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Tuatara: Volume 27, Issue 2, December 1984

Introduced Birds and Mammals in New Zealand and Their Effect on the Environment

Introduced Birds and Mammals in New Zealand and Their Effect on the Environment

Keywords: Introduced birds, mammals, distribution, population numbers, environmental effects, control, farming.


Thirty three species of introduced birds and thirty two species of introduced mammals are now widely accepted as a part of New Zealand fauna. The history of the introduction of these vertebrates into New Zealand is documented, and consideration is given to their effect on the native vegetation and fauna. The status of introduced mammals in the late 1940's (Wodzicki 1950) is compared with the present, and the success of control is discussed. Improved technology and the high export value of quality animal products led to a population decline of certain mammals as farming became commercially viable. While the distribution of many introduced mammals has expanded, population numbers have generally decreased since the first survey. A notable exception is the possum.


New Zealand is a marginal fragment of the ancient continent of Gondwanaland. The long isolation of New Zealand has resulted in a considerably diverse flora which has been derived from two sources. Firstly, from ancient species that were present when New Zealand formed a part of Gondwanaland. The remainder consist of species which arrived during the subsequent isolation.

Although some browsing herbivores such as the moa (Greenwood and Atkinson, 1977), native pigeon (Hemiphaga novaeseelandiae) and takahe (Notornis mantelli) had an influence on the vegetation, the New Zealand flora developed in the absence of browsing mammals. Since the arrival of man the indigenous vegetation has been modified by destruction and fire, through the introduction of mammals, and by the establishment of many exotic plant species.

The long isolation of New Zealand has also given rise to a considerably diverse endemic fauna. There are sixty-five native species of extant land and freshwater birds of which 57% are endemic (Bull & Whitaker, 1975). New Zealand's long isolation is also shown by the archaic endemic genus of frogs Leiopelma; the tuatara Sphenodon punctatus, the so-called “living fossil”; and the existence of only two terrestrial mammals, the two bats.

Editor's Note: This paper was originally presented to an International Conference on the theme ‘Man as a Biogeographic Factor’ organised by the Société de Biogéographie in Paris, 18-22 October, 1982 and published in French (C. R. Soc. Biogéogr. 59 (2): 231-256, 1983). The English translation is published here to make it available to New Zealanders as an update to Wodzicki (1950).

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Man arrived in New Zealand in three influxes. The first arrivals were the Polynesian settlers who probably came as early as the tenth century A.D. (Davidson, 1976), and brought with them two mammals, the Maori dog and kiore rat.

Centuries later, Captain Cook arrived on the 6th October 1769. From 1792, for about 20 years, European sealing and whaling gangs ruthlessly destroyed their quarry until the animal stocks became exhausted. European rats and mice were presumably introduced into New Zealand by the sealing and whaling ships. Finally the first large influx of European settlers took place in the 1840s and with them came the remaining introduced terrestrial vertebrates of New Zealand.

Table 1 shows the composition of the New Zealand vertebrate fauna including terrestrial, freshwater, native and introduced species. The table also illustrates the success of establishment of the various vertebrate groups, which varies considerably.

Table 1
Endemic and Introduced
Vertebrates of New ZealandM
Fishes Amphibia Reptiles Birds Mammals
27 18 3 3 36 1 65 33 2 32
N — Native
I — Introduced

The origin of vertebrate introductions is of interest. Fig. 1. depicts these introductions and shows that the largest number of successful introductions came from the British Isles and Europe. Comparatively fewer came from the continent of Australia and the Pacific Islands. Obviously sentimental ties of the settlers with their country of origin has prevailed and this is shown by the large number of introductions from Europe. The dates of introduction of freshwater and terrestrial vertebrates are shown in Table 2. The prehistoric period prior to Captain Cook's visit had only two introductions (kiore or the Polynesian rat and the kuri or Maori dog). The largest number of introductions occurred about a quarter of a century after the settlement of New Zealand in the 1840s when most of the settlers were already established.

Table 2
Dates of Introduction of Freshwater
Fish and Terrestrial Vertebrates
Period Prior to Freshwater Fish Amphibia Reptiles Birds Mammals
Captain Cook 2
1773—1840 2 11
1840—1860 3 2
1860—1880 22 3 90 22
1880—1900 8 1 14 8
After 1900 11 7 8
Unknown 1 9

Introduced Birds

A total of 144 bird species were introduced by man into New Zealand between 1840 and the present day and of this total 33 species have survived and are now part of New Zealand's avifauna. The 33 introduced species page 79
Fig. 1. The Origin of Birds and Mammals Introduced into New Zealand (After Wodzicki, 1950). A. Countries of Origin of Introduced Birds. B. Countries of Origin of Introduced Mammals.

Fig. 1. The Origin of Birds and Mammals Introduced into New Zealand (After Wodzicki, 1950). A. Countries of Origin of Introduced Birds. B. Countries of Origin of Introduced Mammals.

that became established belong to 7 orders, the majority being passeriformes (Fig. 2). The species were introduced for several reasons: 7 species were introduced for biological control; 9 as game birds; 10 for sentimental reasons and 6 for reasons unknown. The failure of some species to establish was probably due to factors such as: (i) the initial stock was small and the birds scattered; (ii) some species were migratory; or (iii) the habitat was not suitable (Dr P. C. Bull, pers. comm. 1982). The majority of introductions occurred at a time when the forests were being cleared and other environments, such as wetlands, were being eliminated. Drummond's (1907) survey of introduced birds has shown that most species increased to large numbers soon after their liberation and were soon reported to cause damage to crops.
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In considering the effects of the introduced birds on New Zealand's ecosystem several aspects have to be mentioned. The first is the effect foreign bird species may have had on the native bird fauna. It is claimed that avian diseases introduced by the new arrivals may have caused the disappearance of some susceptible native species. Turbott (1961) reported that aggressive behaviour and predation occurred between native and exotic bird species although only in a few cases. Another consideration is interspecific competition between the native forest species and the introduced bird species. According to Turbott (1961), only three introduced species, the blackbird (Turdus merula), the song thrush (T. philomelos) and the chaffinch (Fringilla coelebs) are found in the native forest, all being more common along the forest edge than deep in the forest (Kikkawa, 1966). The hedgesparrow (Prunella modularis) is found only in the forest edge and the redpoll (Carduelis flammea) is present in the subalpine scrub zone.

