Читать книгу Pesticides and Pollution - Kenneth Mellanby - Страница 6

This book is essentially an account of the way in which man is unintentionally contaminating his environment. This is a world-wide problem, but for the most part I am restricting my scope to consider the situation in Britain. I do this for two reasons, first because this treatment seems appropriate to a volume in the New Naturalist series, and secondly because although the problem is more acute in these small, developed and densely populated islands than in many other parts of the world, it is possible that what we do can help and guide others, and serve as a warning to prevent damage in the countries at present undeveloped which might otherwise accompany their economic development.

I think it is quite logical to bring together in one book the effects on our environments and on our wild life of pollutions which arise from urban conditions and from industry, and the rather special case of pesticides, which have recently been shown to constitute such an important contribution to environmental pollution. In all cases we are dealing with effects which were not deliberately planned by those who produce them. Pollution, including radiation, has grown as a side effect of the increase in human population, and of increased material productivity. Pest control began in quite a different way, but it is the side effects of modern pesticides, substances which could only have been produced and developed in an industrial system, and which may be effective weapons when properly used, which are causing so much concern.

In this book I have attempted to review the present situation, show how it has arisen, and discuss how man may either continue to pollute the earth and to impoverish it by destroying and restricting its flora and fauna, or alternatively may control his own activities for the benefit of posterity. The situation is serious. Much irremediable damage has already been done. On the other hand, we have learned how to avoid the worst types of pollution. Public opinion is strongly in favour of controlling pollution, and we are not so dogmatically in favour of eradicating any forms of life we consider to be “pests” as perhaps we used to be. If mankind is prepared to make a determined effort, to support much more research to make that effort effective, our descendants may not be condemned to live on an impoverished planet devoid of so much of the varied life which has made it so interesting and so beautiful. There is one other point which should be noted. Some pollutions, particularly from insecticides, seem to be harmful to wild life but not, as yet, to man. Air pollution is more often seen to damage plants than to harm man. Perhaps we should look upon these as useful “early warning” systems, and use the opportunity to reduce the chances of future damage to our own species.

Man deliberately alters the face of the earth, to a degree to which no other species of animal has aspired. He builds cities, factories and roads which wipe out wild life over large areas. He does this quite intentionally, assuming that the gain far outweighs the loss, though his pollution of the surrounding countryside with domestic and industrial effluents is not intentional. However, important as are the effects of urban and industrial development, it is as a farmer that man has the most profound effects on the landscape, and on the plants and animals which live there. An excellent account of these changes is given in Man and the Land by Sir Dudley Stamp. Farmers destroy the natural vegetation, and substitute wide areas of alien plants grown in monoculture. They deliberately try to get rid of plants which compete with their crops – that is, weeds. They try to destroy animals which eat their crops and so reduce the amount harvested – that is, pests. As agriculture has developed, new methods of dealing with weeds and pests have been discovered and put into use. To-day we are all conscious of the fact that chemical controls have been developed, and that poisonous substances are widely used to achieve control. We know that these poisons may kill birds and other forms of wild life, and we fear that they may endanger human health. However, we tend to forget that these possible toxic side effects may be less catastrophic to wild life than are the ecological effects of “traditional” agriculture.

Man has existed for hundreds of thousands of years, but it is only within the last ten thousand that he has had any considerable ecological effect, and only within the last millenium that he has really begun to change the whole appearance of the earth’s surface. Until about 8,000 B.C. man was a hunter and a gatherer of wild plants, with no domestic animals and no crops. Life, as Hobbes has said, was “nasty, brutish and short.” Man had little effect on his environment. He ate fruit and berries, and helped to disperse the seeds as do birds and animals to-day. His waste-products were probably dispersed over the land and helped to fertilise it. When he died his remains decayed and the nutrients were returned to the system. The nastiness of his life was probably made worse by pollution, even at that stage of his development, and pests contributed to its shortness. Some animals are naturally clean. They excrete in recognised latrines some distance from their usual resting places, and they do not leave the decaying remains of food in inconvenient places. Other animals, and primitive man was probably one of them, are not so hygienic, and if he lived in a cave it probably stank disgustingly. Even when he buried his dead he sometimes did so beneath his dwelling, just where decay would be most objectionable. Under these conditions many diseases must have been rife.

