Stoat in the dock

Forest and Bird, Volume 16, Issue 3, 1 August 1985, Page 7

Stoat in the dock

by Dr

Carolyn M King

M:: people in New Zealand dislike stoats, not only because they are introduced aliens; there are many other introduced animals that are accepted without question, and granted to have a permanent and deserved place here — for example, sheep, cattle and people. It is not even simply because stoats are carnivores; there are other introduced carnivores which are regarded as positively desirable — for example, domestic cats, sheep dogs and people. No, the problem with stoats is that they have long, thin, sneaky bodies, sharp, mean, sneaky faces and they live in the bush and eat birds. People tend to distrust them because of their appearance, and so are more than ready to overestimate the damage they can do to forest wildlife. The argument runs something like this: there are fewer birds than there used to be; stoats are nasty, vicious predators that kill a lot of birds; therefore, in order to protect birds, we must control stoats. Logical, reasonable, true? No, no, no. Even if the first two statements were perfectly correct, the third is not a valid deduction from them: and in fact, the first two statements are only partially right anyway.

Fewer birds? How often have you heard it said, or said to yourself, that there are fewer birds in New Zealand now than in primeval times? This lament is generally true, but it can still be misleading, because there are some respects in which it is not true. For example, you must first state clearly whether you mean that there are fewer species of birds, or fewer individual birds of all or any species; and also whether you are talking about native species only or introduced ones as well. If you mean there are fewer native

species, the facts will bear you out, at least for the inhabitants of land and freshwater; on the main islands, about 50 native land and freshwater species have disappeared, or have _ been drastically reduced, since around 1000 AD (including some which survive on offshore islands), whereas only 10 species have arrived unaided (and are therefore included with the natives) since the beginning of European settlement. Some of the 12 or so species regarded . as. ‘native’. but... in= distinguishable from their Australian relatives (eg the pied stilt) are probably self-introductions dating from between 1000 and 1840, so the total number of species arriving since 1000 is probably more than 10; but it certainly isn’t enough to balance the loss of 50, unless you add the 36 species introduced by man. These would be almost enough to balance the gains against the losses, at least in terms of total numbers, though not, of course, in quality and distribution: sparrows and starlings are no substitute for bush wrens and huias, in appeal or in replacing the losses in our forests. So, in gross terms there are fewer native land and freshwater species than there were, and of course those that have gone are often the most unique and precious, while those that replace them are often common in other countries. If you mean that there are fewer individual birds now than in primeval times, we immediately hit the problem of making assumptions about what the undisturbed numbers were like. Before about 750 AD there were no observers; then there were observers but no records; and by the time there were records, the birds were no longer undisturbed. Still, it would be reasonable to suppose that the average density of birds in the bush was higher a thousand years ago than now; but again, that is a general statement that needs qualifying. Some species are a lot more common now than then, mostly those that have thrived on the extended open spaces created by man, or on the food he provides — the herring gull and the harrier hawk, to name only two. But regrettably, these are in the minority: the list of species that have suffered losses in their numbers is far longer than the list of those that have benefitted. And again, the scales are weighted against the old endemics, the most precious and unique of the native species. It is the robins, the blue duck, the black stilt and the takahe (all found only in New Zealand) that have become scarcer, while the pied stilt and the white-eye (indistinguishable from their Australian relatives) thrive.

Stoats eat birds Stoats are killers, yes, but not vicious, since only people, not animals, can be ‘*addicted to vice’’. Stoats are efficient, yes — at least as far as any predator is, which means that a hunting stoat does not always catch everything it stalks, and probably knows all too well how it feels to be hungry. Stoats kill a lot of birds — well, it depends how you define ‘a lot’’. If you collect a sample of dead stoats and cut open their stomachs, the chances are that you will find feathers in about half of them. That may mean the stoats often kill birds, but it does not necessarily mean that the stoat living in a given place must be killing a large proportion of the total numbers of local birds. Stoats are relatively scarce animals compared with birds. For example, in 40 ha of forest in the Hollyford Valley in summer there might be around 150 pairs of birds, plus their young, and, at the very most, one stoat. This one would probably not be there all the time either, because usually the density of stoats is much lower than one per 40 ha — which means that the local resident animal might visit any particular 40 ha only every second or third day. So even if the stoat kills a bird every day of its life, the amount of damage it can do is limited from the outset by the fact that potential kills are not only hard to catch, but also. much more numerous and more quickly replaced than are stoats. Most of the 150 pairs and broods of birds in that 40 ha will never be at serious risk from it, and most will die from causes other than predation. Conversely, birds do not form the staple diet of stoats, the main contribution towards the nutritive requirements of each day, largely because the ones most frequently eaten, the small bush birds, are light in weight and supply few calories. The really important items for stoats are mammals — especially rabbits in the open country, possums and rats in the bush. In the northern hemisphere, there are many different kinds of wild small mammals — field mice (distinct from and more abundant than the feral house mice we have), voles and lemmings — which are the most important foods for stoats there. People have tended to assume that stoats in New Zealand must eat disproportionately more birds in order to make up for the lack of field mice, voles and lemmings here. But even that quite reasonable assumption has turned out to be wrong, at least when you compare the proportion of birds eaten by stoats in our National Parks with the proportion eaten by stoats on British game estates.