Although the destruction of native forest by man has reduced or even eliminated certain native species, other native species entered modified environments and are now found together with introduced species. For instance in the Kaingaroa exotic plantation there are vigorous populations of native and introduced species (Gibb, 1961).
Fig. 2. Numbers of Introduced Bird Species According to Order. (After Kinsky, 1970 and 1980).

Fig. 2. Numbers of Introduced Bird Species According to Order. (After Kinsky, 1970 and 1980).

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Williams (1953), in his survey of introduced passerines on offshore islands of New Zealand, reported that 10 or 11 out of 13 species then established in New Zealand were self-introduced to the various islands up to 885km distant. These self-introductions occurred within 30 or 40 years of their liberation in New Zealand.

Over a century has elapsed since the liberation of most introduced birds in New Zealand and the distribution and populations of most have for some time remained static (see Table 3). Nevertheless, a few species are still subject to changes in distribution and numbers. The rook (Corvus frugilegus), which was introduced to North and South Island has spread considerably since 1969, although for several years it remained static in its distribution. The rosella (Platycercus eximius) has also widened its distribution, while the populations of other species such as the myna (Acridotheres tristis) and the magpie (Gymnorhina tibicen) have increased (Dawson and Bull, 1970).
Table 3
General Distribution of Introduced
Birds in New Zealand
BlackbirdKookaburraBlack-backed Magpie
ThrushMalay Spotted DoveWhite-backed Magpie
HedgesparrowBarbary DoveCanada goose
HousesparrowCrimson RosellaMyna
GoldfinchEastern Rosella
GreenfinchWhite Cockatoo
StarlingCirl Bunting
RedpollBrown Quail
California QuailLittle Owl
Black SwanPeafowl
YellowhammerMute Swan
ChaffinchFeral Pigeon

Introduced species in New Zealand display several characteristics that contrast with the same species in its native country. For instance in New Zealand the goldfinch is a very common bird whereas in England it is fairly local and generally in low numbers. Introduced species in New Zealand tend to have high population densities and therefore more intraspecific competition occurs (Dr P. C. Bull, pers. comm. 1982). A study by Niethammer (1970) found that clutches are smaller here than in Britain but adults tend to live longer, possibly because of the lack of predators. In some birds such as the redpoll (Carduelis flammea) and yellowhammer (Emberiza citrinella) the plumage of New Zealand specimens is significantly brighter than comparable specimens from Europe (Mr J. A. Bartle, pers. comm. 1982). Generally, introduced species have fewer parasites than the same species in the country of origin thus increasing their rate of survival.

The birds introduced into New Zealand may be divided into agriculturally important species, game birds and others. The destruction of wetlands occurred at the same time as forest destruction and reduced numbers of the native waterbird species such as the grey duck (Anas superciliosa). To satisfy the demands of sportsmen the following game birds were introduced: mallard (Anas platyrhynchos), Canada goose (Branta canadensis), mute swan (Cygnus olori), black swan (Cygnus atratus), quail (Synoicus ypsilophorus), California quail (Lophortyx page 82 californica), pheasant (Phasianus colchicus). Birds that have adapted to pasture grazing such as the Canada goose, the native paradise duck and later also the mallard, have benefitted from the great increase in pastoral land in New Zealand. Consequently their populations, despite annual shooting, have increased. The populations of other game birds, such as the black swan or pheasants remain reasonably stable (Mr T. A. Caithness, pers. comm. 1982).

As in their countries of origin, birds introduced into New Zealand are at times considered to be pests to crops. Dawson and Bull (1970) have found that serious damage is done by nine species of introduced passerines to various fruit and fruit buds. Some bird species, e.g. the starling (Sturnus vulgaris) are considered both noxious and useful on the New Zealand agricultural scene. Orchardists consider it to be a pest but in pastoral areas this species is beneficial as it preys upon grass grub (Costelytra zealandica).

A special class of birds influenced by man are the self-introduced Australian birds, which have colonized New Zealand in recent years and are established in this country. An early example is the Silvereye (Zosterops lateralis), large flocks of which migrated to New Zealand in the 1850's.

More recent migrants include the spur-winged plover (Vanellus miles), the white-faced heron (Ardea novaehollandiae), the welcome swallow (Hirundo tahitica) and the Australian coot (Fulica atra.). It is believed that some of them, like the white-faced heron and the welcome swallow, were able to establish themselves and breed because man had modified the environment. Colonization by most of these species is proceeding extremely quickly and many are now breeding in both islands.


The 32 species of mammals introduced and established in New Zealand belong to seven orders. These mammals were introduced for a variety of reasons, the majority for sport (Table 4). An interesting characteristic of a number of species was their ability to penetrate into the native forest and to become established there. It should be added that the New Zealand native forests grew and developed without browsing mammals. Their populations expanded rapidly and contributed to the destruction of forests and the initiation of erosion. However, other introduced species took to the man-made pastoral country and increased rapidly until they became a menace to farming in hilly areas. More recently in certain species a combination of high technology and high prices for animal products has resulted in a simultaneous decline of animal populations and a rise in export earnings from the animal products.

The Brushtail Possum (Trichosurus vulpecula)

A denizen of Australia and the greatest wildlife pest of New Zealand at present, the possum was repeatedly liberated from Australia between 1858 and 1930 to establish a fur industry. The subsequent spread of this species was greatly enchanced by numerous liberations of New Zealand stock in both islands, particularly between 1890 and 1930 (Pracy, 1962).

The first map showing the distribution of possums in New Zealand was produced by Wodzicki (1950). At this time possums were well distributed in both islands and Stewart Island but there were still large areas without possums, such as North Auckland, most of the central North Island, the central South Island and the west coast of the South Island. At present possums have expanded north of Auckland and from Urewera to the East page 83
Table 4
Successful and Unsuccessful Introduced
Mammals and the Reasons for their introduction
UtilitySport“Escapees”Biological ControlStowawayUnknown
PossumHareFeral CatHedgehogBlack rat
RabbitSwampFeral SheepFerretNorway rat
Feral goatBlack stripedFeral CattleStoatMouse
Feral pigDama wallabyFeral HorseWeasel
Pacific ratRock
Red Deer
Javan Rusa
Ring-tailedAxis DeerRaccoonGuineapigBandicoot
Blue SheepLlama
Coast. They are also moving south down the West Coast though the heavy rainfall of Fiordland prevents their penetration there (Fig. 3). The possum population of New Zealand is estimated at 30 million (Dr R. E. Brockie, pers. comm. 1982).