Early man probably thought of the larger carnivores as the most serious pests. He competed with them for animal food when he became a hunter, and he himself formed a part of their prey. At first he did little positive to control these pests; his main object was to avoid them. Later, but long before he became a farmer, man in some parts of the world learned to trap and hunt even the most ferocious wild animals, and he probably accelerated the extinction of several species. Recent work on the so-called “Pleistocene overkill” goes so far as to suggest that man exterminated nearly half the larger mammals in Africa some 50,000 years ago, and that in North Africa (where man arrived much later) he similarly killed off at least sixty per cent of the species of large mammals around 10,000 B.C. If these conclusions are finally substantiated, they will have a profound effect on ecological thinking. Early man probably paid little attention to the smaller insect pests, lice, bugs and fleas, from which he no doubt suffered. He did not realise that these were not only a nuisance, but were also the carriers of diseases which were far more deadly than all the lions and tigers and snakes which he so greatly feared. At this stage man was simply an animal, competing with other animals, and doing little to upset the uneasy balance of nature.

There were cases where hunters profoundly changed the landscape. It is likely that North American Indians deliberately burned the forest, and so encouraged grassland which could maintain larger herds of buffalo. This could be considered an early example of wild life conservation! It must also have had profound effects on all the other animals and plants in the region. Incidentally, at a much later date, when the prairies were cultivated to grow cereal crops, the buffaloes became “pests” and were almost completely exterminated.

Primitive man suffered from pests and pollutions, even if he was not always aware of this. When he became an agriculturalist, he recognised the fact. Settled agriculture, with the growing of some crops, has gone on in parts of the world for perhaps ten thousand years, but extensive farming has only existed for about five thousand years. Arable farming is essentially a process where certain plants are encouraged, and others, which would compete with them, are discouraged. The unwanted plants are pests or weeds.

It is difficult to find a satisfactory definition of a pest, other than to describe it as a plant or animal living where man does not want it to live. The same animal may sometimes be treated as economically valuable, at other times as a dangerous competitor. Thus mink, escaped from fur farms in England, where they are prized, are considered as dangerous pests in other parts of the country. The same plant may be a valued crop at one time and a weed at another; an obvious example is the potato, for a few tubers, accidentally left in the soil when this crop is harvested, are troublesome weeds in cereals grown in succeeding years.

A great part of the energy expended in arable farming goes in weed control. Ploughing, harrowing and cultivation are all means of reducing weeds and their growth, as well as of making conditions suitable for planting crops. It is therefore somewhat ironic that weeds are important largely because man has produced conditions in which they flourish. Most weeds were rare plants before man became a farmer, and some are now uncommon or even extinct except on farm land. The history of weeds and their development has been fully described by Sir Edward Salisbury in his masterly book Weeds and Aliens, to which any interested reader must refer.

At different times farmers have used different methods of weed control, and different species of plant have been economically important. When wheat and other corn crops were sown broadcast, hand weeding was the only practicable method. When drilling in rows was introduced in the eighteenth century, it became much easier to keep crops clean. At the same time improved methods of separating crop from weed seeds were devised, so that sowing did not itself greatly contaminate the ground. Thus with clean seed and properly planted crops, cheap labour and comparatively simple horse-drawn machines kept the fields clean. At the end of the nineteenth century there was no serious weed problem for the good farmer in most parts of Britain. As agricultural wages rose, mechanisation was introduced, and some farming processes were improved, but many crops became weedier and weedier, so that different rotations had to be developed, not always with success. In recent years the situation, for the farmer, has been saved by the introduction of selective weedkillers. These have revolutionised agriculture, but have produced their own problems, as will be seen in chapter 6.

