It must not be assumed that every one regards the rabbit as a nuisance. Many a successful farmer of to-day got a start as a rabbiter. The killing of rabbits actually became one of the principal industries of the province. Their presence directly led to the subdivision of large estates, and may have been quite as effective in this direction as all the legislation on the subject. Since the war rabbit-skins have become extraordinarily valuable, so that, instead of landholders paying for the destruction of rabbits, rabbiters offer premiums for permission to go on to land to trap the rabbits.
The introduction of rabbits had a lasting effect on acclimatization generally. Before their advent partridges and pheasants had become numerous, but they have entirely disappeared in Otago. In the effort to cope with the rabbits the country was annually sown with poisoned grain. This had a disastrous effect on both native and imported game. Had rabbits not become a nuisance it is unlikely that weasels and other vermin would have been introduced. These animals are largely responsible for the decrease in the numbers of native birds, and also make the successful introduction of new varieties more difficult.
The economic waste caused by the vast increase of rabbits in New Zealand is incalculable, and certainly represents a loss in the stock-carrying capacity of the country which probably runs every year into millions of pounds. It is not only that they eat up food which would support some millions more sheep than are at present reared, but they destroy large areas of country, and yield very little return for the damage they do. The annual export of approximately three million rabbits, valued at (in pre-war times) about £70,000, and of some eight millions of skins, valued at about £115,000, is all the return they give, but it represents only a small proportion of the pest. In all parts where rabbits abound their destruction entails a heavy expense on the occupiers of the land. There are no data available to enable any one to estimate how many rabbits are destroyed every year, but far more are killed by phosphorus than by trapping. The latter method alone furnishes any statistical data; the former is an unknown quantity, but it represents a very large figure.
Probably the most ghastly exhibition of the work of rabbits is to be found in the grass-denuded districts of Central Otago, parts of which have been reduced to the condition of a desert. It is improbable that this state of affairs could have been brought about by rabbits alone. Before their advent the runholders who had possession of the arid regions, in which the rainfall probably averages 10 in. to 12 in. annually, and certainly never exceeds 15 in., were doing their best to denude the surface of the ground by overstocking with sheep and by frequent burning. The latter was resorted to because many of the large tussock-forming grasses, especially such as the silver-tussock (Poa caespitosa), yielded coarse and rather unpalatable fodder, but after burning the tufts a crop of tender green leaves sprung up, which were very readily eaten. Unfortunately the burning not only got rid of most of the coarse growth of the tussocks, but it also swept off the numerous bottom grasses which occupied the intervening spaces, which were the mainstay of the depasturing flocks. Even before the rabbits arrived the work of denudation of the grass covering had been proceeding apace through the causes mentioned. Thus Buchanan, writing in 1865, said, “It is no wonder that many of the runs require 8 acres to feed one sheep, according to an official estimate.” Mr. Petrie thought this an unduly severe estimate, “as in the mid-‘seventies’ the sheep-runs of Central Otago were reputed to carry at least one sheep to 3 acres, or somewhat less.”
Mr. Petrie, who has reported to the Department of Agriculture on these grass-denuded lands of Central Otago, knows more about this subject than any one else who has written on it, and I quote him at some length. He says, “Before the rabbit invasion began the hill-slopes carried a fairly rich and varied covering of tussock and other grasses, and, except on the steeper rock and sun-baked faces, had not been seriously depleted even in the early ‘nineties.’ The earlier stages of this depletion may now be seen in several of the Central Otago ranges, as on the spurs of the Rough Ridge and the Morven Hills districts. The northern slopes of the spurs are almost, in many instances, entirely bare of grass, while the southern shaded slopes still carry a fair amount of pasture. The grass covering generally stops abruptly at the bottoms of the valleys, even when those are not worn into water-channels. The vastly greater depletion of the pasture on the northern slopes is easy enough to understand. They are more exposed to the sun and to the frequent violent parching north-west winds; they lose their covering of snow earlier in spring than the southern slopes, and are thus more closely grazed at a critical season for the pasture; and sheep at all times show a preference for feeding on the warmer sunny slopes. When the pasture on the exposed slopes fails, that on the shaded slopes has to feed all the stock that is about, and unless the stocking is reduced to meet the new conditions the remaining grasses are sooner or later eaten out. The desert, with all its problems, is then established.”
