THE RABBIT PEST
POTENTIAL INCREASE AND MIGRATION. (BY MR D. MUNRO, INSPECTOR OF STOCK, WANGiANUI.) (In the New Zealand Journal of Agriculture.) The present article is contributed in the hope that my experience may be of use to settlers, particularly those situated in districts where rabbits have not yet become permanently established, and wlho have therefore not had the opportunity of becoming acquainted with the peculiarities and possibilities of the rabbit. One might suppose in this country,
where the rabbit menace has driven numbers of men off the land and has been respon ible '.for such, a great national loss, that every settler would be conversant with the subject, and at least have sufficient knowledge to ef-
fectively prevent the pest from making further encroachment to clean territory. This, however, is not the case. It is remarkable to find how little is known concerning the rabbit in districts where it has not actually become a nuisance, and I believe that it is largely due to this lack of knowledge that the pest is still permitted
to spread on to new country. I am firmly convinced that if those settlers in infected areas who have had experience of the rabbit nuisance could be given 'the opportunity which settlers in clean or comparatively clean districts still enjoy they would go to the very limit of their resources rather than allow the pest to gain a footing. Yet in districts where there are only a. few small scattered colonies one frequently hears the statement made (generally by men who have no knowledge of the subject) that rabbits will never do any harm in their particular district, the reasons given being that there is too much grass, the country is too rich or too wet, the subsoil too hard to burrow in, the country too closely settled, etc.—all reasons which I-heard thirty years ago in districts which are to-day rabbit-infested areas. There is no part of New Zealand in which it is either too wet, too dry, too hot, or too cold for the rabbit to prove a very serious nuisance if given the opportunity of becoming established. It would be a very doubtful recommendation to any country to say that it will not carry rabbits.
An argument which is frequently put forward in such districts, and one which appears to carry a good deal of weight with some settlers, is that odd rabbits have been seen for many years, but that they have never increased to any extent—facts which they consider to be ample proof that for some reason or other the country is not suitable for rabbits, and that there is therefore no cause for alarm. There appears to be an impression prevalent in the minds of settlers inexperienced in the ways of the rabbit that for an invasion the pest is going to advance in massed 'formation of battalions and divisions. I have heard it said by settlers in rabb'it-infested
districts that for several years there were only a few rabbits, and then quite suddenly .they came in thousands, ©uclh statements, no doubt, are largely responsible for the mistaken idea that the pest suddenly migrates en masse to new country. If such were the case one would naturally expect to find a corresponding diminution in the country from where they came; but a diminution of th'is nature is never to be found. These odd pairs which appear in new country, and which to the casual observer may not be a matter of any great concern, do and always will constitute a very real and serious danger to the district in which they are allowed to exist. The slow progress made by the pest in the earlier stages of colonisation gives a certain degree of confide ence to the inexperienced settler, and misleads him dnto the belief that the country cannot be suitable. There may not be any appreciable increase for several at any moment the pest may overcome certain forces which have been | operating against it, and from then on will increase at an alarming rate. Such is the early history of the rabb'it in almost every district in New Zealand where the pest has ultimately become established.
This process or period of colonisation is necessarily governed to a very large extent by the opposition offered. In the early period of the rabbit in New Zealand there was little or no opposition offered to its increase. The natural enemies were few in number—stoats, weasels, and ferrets had not then been imported—consequently the colonising process was more rapid. Now, and for many years past, there have been, and are, thousands of these animals in every district in which the rabbits have sought to extend operations. The pioneers -are therefore met with a much greater degree of resistance, and it may take several years before they have reached that point when they are able to satisfy all the demands of their natural enemies and sportsmen and yet have a surplus to carry on freely the process of reproducing their kind. This point may be reached as the result of several causes or by a combination of causes. Principal among the forces acting
against the rabbit in the earlier stages ■is the natural enemy—stoats, weasels, and ferrets—all animals which are particularly susceptible to distemper, a disease which to them almost invariahly proves fatal. Oats are also susceptible to this disease. It is quite possible, therefore, that as a result of an epidemic among the natural enemy, combined perhaps with a particularly favourable season, the rabbits are able to reach that point just mentioned when they have gained the balance of power—a point which will mark the beginning of the rabbit nuisance in the district concerned unless the settlers very quickly take a hand in the game. Potential Increase of Rabbits.
In order to fully appreciate the danger of the pest it is necessary to realise the extent to which rabbits are capable of increasing in a given time, and I have worked out a propagation chart showing in detail what may be considered a reasonably conservative estimate of the increase from one pair of rabbits in one, two, and three years. Many authorities may be quoted in support of the figures on which I base my calculations, among others, the Encyclopaedia Britannica, " Treasury of Nature" (Samuel Mander), "Veterinary Posology" (Banham), page 251, "Veterinary Obstetrics," and Harmsworth's Encyclopaedia (this latter authority stating that one pair of rabbits left undisturbed for three years would breed to over thirteen millions). All these authorities agree on the main 'issue, that is, the period of gestation, average litter, etc.