Control of stoats All discussions of control of stoats are confounded by irrelevant human prejudices. Stoats have a reputation as vicious, alien killers that seems to have an extra-ordinary effect on people. The sight of a helpless little feathered bundle clamped between the jaws of a steelyeyed stoat is virtually guaranteed to get even the calmest, nicest, most logical member of the Forest and Bird Society all steamed up in a matter of seconds. We have to recognise how important this human reaction is, because it goes a long way toward explaining why some people simply cannot accept the rational arguments against conducting a general programme of stoat control, not even in National Parks. Yet all the evidence points towards the conclusions that stoats have in fact had very little to do with the catastrophic extinction of our native birds in the past, and that, except in two or three specific places, control of stoats now would do practically nothing to protect the native birds that remain. However much these statements may seem to contradict all you ever believed about stoats, yet the reasons for them are simple and logical, they have nothing to do with compassion or prevention of cruelty, only with the necessity of avoiding unwise expenditure of the scarce funds available for protection of our wildlife. First, over most of the country, stoats have played a very minor part in these historic extinctions, partly because they arrived late (1884 onwards), long after Norway and ship rats, cats, dogs and human hunters had removed all the most vulnerable birds on the mainland, and partly because stoats have never reached most of the offshore islands where many more very vulnerable birds lived. Westland and Fiordland were the only mainland areas still relatively free of disturbance by the time the stoats arrived, and the destruction of the last of the ancient ground-dwelling birds (the kakapo, saddlebacks, thrushes and bushwrens) of the south and west from 1890 onwards was eloquently described by many contemporary writers. Observers such as Douglas and Harper were in no doubt that stoats were to blame, but they overlooked another, far

more dangerous enemy, ship rats, which arrived at about the same time and certainly could have achieved the same effect with or without the help of stoats. The evidence against stoats as contributing to extinctions in the south and west is circumstantial, and would never stand up in a court of law; that does not mean they were innocent, only that they were not solely to blame. Elsewhere in the country, there is no evidence against them at all. In fact, attacks by stoats on the Westland thrushes and saddlebacks, though well known, were quite exceptional; of the 153 distinct populations of birds known to have disappeared from the islands of the New Zealand group (excluding the outlying islands) since 1000 AD, stoats could have come into contact with only five (3 percent) that are now extinct and 11 (7 percent) that are still threatened. Stoats were perfectly capable of causing wholesale slaughter, but they never had the opportunity; it was the Polynesian hunters and the European sailors and their rats and cats which had the luck to discover undisturbed, tame and defenceless birds on every island they visited. Second, the natural environment in New Zealand is totally different now from the way it was when the first predators arrived and the old conditions cannot be restored by predator control or any other means. The forests have been radically diminished, dissected and irretrievably changed by browsing deer, goats and possums; the old network of relationships to which the original forest dwellers were so well adapted has been torn apart. The past has gone as permanently as if it had never been; and even if the predators could be totally exterminated, the most sensitive of the native birds now confined to predatorfree offshore islands, have no recognisable home to return to. The effects of predators in the past cannot be undone by predator control now; it is too late to slam the stable door, since there is now not only no horse, but no stable. Third, control of stoats would probably not have any effect on the numbers of those hardy species of birds that still manage to survive in the bush today. Not only is it very unlikely that stoats can, in fact, be kept at artificially low numbers, but also, there is no guarantee that individual birds of the non-endangered species saved from predation will necessarily live much longer. Other factors besides trapping determine the numbers of stoats, and other factors besides predation determine the numbers of birds. The trick to understanding both these statements (which are really the same) is to think of life and death among animals as a bank balance. Bankrupt birds If you see people spending lavishly on luxurious cars, trips abroad and dinners at expensive restaurants, you know it could mean that either they must have a