In his account of how desert conditions arise in Central Otago Mr. Petrie refers only to the effects produced by sheep, because it is the loss in sheep-carrying capacity which is so serious; but later on, after describing a typical specimen of the country, and showing that in inaccessible situations a considerable variety of vigorous grasses live on, he adds, “This is one of the facts that go to indicate that the extermination of the grasses in this desert country is mainly due to eating out by overstocking, rabbits as well as sheep being included among the stock carried.... The desert and the greatly denuded lands are not wholly destitute of vegetation. In most of their lower areas greyish, flattened, firm, nearly circular patches of scabweed (Raoulia australis and R. lutescens) are thickly dotted about the bare ground. Though otherwise useless, these moss-like composite plants help to keep the soil from being blown or washed away, and when old supply, in the decayed centres of the patches, spots with some amount of humus where grass-seeds can more readily settle and grow.” These plants are never eaten by either sheep or rabbits.
In regard to other native plants, rabbits have nearly exterminated the wild spear-grasses (Aciphylla squarrosa and A. Colensoi), which used to be so abundant. They particularly attack these plants when the ground is covered with snow. Mr. Petrie, writing me three years ago, said, “When I first visited inland Otago, in 1874, Aciphylla Colensoi was most abundant. In riding about it was almost impossible to deviate from well-beaten tracks or roads because the spines pricked the legs and feet of the horses.” In later years these plants have become rare. Captain Hutton, writing me in March, 1892, said, “As to the extermination of the wild-spaniards (Aciphylla), I believe it to be due to rabbits. When I was in the Nelson District in 1872–73 there were no rabbits on the eastern side of the Upper Wairau near Tarndale, but they were abundant on the western side. Spaniards were abundant on the eastern side, but almost destroyed on the western. The rabbits seemed to burrow under the plants, and then eat the roots.”
Several species of Celmisia (notably C. densiflora) have been greatly checked, and others are almost exterminated. Mr. B. C. Aston, in his ascent of the Kaimanawas in 1914–15, found that at a height of 4,200 ft. Panax Colensoi was nearly exterminated by rabbits, which had ring-barked all the young trees. This mischief is done after heavy falls of snow, when the rabbits are driven down from the tussock-land into the gullies of the scrub and forest zones. Trees of Panax Edgerleyi from 19 ft. to 20 ft. high were found to be ring-barked and dead.
In a good many rabbit-infested districts, particularly in the North Island, these animals have aided very materially in producing a certain amount of erosion and washing-down of alluvium by burrowing extensively in the banks of rivers and small streams. When floods came down, these undermined portions were commonly swept away where the firmer banks resisted the impact of the water. Dr. C. A. Cotton, of Wellington, considers that this action has caused a slight rejuvenation of erosion in certain districts and river-systems. Cattle, sheep, and goats assist in this work, but rabbits are the most active agents in it. The Rev. A. Don, writing to me in 1901, said, “The rabbits, by stripping the ground of vegetation and burrowing into the faces of the slopes, are converting what were once nice green hillsides into shingle-slopes, because when once the face is so bared and its surface broken it begins to slip.” Mr. Petrie also refers to this process in his report, as follows: “The soil on the grass-denuded slopes, which is by no means infertile, being no longer held together by the roots of plants, is being rapidly removed by wind and rain, and pebbles and angular stones are now closely dotted over great stretches of hillside that not many years ago were covered with soil. On the steeper slopes, indeed, the soil is being rapidly sluiced down into the gullies and thence into the river, and deep, narrow, chasm-like watercourses are being dug out.”