The effect of the possum on the vegetation has been the object of several studies as discussed below. Possums have been found to thrive in a variety of habitats including native bush, Pinus radiata, orchards, pasture, scrub and urban gardens. In exotic plantations when the trees are growing they suppress the underneath shrub layer and the possum populations decline. However, when the trees become mature more light is available to the shrubs and the possum population increases (Clout, 1977).

Alice Fitzgerald (1981) provided a list of native plant species eaten by the possum and found eight native species which occurred regularly in the page 84
Fig. 3. Distribution of the Brush-tailed Possum (Trichosurus vulpecula) in New Zealand 1947-1980. (After Wodzicki, 1950 and Unpublished Report of Agricultural Pests Destruction Council, 1980).

Fig. 3. Distribution of the Brush-tailed Possum (Trichosurus vulpecula) in New Zealand 1947-1980. (After Wodzicki, 1950 and Unpublished Report of Agricultural Pests Destruction Council, 1980).

possum diet. In addition grass, clover and insects if available are also included in the diet. According to her (1976) possums are a serious menace because they eat favoured species such as mistletoe (Elytranthe sp.), rata (Metrosideros robusta) and fuchsia (Fuchsia excorticata), and thus actually page 85 change the composition of forest e.g. the closed forest of the Ruahines since 1950 has become open woodland. This transformation was supported by the work of Meads (1976) who traced 50 rata trees in Orongorongo Valley attacked by possums during five years. All trees showed damage and 11 trees were killed by continued browsing of possums. Spurr and Jolly (1981) reviewed reports of possum damage to crops and found that it included turnips, choumollier, lucerne, clover and pasture.

The problem of bovine tuberculosis in the possum is even more important. The first case of tuberculosis in possums was reported in 1970 and since then the disease has been discovered in 23 general localities (Julian, 1981). In 1982 $430,000 were spent controlling tubercular possums (Mr P. Grace, Ministry of Agriculture & Fisheries, pers. comm. 1983). It was known at that time that tuberculosis could maintain itself in possum populations and serve as a source of reinfection to cattle. At present there are many areas of scattered infection in both islands, the worst area being in the Masterton district, North Island (Ministry of Agriculture and Fisheries, 1978). The New Zealand strain is particularly virulent and infected possums die within 3 weeks (Corner and Présidente, 1981).

It should be mentioned that possums are also an economic asset. As soon as possums were established in the last century, possum skins were exported. Since 1970 the price of skins has risen and in 1980 the skin trade was worth 23 million dollars (N.Z. Yearbook, 1981).

In recent years possum farms have been established in New Zealand. The ranching of possums on farms produces skins which fetch higher prices $18–$20 per skin) than the skins of wild animals ($6–$8 per skin) (N.Z. Dept. Statistics, 1980).

Like so many mammal species introduced into New Zealand the possum requires control to reduce damage caused to forests, and also the infection of cattle with tuberculosis.

The control of possums is largely in the hands of Pest Control Boards and commerical trappers. Where skins are of high quality the trappers make deep inroads into their numbers but where the skins are of poor quality, the trappers leave large numbers behind. The very wide spread of possums in both islands makes, despite the existence of satisfactory techniques, an efficient control very difficult and costly. As Freeland and Winter (1975) suggested, in Australia the possum is not a pest for two reasons: firstly the flora has built many chemical defences and secondly there are other marsupials that the possum has to compete with.

In summing up, we can say that the possum is the only introduced mammal species that is still unmanageable.


Twelve species of marsupials were introduced into New Zealand between 1858 and 1870 but only six wallaby species and the possum (Trichosurus vulpecula) became established (Wodzicki, 1950). Five species (Macropus bicolor, M. dorsalis, M. eugenii, M. parma and Petrogale penicillata) are found on Kawau Island and on a few other islands in the Hauraki Gulf, and (M. eugenii) is present in the Rotorua area, North Island. The sixth species (M. rufogriseus) is located in South Canterbury, South Island. Populations of the dama wallaby (M. eugenii) near Rotorua, and the red-necked wallaby (M. rufogriseus), near Waimate have expanded considerably in recent years. The parma wallaby (M.parma), nearly exterminated in its homeland, Australia, is found in satisfactory numbers on Kawau Island.

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Wallabies compete with sheep for food and also cause damage to young trees. Stomach contents of wallabies in South Canterbury included native trees, shrubs, ferns and various grasses and weeds (Wodzicki, 1950). With regard to control, in the Waimate area, $70-90 per hectare was spent in one year on wallaby control to ensure that trees grew high enough to prevent browsing (Mr K. Miers, pers. comm. 1982).

Problems are caused if wallabies are kept as pets as they are often taken to new areas where they can establish themselves if they escape. For example at Lake Hawea in the South Island, wallabies released during the 1930's and 1940's built up a substantial population. The cost of their control thus far is about $100,000.

The Hedgehog (Erinaceus europaeus)

Hedgehogs were introduced from England between 1869 and 1900 to the South Island and between 1900 and 1909 to the North Island. They were introduced to control various insects and other garden pests. The spread of hedgehogs was much assisted by man who continued to liberate them in new localities (Wodzicki, 1950).

Brockie (1975) has found that over 250 frost days per annum or rainfall exceeding 2500mm/year limits hedgehog distribution. In comparison with distribution maps of hedgehogs in the late 1940's (Wodzicki, 1950) it appears that hedgehogs have colonized all suitable areas in both islands. According to Dr R. E. Brockie (pers. comm. 1982) at present the population is static.

With regard to the food of the hedgehog Brockie (1959) has shown that slugs, moths, caterpillars, snails, millipedes, frogs and earwigs make up most of the food of hedgehogs and that the diet varies with habitat. A particularly interesting finding is the lack of predation by this species on ground nesting birds, although hedgehogs are a minor nuisance to farmers by carrying off chicken eggs.