large income, or that they will soon be broke. Only if you are allowed to see their bank statement, and read both the credit and the debit columns, will you be able to tell whether they are genuinely wealthy or just spendthrifts. Everyone understands that a person’s financial position is determined not by expenditure alone, but by the relationship between expenditure and income. So it should be easy to see that populations of birds are just the same, and can be easily understood if we think of breeding as income and death as expenditure. The common small bush birds live short lives, and many of them die every year; but those deaths are usually balanced more or less by the large number of young they rear each season. Take for example the fantail: adult females commonly lay 3, 4 or even 5 clutches a year, averaging 3-5 eggs per clutch, but fewer than one in a hundred adults survive long enough to breed in more than one season. They have high incomes and high expenditure every year, and their bank balance (the population density) tends to be unsteady, but over the long term there is plenty of income available to counter the heavy outgoings. Birds in this position are like the genuinely wealthy — they can suffer heavy mortality year after year without serious effects. But not all birds are adapted to produce a lot of young. Some, like the takahe, lay only one clutch a season, of 1-3 eggs each, so obviously their income is small, and they normally match it with very small expenditure. Their bank balance is steady and reliable — until something happens to increase the cost of living too much. Then there is no way of increasing income sufficient to restore the balance, and disaster follows. Birds in this position are like the spendthrift to whom no bank manager will grant an overdraft — they can suffer heavy mortality only for a limited time, and then they become extinct. Unfortunately, many of the most unique of New Zealand’s ancient birds, such as the moa, huia, thrush, bush wren and hosts of others, had evolved the low-income, low-expenditure pattern. The arrival of the first predators suddenly increased the expenditure; no possible increase in income could compensate; the birds swiftly disappeared. But the common bush birds that still survive were used to heavy mortality, and the predators merely took those that would soon have died from some other cause. This is why the coming of the predators did not have the same results for all birds; it is also why it is difficult to control predators such as stoats, which have the same high population turnover characteristic of small bush birds. Stoats killed in traps are merely rescued from dying of some other cause — usually starvation — just as small bush birds killed by stoats are rescued from dying of some other cause — most often causes that were operating long before the stoats arrived.

So people who demand that we should control stoats in our National Parks in order to protect the last of our native bush birds are really asking the impossible; it is totally impracticable (or at least, outrageously expensive) to achieve any real control over stoat populations, and even if it were possible, it is probably not necessary in the great majority of reserves, in which the native birds that remain have proved themselves able to cope with all the changes brought by the human invasion of their home — not only stoats, but also the loss and modification of the forest and the whole range of alien mammal and bird intruders. The only exceptions are the takahe and the North Island kokako, the only two endangered species on the mainland to whom stoats may still be a hazard. Even for these, stoat control is lower on the present list of management priorities than the bird’s primary needs, which are for adequate secure habitat and food supplies; and no other endangered species on our list is threatened by stoats at all. The verdict Surprising though it may seem, the verdict must be that stoats are in fact responsible for relatively little damage to our bird populations, either in the past or the present. They have contributed to only a handful of extinctions, all in

Westland and Fiordland; as far as we know they have little effect on the densities of the surviving bush birds; and attempts to control them now would be unjustifiable everywhere except as part of the integrated programmes to save the takahe and North Island kokako. Stoats arrived long after the most vulnerable birds had already been removed by other predators, and the populations of birds that are left are controlled more often by other factors — normally habitat and food supplies — than by predation by stoats. There is a limit to the damage that predators can do, because only certain kinds of birds are vulnerable to it, but there is no effective limit to the damage that habitat destruction can do, because all birds are vulnerable to that. We should not waste our energy and resources worrying about stoats, while there are any native forests and wetlands still at risk. oe FOOTNOTE: Further details and full references to the information in this article can be found in the author’s recently published book. Jmmigrant Killers : Introduced predators and the conservation of birds in New Zealand (Oxford University Press, 1984). Carolyn King is the author of some 35 scientific papers on mustelids, and is an acknowledged expert on the stoat and weasel in particular. She has worked as a scientist with DSIR Ecology Division and the New Zealand National parks Authority, and is currently engaged in research at Pureora Forest Park for the Forest Service. Dr King is also scientific editor for the Journal of the Royal Society of New Zealand.