Parasitic fungi cause a great deal of crop damage. Early man was aware of some of these diseases, and the danger of eating corn made poisonous, for instance, by the fungus causing ergot, particularly in rye. However, many fungus diseases were not recognised as such, and their damage was accepted as a normal risk of farming, until the latter part of the nineteenth century. As shown in chapter 7, many fungus diseases are now controlled with little risk to other forms of life.

Farmers, from neolithic times onwards, were aware of mammalian pests, ranging from deer which damaged their crops to wolves which preyed on their herds. Rats and other rodents were known to consume much of the stored grain, and many ingenious methods of excluding them were devised. Early pest control was in effect hunting; the results were sometimes successful, as in the case of large and slow-breeding animals, and quite ineffective against small mammals which bred rapidly. Brown bears were exterminated in England in Roman times, and in Scotland before the Norman invasion. Wolves continued much longer. They were quite common, particularly in Wales, into the medieval period, and the last survivor is believed to have been killed in 1740; a few lingered on in Ireland for another thirty years.

Pest control in the English countryside has usually been related to game preservation and sport as well as to agriculture. The wolf was clearly too large and voracious an animal to be tolerated, and so it was eliminated. No one to-day seriously suggests its reintroduction, though conservationists (if not the local farmers) are concerned about its future in Spain. The fox, however, is still quite abundant, although it undoubtedly kills poultry. It would be difficult, though not impossible, to exterminate all the hill foxes in the wilder parts of Britain, but a determined effort could get rid of this animal in areas of intensive farming within a couple of years. Foxes may not be deliberately preserved, but they have been tolerated for many years because of fox-hunting, and those naturalists who are opposed to blood sports should realise their debt in this connection. Recent research has shown that foxes live as much on carrion and on small animals and insects as upon poultry and game, so they are likely to continue to survive unless accidentally wiped out by chemicals (see here). Recently foxes have been reported in increasing numbers in suburbia, raiding dustbins and feeding on garbage. These habits will not endear them to the more sentimental members of the public, who may equate garbage-eating foxes with rats. More of the animals are likely to survive, and they may eventually become pests in a new role if they become too common.

Gamekeepers were for a long time the main enemies of carnivorous animals and birds, which they spoke of as “vermin” on the assumption that they lived mainly on game. Most keepers until recently had their “gallows” on which the rotting corpses of stoats, weasels, hawks and owls were hung, presumably pour encourager les autres. Systematic shooting of slow-breeding predatory birds effectively controlled their numbers. In the nineteenth century kites, formerly distributed throughout the country, were eliminated except from a few mountainous areas. A careful investigation by Dr. N. W. Moore shows how the buzzard has fared in the last 150 years. At the beginning of the nineteenth century buzzards were quite common breeding birds over most of Britain. By 1865 they had been exterminated in most of East Anglia and the Midlands, and by 1900 they were only to be found in Cornwall, Wales, the Lake District and Western Scotland. By 1954 the situation had somewhat improved, and the birds had recolonised many of the areas occupied in 1865. This spread was clearly due to the decrease in game preservation during and after the 1914-18 war. If the data for buzzards and the numbers of gamekeepers are mapped side by side, this shows a good negative correlation.

To-day many gamekeepers are more enlightened. Although hawks and other carnivorous birds do eat some game birds, they prey much more on small mammals (mice, voles, etc.) which compete with game for food. Most predators are now themselves legally “preserved,” though they can be shot if caught in the act of taking poultry or game. We may see an improvement in numbers, similar to that manifested by the buzzard between 1900 and 1954, for many other species, if the new danger from agricultural chemicals can be overcome.

Smaller, non-carnivorous mammals and birds do a lot of damage, the amount of which is not always recognised. Rabbits were probably introduced into England by the Normans in the eleventh or twelfth century and were comparatively uncommon, prized as game animals until the nineteenth century. Then, for some reason which we do not yet understand, they suddenly increased in numbers so that a density of twenty animals to an acre was not uncommon, and as many as 100,000,000 carcasses were sold in a year, without noticeably depleting the numbers still at large. It was not until rabbits were wiped out in many areas by myxomatosis in 1954 that the extent of the damage they had done was recognised. Pre-myxomatosis control, by shooting and trapping, was rarely effective, and merely served to “crop” the population. The importance of myxomatosis is discussed in chapter 10.