During the long voyage from Europe to New Zealand many parasitic fleas and tapeworms of the hedgehog were lost. However, the scab mite disease spread widely in New Zealand hedgehogs during a population peak in 1970. A ring worm from hedgehogs sometimes affects children and the hedgehog is also a maintenance host for Leptospirosis ballum (Brockie and Till, 1975).


Three mustelids—the weasel (Mustela nivalis), the stoat or ermine (M. erminea) and the ferret (M. putorius) were liberated in New Zealand (Wodzicki, 1950; Marshall, 1963) between 1867 and about 1897 for the biological control of rabbits.

The distribution of the three species of mustelids varies greatly. The ferret and weasel are both locally distributed with the distribution of the ferret being related to that of rabbits. Stoats are the most common mustelid, as shown by Marshall (1963).

Recent publications provide details of the food of the mustelids in New Zealand (Marshall, 1963), particularly in forests of Fiordland (King and Moody, 1982). The principal foods of stoats were insects, mice, birds (10-20 species) and mammals such as rats and possums. Also, of considerable importance was the fact that stoats “did not eat significantly fewer birds when there were plenty of mice” (King and Moody, 1982). However, the real effect of mustelids on birds remains unknown. Ecological studies in the Orongorongo Valley, near Wellington by the page 87 Ecology Division, D.S.I.R. revealed a possible competitive interaction between stoats and cats. Cats prey on stoats and where feral cats are present, the stoat population will not fluctuate (Dr B. M. Fitzgerald, pers. comm. 1982).

The problem of mustelid control has recently been reviewed by King and Moors (1979). Drawing on the results achieved in the control of mustelids in England, they doubt whether trapping, particularly in National Parks, will lead to their extermination. Active management of rare native birds would be the alternative.

Finally, the latest economic development with regard to mustelids is fitch farming. Within the last few years approximately ten farms have been established. Imported ferrets from Scotland have been bred with New Zealand feral animals to improve fur quality and colour (Dr C. M. King, pers. comm. 1982).

Feral Cat (Felis catus)

Since the introduction of the house cat in the early whaling days, the species has become feral and colonized most of mainland New Zealand and some islands. It occupies a wide range of habitats from sea-level to 1500m. (Collins and Charleston, 1979). Feral cat populations are probably self maintaining although straying cats may augment the feral numbers occasionally (Fitzgerald and Karl, 1979). In mixed forest of the Orongorongo Valley near Wellington the food of feral cats has been found to consist primarily of small mammals such as rabbits, rats, mice and even possums and stoats. Although several species of bird are eaten, they form only a small part of the diet. However cat predation has been suggested as one of the reasons for the decline of ground-feeding bird species in the past (Fitzgerald and Karl, 1979). On Stewart Island the continued survival of the rare flightless parrot, the kakapo (Strigops habroptilus) is threatened by feral cats. The cats are estimated to kill 25-50% of the population of 100 kakapo, reducing an already endangered species (Mr H. A. Best, pers. comm. 1982). Recently the Wildlife Service, Department of Internal Affairs, New Zealand eradicated a long established population of feral cats from Little Barrier Island, a wildlife sanctuary. This has resulted in a dramatic increase in native bird species, in particular the endangered stitchbird (Notiomystis cincta) which increased six-fold on some parts of the island over approximately four years. (Veitch unpubl.)

A recent survey has found that feral cats are definitive hosts for several sporozoa parasites including Toxoplasma gondii and Sarcocystis both of which have a wide range of intermediate hosts including sheep (Collins and Charleston, 1979) and therefore may be of economic importance to the meat industry.


Four species of rodents have been introduced into New Zealand; the kiore (Rattus exulans), Norway rat (R. norvegicus), the ship or black rat (R. rattus) and the house mouse (Mus musculus). The kiore has been reported to have been brought to New Zealand by Maoris. It is assumed that the Norway rat came ashore during the first visit of Captain Cook but the ship rat arrived later and was not widely distributed until the 1880's (Atkinson, 1973). The house mouse apparently established itself during the early 1800's (Thomson, 1922).

All three rodents introduced from Europe soon became widespread. An account of the present distribution of the four rodent species was recently page 88 given by Taylor (1978). The kiore, once widely distributed, is at present confined to outlying islands, Stewart Island and Fiordland, South Island. Its disappearance from most of New Zealand was due to the arrival and spread of the mouse. The Norway rat presents, according to Taylor, both a ubiquitous and patchy distribution. It is present on some parts of the mainland where it is largely commensal with man and on some offshore islands. Norway rats appear to be effectively controlled by predation by stoats and thrive only where abundant food is available close to cover, or where stoats are absent.

The mouse is very widespread and is also present on several outlying islands.

Rodents have a very important effect on birds and reptiles as reported by Atkinson (1978). Predation by kiore has been reported on seven bird species, by the ship rat on 10 species and by the Norway rat on nine species. It will be of particular interest to note that nine bird species have been brought to local extinction by ship rats but none by kiore and Norway rats. Fig. 4 illustrates predation by the three rat species on various bird groups (Atkinson, 1978).

Other groups of vertebrates are also subject to predation by the rodents. The effects of kiore on the tuatara (Sphenodon punctatus) and other reptiles have been described by Crook (1973) and Whitaker (1978). It was found that tuataras were “common” on islands without kiore. On islands with kiore, tuataras were rare and often only old large specimens were present. Similarly the diversity of the lizard fauna was reduced in the presence of kiore. No research work has been carried out on the effect of kiore on frogs but it is very likely that they are subject to predation by this species too.
Fig. 4. Differences in the Bird Groups Preyed Upon by Different Species of Rats. (After Atkinson, 1978.)

Fig. 4. Differences in the Bird Groups Preyed Upon by Different Species of Rats. (After Atkinson, 1978.)

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The house mouse is generally distributed through the main and many outlying islands, usually in low densities (Fitzgerald, 1978). However in beech forests where the trees seed irregularly, mice populations will erupt in a good seed year (see e.g. King, 1982).

Rabbit (Oryctolagus cuniculus)

The rabbit is of particular interest as it became a pest in New Zealand very soon after its liberation. The presence of rabbit for over a century has proved to be a most expensive undertaking with regards to control.

