Research on stoat control in forests

Forest and Bird, Volume 14, Issue 3, 1 August 1982, Page 37

Research on stoat control in forests

By

C. M. King

MOST PEOPLE know that stoats are small, rather fierce carnivores of the Mustelid family, which were introduced to New Zealand farmlands in the 1880s to control rabbits. Most will also know that they failed in this task and instead spread into the forests, where they are now common and eat many birds. The possible effects of their preying on native birds are a source of great concern, especially in the national parks.

AS REQUIRED under the National Parks Act of 1952, park rangers have made some attempts from time to time to control stoats, but without any way of knowing whether their efforts were efficient, effective, or even necessary. There was obviously a great need for some basic information about the biology of stoats in national parks; so I began a general survey in 1972. Rangers in all the parks coOperated most generously in collecting dead stoats, in their own trap-lines and from opossum hunters, and so did the Forest Service meteorological officers on their daily rounds in Craigieburn Forest Park and Wildlife Service officers at Mount Bruce, Kaikoura, and Takahe Valley. By March 1976 more than 1600 stoats had been sent to me at the D.S.I.R. Ecology Division in Lower Hutt, not all in the freshest condition. Most were processed by J. E. Moody, surely the most patient and uncomplaining technician the D.S.I.R. has ever had. By mid 1977 all the laboratory work was done (autopsying, skull cleaning and measuring, weighing and histology of reproductive organs, and identification of gut contents) and all the data were entered into a computer. The analysis of this considerable amount of information took a year even in the hands of a most competent statistician, M. G. Efford, and

the writing and reviewing and rewriting of the seven papers describing the results have taken me and various expert critics a further 3 years. Now at last the papers are ready and are to appear in volume 9 (1982) of the New Zealand Journal of Zoology. They go under the general heading ‘‘The biology of the stoat (Mustela erminea) in the national parks of New Zealand’’, and they describe the food habits, body size, reproduction, moult and winter whitening, and two of the major parasites of the 1599 stoats included in this general survey. Though we ceased to collect stoats from the national parks generally in March 1976, we did not stop field work altogether. Instead we concentrated on three areas where we had some chance of unravelling at least some of the factors which control the populations of stoats in nature. If we are ever to control stoats artificially, we must first know what controls them naturally; otherwise we may waste time and money in attempting artificial control, which merely replaces natural control or is totally ineffective. Beech seedfalls From earlier observations it seemed that there might be a link between heavy beech seedfalls and increases in forest

rodents and thence from increases in forest rodents to increases in stoats. So in Craigieburn Forest Park and in two valleys in Fiordland National Park we collected information On: -peecn -Sseearal. eacn season), and on populations of rodents (quarterly) and of stoats (monthly), for most of the years between 1974 and 1980. It turned out that a good beech seedfall has quite predictable results. Most of the seed falls between March and June; by August mice and rats are increasing in numbers, largely because, when there is plenty of seed on the ground, the young rodents survive better than usual and the adult females continue to breed through the winter instead of stopping as usual after May. Most young stoats are born in mid to late October in the South Island, and in years when mice are numerous at that time, their mothers have plenty of mice to eat, and many more young stoats than usual are born or more that are born survive. The young stoats begin to appear "it "ihe 7iraps: 1p December, and in the December after a good seedfall the sudden increase in numbers of stoats caught (mostly young ones) can be staggering. But the peak does not last long: natural processes begin to cut down the numbers of young stoats as soon as they leave the nest, and by the following Summer most of the extra

young produced after the seedfall have disappeared. The same sequence of events followed good seedfalls in all three study areas, though less clearly in areas when there were Other kinds of food available (for example, rabbits) as well as rodents. In places like Mount Cook, where rabbits are much more important prey for stoats than mice, seasons of high numbers of rabbits have the same effect on the breeding success of stoats as seasons of high numbers of mice in beech forests. Implications for birds This sequence of events has important implications for the conservation of birds. First, in summers when stoats are numerous more of the common bush birds than usual are eaten by stoats, not because they eat more birds per head, but because there are many more stoats out hunting, each for the usual number of birds. The high numbers of mice available at the same time do not seem to be enough to distract stoats from paying their customary considerable attention to birds. Individual stoats eat many birds every summer, whether or not mice are plentiful; it is just that after a seedfall there are many more stoats around. second, the: chances of stoats finding the nests or young of one of the endangered mainland species that live near beech forests, such as the takahe, are greatly increased after a seedfall. Third, rats and mice also increase after a seedfall and both are known to interfere with nesting birds. It is important to note that we do not know what effect these events have on the breeding success or the population density of the common bush birds; we do not know if

the birds we find in stoats’ guts were killed or found dead, or what proportion of those killed would otherwise have survived and bred; we do not even know which are native and which introduced species. So it does not necessarily follow that control of stoats after a seedfall would have any beneficial effect on the bush birds generally. (Perhaps some day the field research will be done to find out.) Neither do we have any firm evidence of increased preying on less common or endangered birds after a seedfall; but for them it is perhaps less important to have actual evidence than it is to know when a special effort at predator control in their nesting areas would be worth while. Kinds of control Very many people would like to see stoats ‘‘controlled’’ (that is, reduced in numbers), at least in the national parks, and one of the long-term aims of my research has always been to help work out how far control is possible and, to the extent that it is, how best to go about it. There are two kinds of predator control. The one that most people mean when they talk about getting rid of predators is population control, the reduction by artificial means of the average level around which a stoat population fluctuates. To do this we have to reduce the breeding stock to some density lower than could naturally be supported in that environment. The other kind of control is damage prevention, which aims merely to reduce the number of stoats present at a time or in a place where they could be a particular nuisance whether or not the breeding stock is affected.

