Pesticides and Pollution. Kenneth Mellanby
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be similarly affected. This could be of considerable ecological importance, but there seems little information on the subject. However, as rose mildew soon manifests itself in the suburbs, it would seem likely that other fungi, and susceptible plants of other groups, suffer little damage outside very polluted areas. Nevertheless it would be wrong to be dogmatic about this. Small quantities of sulphur or of other gaseous and solid pollutants which are dispersing through our environment may be more harmful than is generally realised.
It should be noted that although trees may suffer from the effects of pollution, at the same time they do something to alleviate this condition. It has been shown that trees growing in industrial areas may do this in several ways. They filter the air, so the deposits on their leaves are removed from general circulation. They cause turbulence and deviation of the air flow, which may help to ventilate (with less contaminated air) an area of otherwise stagnant pollution. They also remove carbon dioxide and liberate oxygen, an important function on a global scale, but, as mentioned below (see here) even seriously polluted air is almost never deficient in oxygen and dangerous concentrations of carbon dioxide are uncommon. Incidentally, in a highly polluted area where trees are likely to improve conditions, it may be best to plant broad-leaved deciduous species, which are harmed less than evergreen conifers, even though they have less effect in winter when the branches are bare. Conifers will be more efficient, and in some circumstances may be used and considered as “expendable.”
Motor vehicles are responsible for widespread pollution in town and country. The exhaust gases contain a high concentration of carbon monoxide, which is very poisonous to mammals and birds. This gas may reach dangerous levels, particularly to car drivers in traffic blocks in towns, but it is probably dispersed too rapidly in the country to have an appreciable effect. Some three thousand tons of lead are emitted with the exhaust gases of cars in Britain each year. This has been found to accumulate in the vegetation and soil along roadside verges, and although serious damage has been seldom reported up to now, a dangerous concentration could build up locally over a period of years. Lead could possibly enter food chains and have damaging effects far from the source of pollution.
Carbon dioxide is another common constituent of the exhaust from fires, factories and vehicles. It has seldom been found in the high concentrations which are harmful to life, and its presence may even promote plant growth in the way it has been shown to do when CO2-enriched air is pumped into a glasshouse. Thus if the CO2, which is normally only some 0·03-0·04 per cent of the total air, is increased to 0·15 per cent, the rate of photosynthesis in a glasshouse may be more than doubled, and crop yields can be substantially increased. The effects of CO2 from industrial pollution on outdoor crops and on natural vegetation have not yet been thoroughly investigated. It is possible that quite small differences in CO2 may affect the whole pattern of vegetation by stimulating one species of plant more than another. More work on this problem is clearly required.
Recently it has been suggested that CO2 may eventually have a drastic effect on world climate. Coal and other “fossil fuels” are being burned at such a rate that the CO2 content of the whole atmosphere may be raised as much as 25 per cent by the year A.D. 2000, and the level will probably continue to rise. The effects of this are not fully understood but some scientists think the temperature and other properties of the stratosphere may be affected. This could alter the world’s radiation balance, possibly melting the polar ice cap. So far little or nothing has been done to reduce the output of CO2, though some research on ameliorating its possible effects has been suggested. So far most scientists have thought that CO2 pollution was of little importance; it now seems possible that it may cause greater changes to the world than any other man-made factor in our environment. On the other hand, this may be a completely false alarm.
Ozone, the form of oxygen with three atoms in the molecule (O3) instead of the normal two (O2), occurs naturally in tiny quantities, and pollution, particularly from motor vehicles, may increase the amount. As little as one part of ozone in 10,000,000 parts of air has been found, in the U.S.A., to harm many plants and trees, and such ozone poisoning is said to be important in both California and Connecticut, in which state an annual loss of $1,000,000 to vegetable crops is reported. So far, I know of no cases of ozone damage to vegetation in Britain, but with the increasing number of motor vehicles it seems likely to occur either now or in the near future.
Air pollution also affects the soil. Near cities the soil is often considered to be “sour,” because of the sulphur dioxide and other acid-forming substances washed in by the rain. This effect probably does not extend very widely, but many of the chemicals found in rain-water may come from industrial pollution. In the moorland areas of the Pennines we know that the rain brings in substantial quantities of minerals, which contribute to the fertility of the soil. Much of this comes from the ocean, but some from pollution, which here may be having an advantageous effect. The quantities of nutrients are significant, but probably not sufficient to have detrimental effects such as those produced by similar nutrients in much larger amounts in purified sewage, which upset the balance in many rivers (see more).
Botanists have studied the effects of pollution on a wide range of plants, mostly with inconclusive results. They have attempted to find “indicator species” which may be used to measure pollution. Such a species would only grow where pollution was below a certain level. The most successful work has been with lichens. Several species of lichen are absent entirely from the industrial areas of high pollution, and reappear on the outskirts. This problem has been studied in Northern Ireland, near Belfast, and around Newcastle upon Tyne. Fig. 3 shows how the lichen cover of tree trunks increases from the city centre of Belfast to its outskirts. It has been reported that the habit of growth of individual species was affected, so that some seemed barely able to exist where others grew normally. The subject is, however, not an easy one. It is necessary to be competent to recognise individual lichen species accurately, and to distinguish these in their sterile sorediate forms which often occur under unfavourable conditions. I must confess that I personally have been disappointed by the potentialities of this group. After reading in the literature that “a salient feature of lichen’s ecology is that these plants are very scarce in the neighbourhood of towns” I visited the Lower Swansea Valley (Plate 1), perhaps the most polluted area in Britain. My first impression of the soil was an almost pure culture of lichen, and a wooden railway bridge was equally encrusted. These were of course resistant species easily recognised by a specialist, but showing that the method used in Belfast and illustrated in Fig. 3 cannot be generally used except by experts. In time botanists may find other plants which are better indicators; in fact there has been some progress in this field, but the confusion to-day relating to the effects of pollution suggests that unless it is very severe it may not be a factor of major importance in the ecology of most regions.
Fig. 3. Increase of lichen cover outside the city of Belfast. (After A. F. Fenton.)
One particular element – fluorine – requires special attention. Fluorine occurs in minute quantities in all plants and animals, and it is one of the essential elements of protoplasm. If the natural level falls below a minimum, and this occurs in nature, harmful effects may be seen. One (but only one) of the reasons for the poor teeth found in many parts of Britain and North America is that the natural water may have a very low fluorine content, less than one tenth part per million, and combined with a “sophisticated” diet this may cause fluorine deficiency. A tiny additional amount, up to 1 part per million, may then be added to the water, and this has been found to improve tooth structure in children and reduce dental decay. This is one instance of a general principle, that a substance essential in small amounts may be toxic when the proper level is exceeded. The toxicity of fluorine in larger doses has made some people oppose the addition of this element to water, though there is no evidence that drinking water with 1 part per million ever does harm.
Fluorine occurs particularly in the smoke from brickworks, which are often surrounded by agricultural land. Other industries, including iron and aluminium production, are also important in this connection. Unlike active