The numerous and deliberate liberations of rabbits in the North and South Islands were due to the desire to export rabbit skins. The liberations took place between 1864 and 1905 and the rabbits multiplied quickly to the effect that “as early as 1869 rabbits were reported as a nuisance in Southland and in Marlborough and Central Otago by 1878” (Wodzicki, 1950).

Rabbits spread more slowly in the North Island because of large forested areas, different patterns of land development and fewer extensive open tracts. It is believed that between 1920 and 1940 rabbits had occupied all the areas suitable to them in both islands.

The relationship of medium and heavy infestations of rabbits to soil types in the North Island was provided by Dr N. Taylor (Wodzicki, 1950). Rabbits affected farming mainly through the reduction of sheep-carrying capacity and the depletion of soils through erosion. It is estimated that fifteen rabbits eat the same amount as one sheep, although this probably overestimates the requirements of a hill country ewe and underestimates the consumption of a rabbit (Wodzicki, 1948).

Rabbits cannot survive in long grass and this may be the reason that the largest rabbit populations occurred in sheep country. Gibb et al. (1945) considered that if other agencies deplete the vegetation, rabbits may become the dominant cause of erosion. Rabbits do not penetrate native forests but they must have crossed them in order to reach the open tops. On the other hand rabbits are known to be present in exotic forests where they eat the foliage of newly planted trees.

In an experiment carried out by Gibb et al. (1969) in the hill country of Wairarapa, indices of rabbit sign were used to measure changes in density in two areas carrying sparse rabbit populations, one with and the other without rabbit control. At the end of a three year period both areas carried similar rabbit populations although the age structures were different and it was suggested that predators were able to control the rabbit population in the protected area.

The damage done by rabbits forced the settlers to form organizations to control the pest and as early as 1876 the Rabbit Nuisance legislation was introduced. Another important step to control rabbits was the introduction of the “natural enemy” in the form of ferrets, stoats and weasels. Although these predatory animals can control rabbits under special circumstances (Gibb et al. 1978), their harmful effect on native bird life was and still is of great ecological significance.

Myxomatosis was introduced in 1951 to control rabbits but failed as there was no vector to spread the virus.

At present Pest Boards have replaced the Rabbit Boards and the control of other mammals is included in their activities. The methods used now include trapping, fumigation, dogging, night shooting and poisoning. The spreading of poisoned 1080 bait by aeroplane has proved to be the most page 90 succcessful. However, in the high country, where most of the rabbits are found, this form of control has become prohibitive, because of the cost involved.

A recent development has been the establishment of rabbit farms for meat and fur. In 1980 the pest legislation was changed enabling the rabbits to be kept commercially and as pets.

The Hare (Lepus europaeus)

The European hare was introduced repeatedly onto both islands of New Zealand from 1851 for food and sport. Protected at first, the species increased quickly in numbers and soon spread. Wodzicki (1950) stated that hares had occupied all suitable areas in both islands from sea level to snowline but were most abundant on the east coast of the South Island. According to Dr J. E. C. Flux (pers. comm. 1982) the species has widened its distribution in Westland but is now absent from North Auckland where gum lands occur. Hare numbers have increased slightly in recent years in many parts of New Zealand (Mr P. C. Nelson, pers. comm. 1982).

The hare, despite coming under the jurisdiction of the Agricultural Pest Destruction Council, has never become a significant pest in New Zealand, although the species can cause damage by nipping off new growth in young trees.

At the beginning of this century 130,000 whole frozen hares were exported and later during the 1940's many skins were exported annually (Wodzicki, 1950). In recent years live hares have been air freighted to France for $80–$90 each to restock game farms (Dr J. E. C. Flux, pers. comm. 1982) and overseas earnings of $4 million a year could be sustained by exports of wild hares (Flux, 1981).

The Wild Pig (Sus scrofa)

Wild pigs were first introduced into New Zealand by Captain Cook in 1773 and later more were liberated by the early settlers. Pigs were also released on various outlying islands as food for castaways (Wodzicki, 1950).

The latest information on wild pig distribution is given by Challies (1976). It shows that wild pigs are more widespread in the North Island than in the South Island, where pigs are absent on the West Coast. In comparison with the distribution maps of Wodzicki (1950), the areas with pigs appear to have become restricted (Mr P. C. Nelson, pers. comm. 1982).

The main vegetation damage caused by wild pigs stems from their feeding habits. Several plant species, particularly those with succulent roots, such as ferns, are uprooted. Rudge (1976) provided information on animal remains in the faeces of pigs on Auckland Islands. Among these remains were feathers of Antarctic prions (Pachyptila desolata).

At present local control operations are required from time to time to reduce wild pig predation on lambs and cast sheep, damage to young plantations, agricultural crops and pasture. Otherwise pigs are hunted for recreation or for meat.

The Wild Goat (Capra hircus)

Goats were first liberated by Captain Cook and for more than a century more introductions followed.

There were three main reasons for liberation of goats: (i) as a supply of page 91 food for visiting ships and for castaways on outlying islands; (ii) in 1867 Angora goats were introduced with the hope of producing skins of commercial value and (iii) using goats for checking the spread of blackberry, particularly on the East Coast of the North Island and in Marlborough (Wodzicki, 1950).

Feral goats have spread widely and their recent distribution has been given by Rudge (1976). A comparison with the distribution in 1947 (Wodzicki, 1950) shows that goats have extended their distribution on the two main islands (Fig. 5). On the other hand, goats have been exterminated from 12 of the 23 outlying islands where they were liberated.
Fig. 5. Distribution of the Wild Goat (Capra hircus) in New Zealand 1947-1970. (After Wodzicki, 1950 and Harris, 1970).

Fig. 5. Distribution of the Wild Goat (Capra hircus) in New Zealand 1947-1970. (After Wodzicki, 1950 and Harris, 1970).

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An account of the feeding habits of the goat and of its effect on the vegetation of islands has been given by Turbott (1948). A goat population left uncontrolled on Great Island, Three Kings Islands for 35 years, reduced a flora of 143 species to only 70 species. Similar destruction was observed on Kermadec Islands by Sykes (1969). Goats are known to feed on the foliage of most trees and plants and can destroy all vegetation. They can reach otherwise inaccessible foliage by climbing on leaning tree trunks.