Fig. 1 Changes in the distribution of the buzzard in the British Isles. (from Dr. N. W. Moore with acknowledgement to British Birds).

KEY: Black: Breeding proved, or good circumstantial evidence of breeding.

? on black: Circumstantial evidence suggests that breeding probably took place.

? on white: Inadequate evidence of breeding.

White: No evidence of breeding.

Fig. 2 a. The breeding population of the buzzard in 1954.

KEY: Black: 1 or more pairs per 10 square miles.

Cross-hatch: More than 1 pair per 100 square miles, but less than 1 pair per 10 square miles.

Diagonal hatch: Less than 1 pair per 100 square miles.

White: No breeding buzzards.

+ means that breeding density may belong to the category higher than that indicated.

—means that the breeding density may belong to the category lower than that indicated.

b. Game preservation in 1955.

KEY: Black: 3 to 6 gamekeepers per 100 square miles.

Cross-hatch: 1 to 2 gamekeepers per 100 square miles.

Diagonal hatch: Less than 1 gamekeeper per 100 square miles but more than 1 per 200 square miles.

White: Less than 1 gamekeeper per 200 square miles.

G. Principal grouse-preserving areas. On these, and also on some very large estates, the numbers of keepers may be higher than shown on this map.

Mice and rats invaded man’s home as soon as there was a home to invade. They also lived in his grain stores and farm buildings. Early man tried to make his granaries rodent-proof, sometimes with remarkable success. Control by trapping and poisoning was generally inefficient, and the rodent population in contact with man was roughly a measure of the amount of food he made available. A mouse-proof larder is more effective than an apparently efficient trap. The domestic cat was probably the most useful method of local control, though cats, like other “pesticides,” have had their side effects. Those that have escaped and become feral have important effects on other wild life. Modern methods of rodent control are much improved, but these animals still do much economic damage in our cities and on our farms to-day. It is perhaps surprising that the black rat (Rattus rattus), which is found mainly in towns, where it is particularly at home in hot-water ducts in tall buildings, was our “original rat,” though some think even it only arrived about A.D. 1200, and the brown rat (Rattus norvegicus), which is the species commonly found in the country, only arrived in Britain in the eighteenth century. The black rat was apparently driven from the rural haunts by the brown invader.

Pigeons and sparrows are serious agricultural pests, against which no really satisfactory control measures have so far been devised. Pigeons become more numerous each year. This increase is probably due to the increased amount of winter food, particularly clover leys on farms, that is available. The recent fall in the number of hawks may also have had some effect. Organised shoots give some sport to the participants, but have negligible effects on the pigeon population. Work on poisoning or narcotising pigeons is progressing, but the danger to other and more desirable species of birds is difficult to prevent. Sparrows probably increase because suburban householders feed them in winter. This enables them to survive in cold weather, and the increased population does more harm on the nearby farms to which it migrates in summer and autumn. Again shooting and trapping has little effect.

Farmers and others were then soon aware that wolves and other large mammals and birds might be pests, competing with them in various ways, even if they often overestimated the damage done by the carnivores and sometimes underestimated the amount of food taken by rabbits and other herbivores. They took active, if sometimes misdirected, steps to control these animals. On the other hand, they almost always underestimated the harm done to their crops and to their health by insect pests, and it is only in recent years that serious attempts have been made at control in this direction.

