any ofthe ports, in New-Zealand andsthensto_travel reund with the™~boaty,ready. to trigger an ecologicalcatastrophe. They donot needa wharf or jetty to.get ashore; althoughthis» would obvidusly hélp~but. are "quite»adept at climbing along -moormpinés.oréeven swimming meodératesdistances*to the shore. Ehé Norway rat;.onee Widespreadwin New Zealand, "now has a very patchy distribution on the-mainlandy, beitgconfinedto \pockéts of infestation usually associated" with humanoccupation NOffshorelitis found only_en StewartIsland, Raoul, Campbell, Kapiti, Mayer)main Chatham Island and Breaksea and’some smallerislands. Rw. Laylor;.ofthe Ecology Division of the DSTR believes that, the imtroduc> tion ofstoats, to the Worth tand\South Islands inthe 1880s.is the main'reason for the*decline im Norway: rat-humbers\and quotes,adjacent\islands inthe Fiordland National Park, to make his points, Both islands Were. inhabited by \Norway rats until ‘about /1900, when stoats \were released on Resolution Island but not on nearby Breaksea, Vhere are now no/NorWayrrats on Resolution Island but they are plentiful on Breaksea. Kiore are also no longer common, be-

ing confined to the south-west part of the South Island, Stewart Island, Cook Strait and about half of the offshore islands in the Northland-Hauraki GulfCoromandel region. They are present on Raoul and Macauley Islands in the Kermadec group but seem to be extinct on Chatham Island. Taylor believes that competition from the house mouse has led to the gradual disappearance of the kiore from the mainland and other inhabited areas. He points out that kiore exist alongside ship rats and Norway rats on Stewart Island, where mice have not become established. Many of the northern islands where kioxe occur were inhabited or visited by Maoris.in pre-European times and the rats. mayhiaye been taken for food or have arrived by accident. Atkinson points out thatuslands with precipitous shores, like the ‘Three Kings and the Poor Knights, are frée of kione\in spite of being inhabited by, Maoris. Island\treasure houses Forty nine ‘of 196 northern islands are rodent-frée:\ This includes only a few of the larger, islands,such \as Great Island (Three\. Kings), \Tawhiti Rahi and Aofangi (Roor \Knights), and Moturoa Island (Mahurangi Heads). Kiore is the most widespread, oceurring on at least 33 islands; while\ the’ more recently introduced Norway \rat, ship rat or the house Mouse Occur on\some others. The rats have had a marked effect on island faunas, eating lizards, land snails, annelid wormssspiders,\centipedes and a large. variety of insects\ ‘They are also beheéved to have eliminated or reduced the, smaller species, of petrels and terns arid aré,probably responsible for the extinction of smaller groundnesting or perching birds in prehistoric times. The distribution of\izards appears to be a reflection*of islandsextinctions caused by ratsy parficularlyskiore. For example, Lizatd Island (Mokéhinau group) had five species of lizards in abundance before the accidental arrival of kiore in 1977 but only three speci¢s,temain today in very low numbers. Some very small rat-free nds support five or more species | of lizard, for example, seven speciés on Motuharakeke (Cavalli group), six species on Pupuha (Chicken Group). Both thése islands are only one hectar€ im"aréad The tuatara 1s: numerous only on ratfree islands@_ Theres€antbe ho doubt human beings are the’ prumary agency for distributing rats‘and,mice — the kiore came to New Zealand with’the Maoris and was spread by them to the offshore islands. The other rodent Species came with European settlers and it is our careless habits which allow their spread to continue. With rats, prevention is the only cure. People who use boats, whether for pleasure or business, have the future of our offshore nature reserves in their hands. Only by their awareness and cooperation can we ensure that these natural treasure houses remain inviolate. es