The relationship of goats to deer is of interest. In North-West Nelson Forest Park deer have been removed by helicopter hunting. As soon as the deer disappeared goats moved in. On the other hand in Southern Ruahine mountains, when goats were shot out, deer came in (Dr M. R. Rudge, pers. comm. 1982).

It is likely that feral goats will eventually be exterminated from further outlying islands, particularly those whose fauna and flora is of special value. According to Dr Rudge the number of goats on the mainland has been reduced by control measures of the Forest Service, but their distribution remains the same. The low export value of goats normally prevents the use of helicopters in their control. The only commerical use of feral goats is in Hawkes Bay and Taranaki where goats are mustered and their meat is exported, with a value of $0.5 million per year to the Caribbean, Pacific Islands and South East Asia.

Despite the economic control applied in these areas the populations remain static.

Feral Farm Animals

Sheep (Ovis aries) were introduced into New Zealand along with cattle (Bos taurus) and horses (Equus caballus) as domestic stock by the early settlers (Thomson, 1922). Because the land was not fenced the animals roamed free and some, escaping periodic mustering, became feral.

In comparison to the situation in the late 1940's when there were several herds in the North Island (Wodzicki, 1950), feral horses are now found only in the Kaimanawa Ranges where a reserve has been established to protect them. Small populations of feral cattle exist only on Campbell and Enderby Islands although in the past they were present in remote areas of the mainland (Taylor, 1976). Feral sheep are still distributed in isolated local areas of the mainland and on one inshore and three outlying islands (Dilks and Wilson, 1979). Some of these populations exhibit features of scientific interest. For example the “merino × longwool” sheep on Campbell Island are resistant to footrot, have a low ratio of secondary to primary follicles in the wool and have increased body temperatures (Rudge, 1982).

Unfortunately some of the feral populations of cattle and sheep are on reserves and their presence conflicts with reserve management policies. Other herds are present on private land and as this is cleared of scrub the animals are eliminated.

Overall the populations of feral farm animals are declining but moves have been made recently to preserve particular breeds so that their unique genetic qualities may be drawn on for future domestic stock (Rudge, 1982).

Thar and Chamois

Himalayan thar (Hemitragus jemlahicus) and chamois (Rupicapra rupicapra) were liberated as game animals near Mt. Cook in the South Island of New Zealand. The thar was released in 1904 and the chamois in 1907 (Wodzicki, 1950).

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Since then the thar has spread north and south to occupy the middle region of the Southern Alps (Caughley, 1970). The chamois increased more rapidly and has dispersed along the Alps at a rate of approximately 10 kilometres per year (Christie, 1964), believed to be the fastest dispersal rate for any game animal in New Zealand (Riney, 1955). The distribution of both species in the late 1940's (Wodzicki, 1950) is compared to that of the late 1960's (Harris, 1970) in Fig. 6 and 7.
Fig. 6. Distribution of the Chamois (Rupicapra rupicapra) in the South Island, New Zealand 1947-1970. (After Wodzicki, 1950 and Harris, 1970.)

Fig. 6. Distribution of the Chamois (Rupicapra rupicapra) in the South Island, New Zealand 1947-1970. (After Wodzicki, 1950 and Harris, 1970.)

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Both species can feed in places inaccessible to red deer (Riney, 1955). Chamois eat a variety of grasses (Christie, 1970) while thar is known to eat out high altitude pastures (Howard, 1965). As both animals live in areas where erosion has accelerated they may be having a prominent effect on the habitat.
Fig. 7. Distribution of the Himalayan Thar (Hemitragus jemlahicus) in the South Island, New Zealand 1947-1970. (After Wodzicki, 1950 and Christie, 1970.)

Fig. 7. Distribution of the Himalayan Thar (Hemitragus jemlahicus) in the South Island, New Zealand 1947-1970. (After Wodzicki, 1950 and Christie, 1970.)

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The effect of chamois and thar on the alpine environment of the South Island made control mandatory. Between 1936 and 1968 official control accounted for over 82,000 chamois and 30,000 thar (Harris, 1970). Many additional thousands of chamois and thar were shot by sportsmen, and helicopter hunting has now virtually eliminated the thar.

Red Deer (Cervus elaphus)

Red deer were liberated in New Zealand for sport, with 29 recorded liberations between 1851 and 1910 in the North, South and Stewart Islands (Wodzicki, 1950).

Red deer expanded their range considerably between 1924 (Forbes, 1924), 1947 (Wodzicki, 1950) and the present time (Fig. 8). They now occupy almost all forested areas. Exceptions in the North Island are parts of the East Cape area, North Auckland, Coromandel Peninsula and the Mount Egmont area. In the South Island the area between Franz Josef and Fox Glacier remains free of deer (Mr K. Miers, pers. comm. 1982).

Most of the deer are found in native forests but substantial numbers live in Pinus radiata forests, e.g. Kaingaroa Forest. The steadily increasing deer population had a dramatic effect on the native vegetation, which had evolved without the presence of browsing mammals. The structure of some forests has been modified by the destruction of the lower tiers of vegetation. By selective browsing the animals have altered the floral composition, with browse-resistant plants replacing palatable species (Howard, 1965). Accounts of the effect of high deer populations on native and exotic forests were given by Wodzicki (1950), and Clarke (1972) who described the habitat destruction resulting from the presence of deer. Possums, by defoliating forest canopies open up the forest, make it less impenetrable for deer, which inflict further serious damage (Wallis and James, 1972).

Red deer were protected after their first liberation in 1851. The damage to vegetation, which became apparent in many parts of the country, led to the removal of protection in 1930 and a year later to the implementation of deer control. This was carried out all over the country firstly by shooters of the Department of Internal Affairs and later of the Forest Service. From 1931 to 1968 a total of 1,067,434 deer were destroyed by Government shooters, although plenty of deer still remained in the forests.

By 1968, because of rising velvet and venison prices, commercial interests had largely taken over the control of deer. This led to a drastic reduction of deer numbers, mostly in the South Island but also in the North Island. According to Dr I. Atkinson (pers. comm. 1982) the removal of large numbers of deer had the most marked effect on the vegetation in alpine areas. As deer move between the forests and the alpine areas the effect is also noticeable in the forests although less pronounced.

