Pesticides and Pollution. Kenneth Mellanby

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.