In Britain we do not have plagues of locusts, which in many countries can consume the whole of a crop, but it is estimated that to-day some £300,000,000’s worth of food is lost each year because of pests of crops and insect damage to farm stock. This sum is almost exactly the same as is spent annually on the support of agricultural prices (“farm subsidies”). Some insects, like the caterpillars of Cabbage White butterflies, eat crop plants, reduce the yield and make many plants unsaleable. Other insects do little damage themselves but carry organisms which cause diseases. Thus aphids carry the virus causing virus yellows in sugar beet; this can seriously reduce the value of the crop. For many years farmers and gardeners accepted insect damage as something they could not prevent. They learned by experience that it could sometimes be avoided or reduced to a minimum by timing their operations carefully. Thus if broad beans in the garden are sown early, seed is set before aphids (“black fly”) are numerous and a good crop is obtained, but in two years out of three a late sown crop will be smothered by insects and prove a failure. Cultural devices such as this are valuable and important, but will seldom allow a late crop of broad beans to be produced. Field beans, which flower and set seed over a long period during the summer, cannot be successfully grown in a way to avoid attack in a bad aphid year. Resistant strains of crop plants have been recognised and used for many years, and in future are likely to prove very important, where resistance is linked with high yield and quality. Farmers generally prefer to be able to grow the most profitable crop at the most convenient time and wish to attack pests by any possible means. Before 1939 most effective insecticides (except general poisons very dangerous to man) could only be produced in relatively small quantities and at a price which made their use on many crops uneconomic. The farmers therefore have welcomed the synthetic insecticides which can be produced in unlimited quantities and which, at first at any rate, seemed the perfect answer. The dangers from their use are described in later chapters.

Insect pests of crops were soon recognised as such, even if little was done about the problem until very recently. The importance of insects as vectors of disease has only been understood for about seventy years, though man and his habitations have provided niches for troublesome parasitic species in the same way that wild animals have supported their own parasites. Man has sometimes controlled insects of medical importance effectively without understanding the problem. Malaria was formerly widespread in Britain, but it was almost eliminated long before man knew the parasitic organism concerned or that it was carried only by the Anopheles mosquito. This was because man avoided marshy areas, thinking that malaria was caught from the “bad air,” and so he kept away from the breeding places of the mosquitoes. He also drained the swamps, usually to produce better agricultural land, but in so doing he got rid of the insects which carried the disease. Incidentally naturalists are now very concerned at the continued draining of marshes and swamps, which are now the last refuges of many species of wild life. This is just one of the ways in which non-chemical pest control can have effects which have end results which may be as disastrous to wild life as the most indiscriminate use of chemicals.

In the Middle Ages most people of all ranks of life harboured body lice on their persons, and as recently as 1940 the majority of the girls in our industrial cities had lousy heads. Personal cleanliness could eliminate these pests, except where conditions were grossly overcrowded, but in war and after any disaster infestation, and the risk from louse-borne typhus fever, grew. Persistent insecticides now control these insects, and properly applied to man and his clothing they present little danger to any other organisms.

Pest insects which attack man, but depend on his having a permanent home, include fleas and bedbugs. Fleas are not only a nuisance, but also carry plague, a disease which died out in Britain many years before effective insecticides were discovered to control the vectors. Improved hygienic conditions rather than chemicals have made fleas uncommon insects.

Human bedbugs are very similar to the species which attack bats and swallows, and primitive man may have become infested first when he also lived in caves. Bugs were common from the earliest times in the warmer parts of the world. However, Britain had no bedbugs before the sixteenth century, possibly because the houses were too cold. Bugs certainly existed in Italy in classical times. When bugs arrived in Britain, they soon spread through overcrowded slums, and before the 1939 war most houses in our cities, except detached surburban villas, harboured at least a few. However, they were only common in unhygienic and overcrowded dwellings, and improved conditions soon reduced their numbers. Modern insecticides, particularly those which put a persistent film in the cracks which the bugs haunt, control the insects effectively without seriously contaminating the environment.

When his numbers were few, pollution was not a serious problem to man. Many pests, both plant and animal, have become common only because man has produced suitable conditions. In some cases pests have been controlled with little harm to the environment, in others pest control has become a new and potent form of pollution. The great difficulty is to assess accurately just how much pollution affects the environment and the plants and animals it contains. We are seldom able to give simple answers. Sometimes an animal has obviously been killed, perhaps from the effluent from a factory, perhaps by accidental contamination with a pesticide. Generally, however, we have to depend on circumstantial evidence of damage, and this is the reason for the controversy which so often surrounds our subject.