Hunting by helicopter has allowed a feral and noxious animal to become a source of a considerable revenue. The total export of venison (feral and farmed) in 1979-1980 amounted to $5.9 million, a total of 1,063 tonnes of which 100 tonnes were farmed venison. (1981 New Zealand Yearbook). West Germany is the main market for New Zealand feral venison which is considered to be of the finest quality.

The high prices obtained for New Zealand deer products on overseas markets provided the incentive for the establishment of over 3,000 deer farms (Fig. 9). It is estimated that a total of 1/4 million deer are present on page 96
Fig. 8. Distribution of the Red Deer (Cervus elaphus) in New Zealand 1947-1970. (After Wodzicki, 1950 and Harris, 1970.)

Fig. 8. Distribution of the Red Deer (Cervus elaphus) in New Zealand 1947-1970. (After Wodzicki, 1950 and Harris, 1970.)

New Zealand farms that vary in size from 4 hectare blocks to properties capable of stocking several thousand deer (Mr D. K. Yerex, pers. comm. 1982).

Red deer comprise 90% of the total farmed deer population while fallow and wapiti constitute the remainder. The red deer are the easiest species to handle and provide the most weight. Initially farms were stocked with wild deer caught from helicopters but now most stock is farm bred (Mr D. K. Yerex, pers. comm. 1982).

New Zealand farmed venison is sold overseas, mainly to Australia and America, countries which require licensed slaughter premises and meat inspection.

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Fig. 9. Red Deer (Cervus elaphus) farmed at Mesopotamia Station on the Southern Banks of the Rangitata. (Taken from the New Zealand Deer Farming Annual, 1976.)

Fig. 9. Red Deer (Cervus elaphus) farmed at Mesopotamia Station on the Southern Banks of the Rangitata. (Taken from the New Zealand Deer Farming Annual, 1976.)

It is interesting to note that a wild, previously noxious animal species has not only become a source of considerable revenue but has been turned into a new domestic breeding stock.

Other Deer Species

Six deer species that have been introduced as game animals are of minor importance to New Zealand. They include the sika (Cervus nippon), virginian (Odocoileus virginianus), fallow (Dama dama), wapiti (Cervus page 98 canadensis), sambar (C. unicolor) and the javan rusa deer (C. timorensis). These species were introduced at various times between 1864 and 1907 and are established in various localities in both islands with varying degrees of success (Wodzicki, 1950) (Fig. 10).

Perhaps the most successful species is the sika deer which has increased both in density and range. Because it is an intensive forager and can inhabit areas where food is limited, the species has managed to colonize areas where red deer are present (Mr K. Miers, pers. comm. 1982). Sporadic control operations attempt to keep the deer at manageable levels (Harris, 1970).

Wapiti which is restricted by natural barriers to an area in Fiordland, South Island, has continued to expand within this area since its release in 1905 (Meirs, 1970). The species is closely related to the red deer and hybridization occurs. Unfortunately the presence of the wapiti in Fiordland is thought to inhibit the expansion in range of the endangered native takahe (Notornis mantelli) and recently some wapiti have been removed from Fiordland. The private consortium responsible for the removal of the deer will receive half of the wapiti caught and these will probably be sold to deer farmers. The remaining deer are to be liberated in another area in the South Island for trophy hunting.

The distributions of both the virginian and fallow deer have remained static since 1950 although their densities have declined. Virginian deer are still present at Lake Wakatipu, in Fiordland and on the coastal fringes of Stewart Island where they have flourished (Harris, 1970). The Forest Service periodically carry out control programmes in an effort to reduce the deer populations (both red and virginian) on Stewart Island. Some fallow herds are presently on privately owned land and are thus afforded some measure of protection (Miers, pers. comm. 1982). This species is also used in deer farming although to a lesser extent than red deer.

The remaining sambar deer present in New Zealand are still found near their point of liberation. Because of their diminishing numbers they are now protected by the Forest Service in one area (Mr J. B. Henderson. pers. comm. 1982).

The Javan rusa deer occupies the smallest range of deer in New Zealand, being present only in shrubland in the Rotorua District (Miers, pers. comm. 1982). They were accidentally introduced into New Zealand as sambar deer in 1907 and it was not until 1955 that they were recognized as a separate species (Harris, 1970).

Wherever any of these six species occur there is a noticeable alteration in the species composition of the flora. Fortunately, because of their limited distribution and small population densities their impact on the vegetation is slight compared to that of the red deer (Howard, 1965).

All the species mentioned are highly regarded as trophy animals for various reasons. The fallow buck is sought after for its palmated antlers and fine skin while the sambar is considered by many sportsmen to be the ultimate trophy because of its natural cunning (Harris, 1970).

In addition to the aforementioned species two other deer species were liberated in New Zealand, although both have now died out. The axis deer (Cervus axis) introduced during the 1860's and early 1900's, was last seen in Fiordland, South Island in 1948 (Harris, 1970). The moose (Alces americanus) was also liberated in Fiordland and in the early years following its introduction appeared to be successful. By 1924 the species was considered to be very scarce. The moose has not been recorded since 1954 when a single bull was shot at Wet Jacket Arm, Fiordland (Harris, 1970).

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Fig. 10. Distribution of Other Deer Species in New Zealand 1947-1970. (After Wodzicki, 1950 and Harris, 1970).

Fig. 10. Distribution of Other Deer Species in New Zealand 1947-1970. (After Wodzicki, 1950 and Harris, 1970).


The aim of this paper has been to decribe the effects of man on the New Zealand environment brought about by the introduction of exotic birds and mammals. (See Table 1).

In trying to assess the present situation it is important to consider three points. Firstly, with regard to mammals, the changes between the late 1940's (Wodzicki, 1950) and 1980. Secondly, to view the presence of the introduced vertebrates in the light of the growing consciousness among the page 100 New Zealand people of the conservation of fauna and flora. And finally the present economic aspects of some introduced animal species and the methods of their control.

The thiry-three species of introduced birds which have become established in New Zealand are now widely accepted as a part of the New Zealand avifauna. The majority of species arrived when the New Zealand landscape was undergoing modification from forest to pasture. As a result many new niches were established which could not be filled by the native species, but suited the new arrivals. Because of a lack of inter-specific competition a large number of these immigrants have become more successful in New Zealand than in their county of origin (Dr P. C. Bull, pers. comm. 1982).