Both are laborious and expensive. In my opinion population control of stoats over any substantial mainland area is impossible with the means we have at present; damage prevention is possible if ornithological evidence can justify the expense. The reasons why population control of stoats is almost impossible are simple. Young female stoats are so precocious that they are sexually mature before their eyes open at about 6 weeks of age, and they are usually fertilised by an adult male before or soon after they leave the nest in midsummer. (Young males mature at 1 year old.) Adult females are fertilised for the next season at about the same time as their female young. So females of all ages leaving the family groups in January are already carrying an average of 9 or 10 blastocysts (early embryos), which, after 9 to 10 months’ dormancy, will resume their development to full-term young and be born in the following spring. Hence, even if every male is killed over the summer, when trapping is most effective, the next generation is already assured. Moreover, as females are usually more difficult to catch than males, more females than males escape the trapping and survive to bear their young. A single female could in theory re-establish the population by herself, but in fact she is unlikely to remain alone for long. Live-trapping work in Fiordland has shown that young males can easily disperse over distances of more than 20 km in the first few weeks of their independence in January and no doubt may travel further still in later months. So any attempt to reduce a local population of

stoats (except on a_ small island), is continually frustrated by the low chance of catching all the stoats present and the very high chances of immigration from surrounding untrapped areas. The average capture rate is low, especially in winter; in summer the capture rate may be higher, but then the traps are often removing (at some effort and expense) only the season’s surplus, which will soon be cut down by natural means anyway. I have heard it suggested that one should always kill stoats whenever the opportunity arises, for example, by setting a few traps around the camp site whenever one is in the bush; but the fact is that such actions are more effective in getting rid of vengeful feelings against stoats than in controlling their numbers. Compromise policy Control for the prevention of damage is a compromise policy which makes no _ attempt. ta. reduce the. -local population of stoats, but meérély- nies: £0- linut "the damage they may do at particularly sensitive times and places. Damage is most likely in summer, when birds are breeding and young stoats are demanding food. The worst damage is likely to be in summers when many young stoats are produced. Trapping for damage control simply aims at removing these young as soon as they ‘appéar: ‘(Because breeding adult female stoats are very difficult to catch, it is not practical to try the alternative approach, which would be better in theory, of removing the breeding females before they have had their young.) The best scheme is to have the traps set out and ready by mid December (early December in the North Island) and operate them intensively for 3 mpage 42

inOnins.. By Bby..tne the eid . o1 . . February most birds are past the vulnerable stages of their nesting cycles, and stoats normally tend to eat fewer birds and more mice in autumn (March-May) than in the rest of the year. Because even a_ limited damage control effort will be expensive in labour and traps, it is probably worth doing only in areas known to contain a particularly vulnerable or endangered species and perhaps only in summers when stoats are expected to be particularly numerous. Fortunately, in beech forests such years can be predicted well in advance. In the takahe area the Wildlife Service now monitors beech seedfall and mouse populations every year and can count on 9 months’ notice of a stoat peak. Fortunately again, the most intensive trapping effort is required from December to February, which means that the entire field

operation for damage prevention could be run with casual labour available in the university vacation. In non-beech forests the population biology of stoats is much less well known; it may be that there is no great variation from year to year in the numbers of young stoats born. Then the only way to protect birds such as the kokako would be to trap for damage control every year. Furthermore, we do not know whether stoats or rats are the main threats to kokako, but since both are likely to increase together after a good fruiting season for the forest trees, a kokako damage control programme should be directed at both. Conclusion There is good evidence that the activities of the introduced predators (including, even especially, man) were among the most important of the

reasons why many of New Zealand’s unique birds (and other fauna) are now extinct or confined to off-shore islands. But we must appreciate that the historic and contemporary situations are entirely different, and it does not necessarily follow that contemporary control of predators on the mainland (even if possible) would lead to increases in the density or distribution of the birds that still survive on the mainland. Even programmes for damage control to assist the takahe and kokako are less urgent than the control of browsing mammals and the improvement and legal protection of these birds’ habitats. Much though we may regret it, stoats are here to stay. Our only hope is that, by learning more about them, we may be able to limit the damage they can do in the most sensitive remaining areas and avoid wasting resources on attempts at general control. I am now preparing a paper for the National Parks Authority setting out this idea with full supporting information and I would welcome comments on It. Further information King, C. M., 1980: Field experiments on the trapping of stoats (Mustela erminea). N.Z. Journal of Zoology 7: 261-266. King, C. M., and Edgar, R. L., 1977: Techniques for trapping and tracking stoats (Mustela erminea L.): A review and a new system. N.Z. Journal of Zoology 4: 193-212. King, C. M., and Moody, J. E. , 1982: The biology of the stoat (Mustela erminea) in the national parks of New Zealand. N.Z. Journal of Zoology 9: 49-144. King, C. M., and Moors, P. J. 1979: The life-history tactics of mustelids, and their significance for predator control and conservation in New Zealand. N.Z. Journal of Zoology 6: 619-622. [Readers are invited to address comments on this article (or on anything to do with stoats) and requests for reprints of it to C. M. King at 3 Waerenga Road, Eastbourne. ] 8