Toxicology is difficult and complicated. The results of analyses of animals’ bodies, where traces of poisonous substances are found, are not easily interpreted. In the case of well-known poisons like arsenic, strychnine or cyanide the situation may be less mysterious if large amounts are found. We know, for instance, that if a man eats five grams of lead arsenate he is likely to be fatally poisoned. If the pathologist finds ten grams of lead arsenate in the stomach of a corpse, he will be almost certain that this poison caused death. If he finds only a few milligrams, he will be almost certain that death was due to some other cause. The finding of intermediate amounts makes diagnosis difficult. Consideration must be given to the site where the poison is found in the body, and to losses due to vomiting or excretion. The situation is even more complicated where poisons are broken down in the body, either as part of the process of damaging the victim, or due to post mortem changes. If we do not know accurately how toxic a chemical is to a particular animal, and if we are not fully familiar with these chemical changes, we cannot usually say for certain whether a small residue of poison in a live or dead specimen has any significance.

It is generally fairly easy to establish the acute toxicity of a substance, that is the amount which, in a single dose, is lethal. Experiments with rats, chicks or fish are commonly made. Groups of animals are given different doses, and the least amount of poison which kills is found. Usually different individuals of a species show a somewhat varied susceptibility, and instead of determining the amount which kills them all, the so-called LD₅₀, that is the amount which kills half of a batch, is determined. In most instances few animals die from a single dose of half the LD₅₀, and twice the LD₅₀ is likely to kill almost every individual. However, this is not always the case. Sometimes a population contains a few individuals which can survive relatively large doses of certain poisons; under certain circumstances these may be selected out and may breed a strain which is more resistant than the normal to a toxic substance. Resistance or susceptibility to poisons is not necessarily correlated with unusual or subnormal “vigour,” and this type of chemical selection may leave a species less well adapted to normal environmental conditions.

Although acute toxicity is not difficult to determine in the laboratory, it can only be done with a limited number of species, and values for others (including man) can usually only be inferred. Also the effects of a specific poison may differ even with the same batch of the same species depending on how it is administered, e.g. neat, in suspension, in oily solution, on an empty stomach, through the skin, by inhalation and so forth. These difficulties have usually meant that, at least where man is exposed, a fairly large “safety factor” has been applied. Thus if work with rats suggests that the LD₅₀ for substance “X” is 50 milligrams per kilogram, it could be assumed that half of a group of 50 kilogram men would probably die if they ate one gram each of “X.” It would generally be found that a single dose of one hundredth of this amount, i.e. of 10 milligrams, would be unlikely to be harmful. In many cases this assumption is quite justified but contamination of food to this extent would not normally be tolerated.

While there are sometimes difficulties in establishing the effects of single, large doses of poisonous substances, the study of the effects of repeated small doses, each of which would probably be harmless, spread over long periods, presents even more serious problems. Poisons which are unstable are unlikely to be very dangerous under these circumstances. Those which are stable, particularly if they are stored in the body, may present great risks even if they are not acutely poisonous in single doses. All these factors are borne in mind when, for instance, new insecticides are tested. Their action on a number of insects, particularly pests, is determined. Then long-term experiments, lasting over severa, years and a number of generations, are then made with rats, chickens and other animals. It is obviously impossible to include more than a few species in such trials, so it is not surprising that sometimes a desirable species of bird, or mammal, is found (too late) to be unexpectedly susceptible. The effects of chronic exposure to low-level industrial and urban pollution is even harder to study. Some, impressed by the complexity of the situation, fear that the ecological effects of pesticides may bear little relation to their gross toxicity.

Everyone wishes to abolish the damage which may be caused to man and to wild life by pollution from every source. As, however, we are not always agreed as to when damage is being caused, or how exactly some obvious damage arose, an easy solution will not be found. Man has always polluted his environment; he has always suffered from pests, but because of the “population explosion,” these problems have become more serious in recent years. The need for more research in these subjects is obvious, if irreparable damage to wild life, and to man, is to be avoided. Equally important, we must make sure that the results of such research are quickly and efficiently applied.