The demise of the native bird fauna is generally accepted as being attributable to the destruction of the native forests and wetlands and the introduction of predators. However, some early reductions of native birds in unmodified forests have been attributed to diseases introduced by the exotic birds. Interactions between native and exotic species are limited. Only a few introduced species occur in the native forest and there is no indication that these species “drive out” native species.

Some species of introduced birds such as the rook (Corvus frugilegus) have become pests. Others such as the starling (Sturnus vulgaris) are useful in controlling invertebrate pests such as the grass grub (Costelytra zealandica) although are a nuisance to orchardists.

Of the six successfully introduced gamebirds the mallard and the Canada goose have benefited greatly from the change in land use for farming purposes and their populations have expanded at an astonishing rate.

The self-introduced birds from Australia owe their colonization success to man. In the past many birds have probably arrived in New Zealand but have been unable to settle as the environment was unsuitable. When European man arrived and modified the environment he provided new habitats previously unavailable. It is therefore likely that New Zealand can expect more additions to the avifauna from Australia.

The introduction by man and establishment of 32 species of exotic mammals is a fact of considerable ecological and economic importance for New Zealand. New Zealand provides an interesting example of the effects, detrimental or otherwise, of introduced mammals in an ecosystem unused to either herbivorous or predatory mammals.

In comparing the ecological and economic situation of the introduced mammals between the late 1940's and the present, we see that their distribution has not changed much during these 35 years. One species (possum) has increased nationally, 13 species have increased locally (red, sika, fallow, virginian, Javan rusa, sambar and wapiti deer, goats, chamois and thar, dama, brush-tail wallabies and hedgehog). The distribution of 14 species (three species of mustelids, three species of rats, mouse, hare and rabbit, feral cat and pig, parma, red-necked and swamp wallabies) appears not to have changed significantly. Finally four species (black-striped wallaby, feral horses, cattle and sheep) have decreased in range and numbers.

For more than a century man has been conscious of the adverse effect of introduced mammals on both the man-made and the native environment and a good deal of research has been devoted to these problems. As early as 1867 rabbits were known to compete successfully with sheep leading to a depletion of pasture and eventually to soil erosion (Wodzicki, 1950). The page 101 effect of red deer on the vegetation and soils in forests was not recognized until much later. Deer by selective browsing prevent regeneration of palatable principal crown plants (Howard, 1965). The possum, which can survive in a variety of habitats is a serious threat to our native flora. By selective browsing the possum can open out a forest and change its floristic composition.

Finally, a study of the impact on the native vegetation by red deer (Cervus elaphus) and the possum (Trichosurus vulpecula) by Veblen and Stewart (1982) attempts to separate animal-induced changes from other types of vegetation change.

Another important aspect of introduced mammals in the New Zealand environment is the effect of rodents (ship and Norway rats) and carnivores (feral cats, weasels, stoats and ferrets) on the bird life. All native bird species in New Zealand have evolved in an ecosystem devoid of mammalian predators and as such are particularly vulnerable to any that are introduced.

Looking further on the essential changes which occurred between the late 1940's and present there has been a substantial transformation in the attitude of the public to the problems of introduced mammals. Today there is little emphasis on the “last rabbit, last deer” policy of the 1930-1968 era, as the public have come to accept the introduced mammals as part of the wildlife scene of New Zealand. This fact combined with a general increase in public awareness of nature conservation has lead to some interesting examples of introduced mammal management. The removal of some wapiti from Fiordland National Park and the protection of sambar deer in one small area are recent examples of this new attitude.

A new and important development in the management of some introduced mammals has been the innovation of helicopter hunting for deer and goats. An increase in the revenue from wild venison sold in Europe and velvet sold in Asia, led to the use of helicopters as a hunting tool. As a result there has been a substantial decline in wild deer numbers throughout New Zealand. However, this indirect form of control will continue only while high prices are paid for venison in West Germany. If the economics of our wild venison fail the uncontrolled deer populations could increase rapidly.

Within recent years the farming of certain wild mammals has attracted much money and attention particularly in the case of deer. At present deer farms number at around 3,000 and contain approximately 250,000 animals.

Unlike deer, the possum has not been controlled by hunting despite the skins being a valuable export article. Unfortunately good exportable skins usually come from areas with light possum populations and not from areas where control is required.

The Pest Destruction Council has relpaced the Rabbit Board and through its Boards controls rabbits, hares, wallabies, possums and rooks.

Finally, the concluding words may be devoted to a new and interesting aspect of introduced, feral farm animals (Dr Rudge, pers. comm. 1982): genetic conservation. The genetic base of our domestic animals is narrowing as breeding programmes reduce the range of genes available. Some of the feral farm animals display advantageous characteristics such as footrot resistance in sheep and these and less important genes should be preserved so that they can be drawn upon by animal breeders for future domestic stock. Unfortunately many New Zealand feral animal herds have already been exterminated.

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We would like to record our sincere gratitude for the invaluable assistance and comments provided to us by Dr I. A. E. Atkinson and Dr A. P. Druce, Botany Division, D.S.I.R., Mr J. A. Bartle, National Museum, Mr H. A. Best, and Mr T. A. Caithness, Wildlife Service, Department of Internal Affairs, Dr R. E. Brockie, Dr P. C. Bull, Dr B. M. Fitzgerald, Dr J. E. C. Flux, Dr J. A. Gibb and Mr R. H. Taylor, Ecology Division, D.S.I.R., Mr J. B. Henderson, President of the N.Z. Deerstalkers Association, Dr C. M. King, Mr Ken Miers, Director, Environmental Forestry, Mr Peter Nelson, Senior Field Officer, Agricultural Pests Destruction Council and Mr D. K. Yerex.

We are grateful to the Cartographic Section of the Science Information Division, D.S.I.R., and the Photographic Section, Department of Zoology, for their co-operation and use of their facilities.

We would also like to thank Dr R. E. Brockie, Dr B. M. Fitzgerald and Miss C. J. T. Alderton who kindly read and commented on the typescript.

The typing of the drafts was kindly undertaken by Mrs J. Bedggood, Miss N. Black and Mrs M. Penning.

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