It may be also mentioned that personally I have had over thirty years' practical experience in dealing with the pest, during which period I have had the best of opportunities of studying the habits and peculiarities of the rabbit. My experience has not been limited to any one locality; during the thirty years of my departmental service I have had occasion to deal with the pest in many districts in both islands and in all classes of country. Moreover, I have had the benefit of exchanging opinions on the subject with hundreds of practical rabbiters and settlers full of experience. The authorities named deal more particularly with the rabbit in European countries, but, like many noxious plants, birds, and animals, the rabbit has under the more congenial climate enjoyed -in New Zealand and Australia speeded up very considerably in the matter of reproduction. There is no doubt that the number of litters which a doe will produce in ft season is to a large extent regulated by climatic conditions, a dry, warm season being, of course, most suitable. This point is well demonstrated in New Zealand, where climatic conditions vary a good deal. In the southern part of the South Island, where the winter months are more severe, there is a distinct break in the breeding seasons of from three to four months. In Southland, Otago, and South Canterbury there is practically no breeding from the beginning of May to the middle of August; but in parts of the North Island, particularly on the coastal country, where the 'winter is usually very mild, it is not uncommon to find numbers of nests 1 and young rabbits all through the winter months. Under ideal conditions it is easily possible for a doe to have ten litters in the year (the period of gestation being twenty-eight
to thirty days), and from records kept over a lengthy period I have found that the average number in the litter is six. In the early spring months the average is; approximately seven, but after November, when the young does come into bearing, the average litter will be found to drop to -about six. I have seen on two occasions a litter of fourteen, and litters of ten to twelve are not uncommon. During last spring I noted that in this district (Wanganui) the main breeding season commenced about the last week in August; quite a number of nests were found and destroyed, and after the first week 'in November the majority of young does were found to be in an advanced state of pregnancy or had already given birth to a litter. Assuming, then, that the average litter is six, that each doe will produce in the year eight litters of equal sexes, and that the young does will breed at fifteen weeks, the total increase for the year from * one pair would be 534. This total is contingent upon there being no casualties; but, allowing that from all causes 50 per cent of this increase will become casualties before the beginning of the second year, the second breeding season would thus commence with a total of 267, plus one of the original pair—--268. Of this number, half—l34—are does, each producing an increase of 534 in the season; 534 plus 134 equals 71,556. The casualties during the second year and succeeding years would be less than in the first year, as the requirements df the natural enemy would not increase in proportion; but, allowing that the casualties still remain at 50 per cent, the third year would start with half of the second year's increase—3s,7Bß, plus 134 (with which the second season started) — 35,912. Half of these are does, or 17,956 does, each giving an increase of 534 in the year; 17,956 multiplied by 534 equals 9,588,504. To this must be added the number at the beginning
of the season —35,912—showing a total of 9,624,416 at the end of the third year.
It will thus be seen that it is reasonably possible for one pair of rabbits to increase in three years to between 9,000,000 and 10,0000,000. This is a fact which it would be well for every settler to fix very clearly in his mind. The figures stated present an interesting problem. It is estimated that six or eight rabbits will eat or destroy as much grass as would graze one sheep; but, allowing that ten rabbits equal one sheep, it is evident that one pair of rabbits is capable of reducing in three years the stock-carrying capacity of our lands to the extent of a million sheep. This affords some 'indication of the enormous annual loss which the rabbit pest .may represent. Control Work. In infested districts the main efforts for the destruction of rabbits are generally made in the winter months, when the skins are of most value, and the approximate cost of the rabbits', destruction then is not less than 3d per head. The cost of destroying 534 rabbits at 3d per head equals £6 13s 6d. It would therefore have paid the settler to have spent this amount in destroying the parent pair at the beginning of t)he season; £he feed saved would represent a profit on the transaction. This point is mentioned merely to illustrate the importance of •control work at the proper season of the year. Poisoning, which is quite undoubtedly the best and cheapest wholesale method of destroying the pest, should be carried out as early as possible in the season, and so allow as much time as possible between the poisoning and the beginning of the next breeding season. During the interval every possible effort should be made to clean up those rabbits which will not take the poison. When a general poisoning is necessary it is essential, in order to obtain the best results; that the rabbits should not be disturbed by dogging, trapping, or shooting for at least two or three months before the poison is laid.- It has been my experience that where rabbits are hunted right up to the time of poisoning the results are never so good as when they are allowed to settle down for, say, two or three months before. ,
In a later issue of the Journal I propose to deal briefly with the various methods of destroying rabbits, and with other related matters which may be of interest to settlers, particularly those in districts where the pest has not yet become permanently established.
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Bibliographic details
Waipa Post, Volume XXIV, Issue 1574, 15 November 1924, Page 2
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2,175THE RABBIT PEST Waipa Post, Volume XXIV, Issue 1574, 15 November 1924, Page 2
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