Aliens in the Forest

Forest and Bird, Issue 107, 1 February 1953, Page 8

Aliens in the Forest

Fi. The Wild Cat Problem]

By

P. C. BULL

'T'HIS is the first of a series of articles on alien animals which have penetrated our A forests as the result of colonization by man. It is contributed by Mr. P. C. Bull Zoologist, Animal Ecology Section, Department of Scientific and Industrial Research.’ Accompanying it is a photograph showing the size to which the feral cat can grow if circumstances are favourable. It is an interesting and informative article. The Society, while consistently condemning the practice of releasing cats into the bush to become pests in a habitat not naturally adapted for mammals, welcomes any effective scientific investigation into the wild life problems of this country

For the purpose of the present article wild cats may be defined as cats which no longer depend on man to provide them with food and shelter. They are merely domestic cats gone wild and should not be confused with the true wild cat of Europe which is a different species and does not occur in New Zealand. There is every gradation in this country between the much-fondled house cat and the cat born of wild parents and living itself far back in the bush. We know from the writings of the early naturalists that there have been wild cats in New Zealand from .the earliest days of settlement. We presume that such animals have descended from cats deserted when the temporary camps of prospectors, bush- - men, sealers and others were abandoned, and it is well known that farmers used to buy up cats in the towns and liberate them on their farms to kill rabbits. In more recent times cats have been left on some of the mutton bird islands off Stewart Island as a rat control measure or through carelessness. That cats can survive and reproduce in the wild is shown at Little Barrier Island where there are still wild cats although no animals have been liberated there for more than half a century. Wild cats are . present over wide areas of the three main islands of New Zealand, but except locally, they are in relatively small numbers which conforms to the well known fact that predators exist in much smaller numbers than do their prey. Wild cats are found in a wide variety of habitats such as farm paddocks, hedgerows, plantations, sand-dunes, scrub and dense bush. They also occur on several of the outlying islands of New Zealand. Their food probably in- • eludes birds, rats, mice, small rabbits, lizards, fish and invertebrate animals but little is known of the relative importance of the various items-of the diet. Recent work

overseas makes it fairly clear that, of the foods suitable for a given carnivorous animal, those which are most readily available are taken. For instance a study of feral house cats in California showed that where .small rodents were abundant they were taken more often than were birds. Most readers of “Forest and Bird” will be interested in the effect of wild cats on our native birds, and it is the purpose of this article to discuss some of the facts which bear on this complicated problem. There is abundant evidence that cats kill birds, and for the individual bird, a cat is certainly an unmitigated menace. The real problem is to determine what effects cats have on the species as opposed to the individual. The problem is different for each species of bird and for each locality. For common species such as blackbirds and silvereyes cats are probably of little importance even though they kill a large number of birds—especially young birds. These species at present produce more young than the environment can support and a large proportion of the young are destined to die in their first year so, as far as the species is concerned, it makes little difference . whether they are killed by cats, shot by orchardists or die of starvation.

Most of our native birds are in a different category. For a number of reasons, of which cats are only one, many of them - are so greatly reduced in numbers that they do not appear to occupy fully the habitats available to them and every bird killed is therefore a real loss to the species. Evolved in the absence of carnivorous land mammals, many of them are ground-frequent-ing and lack the constant' alertness so typical of most of the introduced birds. Under such circumstances it is .probable that cats are a serious hazard to the continued existence of such species. On the

main islands, however, cats have been present for so long that the harm they are responsible for is already done and the mammals and birds appear to be gradually reaching a state of balance. It must not be forgotten that cats are only one of the several introduced predaceous mammals and that these have effects on each other as well as on . the birds. Indeed, .GuthrieSmith, arguing from the fact that near houses where cats are kept the nest of various rails are less molested by rats, believed that the influence of cats may at times be actually protective. It is not possible to exterminate wild cats from the. main islands and, even if this were possible, it would be most unwise to do so until the relationships between cats, rats and birds are more fully understood. If a balance is in fact being established it is essential that nothing be done to upset it, and in this connection the practice of liberating unwanted domestic cats in the country is to be deplored. Such a practice will result in the cat population being higher than the environment can support, and until the' wretched animals die there will be increased predation on native birds and game birds. There can be no excuse for this cruel evasion of responsibility when facilities exist for the humane destruction of unwanted pets.

Finally, I must mention the effect.of cats oil the birdlife of our off-lying islands because it is here that tragedies have occurred in recent times and more may follow unless everyone is watchful. The birds on such islands are often different from their mainland relatives and they are of great scientific interest. Many of the populations are extremely small—only a few dozen individuals in the case of the Chatham Island robin. . Such species as the saddleback, stitchbird and shore plover are extinct or almost so on the mainland and the last survivors are located on some of the less-accessible off-lying' islands. A single cat allowed ashore by a careless yachtsman could cause their extermination. This is demonstrated by the recent tragedy at Herekopare Island near Stewart Island. When Guthrie-Smith visited this island in 1911 he found an extraordinary abundance of birds. - Cats became established some twenty years later, and when the island was visited by Mr. Richdale in 1941, six species of native birds had become extinct and the populations of the remaining three

-species had been decimated. A much older example is provided by the Stephen Island wren which is known from only a few specimens brought in by the lighthouse keeper’s cat in 1894. The bird was apparently never numerous and it was confined to this one small island where its ground-frequenting habits made it an easy prey for the cat which apparently exterminated it. Little Barrier Island presents a slightly different example, for this sanctuary is,still renowned for its wealth of birdlife despite the long established cat and rat populations. The ground-frequenting robin exists there, although it is not particularly numerous and there are special conditions which may assist it to exist despite the cats. In summer, the cats are probably attracted to the ridgetops by the easily

caught Cooks petrels which come ashore to nest in large numbers, and this may allow the robin some respite to rear its young in the gullies more or less free of cats. Saddlebacks were once present on the island but became extinct. Attempts to reintroduce them failed although the species holds out on Hen Island where there are no cats.

To sum up I consider that where longestablished cat and rat populations are both present no action should be taken against the cats until the relation between them and the rats is fully understood. We know that there is a healthy avifauna on Little Barrier, but we can only guess at what would happen if we removed the cats and

left the rats, and our guess might be wrong. Where there are neither cats nor rats every effort should be made to keep them out. There are still a few cat-free islands where rare birds have their last stronghold and such islands should be inspected regularly so that any immigrant mammals can be dealt with before they become thoroughly established. We have missed several chances and there are not many more left to us. The cat problem then, is first to prevent these animals becoming established' on the few islands remaining free of them and secondly to obtain precise information on feeding habits of predaceous animals with respect to their effects on the birds and on each other.