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Problems and Concerns caused by Human Influences on the Environment Agriculture
Man has had an important influence on his environment for a long time. About 10000 years ago people started to farm the land. They caught wild animals, such as cows and sheep, and kept them in fields surrounded by fences. They planted seeds and grew crops, such as wheat and maize. Selective breeding was used to improve the quality and quantity of crops. The first crops were grown from seeds which were taken from wild plants. Selective breeding between different types of wheat, for example, has produced the wheat we see growing in the fields today. Selective breeding has produced strong plants, each of which develops many seeds. Chemicals called fertilizers are added to the soil before the seeds are sown. Fertilizers contain minerals, such as sulphates and phosphates, which are absorbed by the plant roots. Crops must also be protected from herbivores and parasites. Fences usually keep the larger herbivores away from crops. The problem of insect herbivores, however, cannot be solved by fences. To eliminate these insects many farmers use special chemicals called insecticides. To prevent parasites developing on the crops, other chemicals are used, such as herbicides and fungicides. The problems of modern agriculture Farming has become more and more productive because of the use of chemicals and farm machinery. Unfortunately, many of these the chemicals have very bad side-effects. Many insecticides kill all insects, not just the crop eating insects. Some insecticides are also passed through the food chains as one animal eats another. In small quantities the insecticides do not do any serious harm to large animals. The problem is that as the larger animals continue to eat prey which contain insecticides, the amount of insecticide in the bodies of the predators builds up. In larger amounts, the insecticides can kill even the top carnivores. The chemicals which are added to the soil and sprayed on the crops can be washed away by rain water. They find their way into stream and rivers and eventually into the sea. These chemicals are also passed through the food chains to the top carnivores. Examination of penguins, seals and whales which live in the Antarctic has shown the presence of pesticides in their bodies, even though pesticides have never been used in the Antarctic! So many of the insecticides, weed-killers; fungicides and fertilizers which are used to improve the quality of our crops are also pollutants. A pollutant is something which causes harm to a living organism when added to the environment by humans. Biological control and its problems Once the danger of using these chemicals was realized, scientists worked hard to find other chemicals which would only kill the insects and other organisms .which cause damage to crops. The scientists also tried to develop chemicals which would not affect the food webs. This kind of scientific research is still being carried out.
Some farmers have stopped using chemicals altogether. Farming without chemicals is called "organic farming" . Many people prefer to buy "organic" foods, both vegetables and meats, even though these foods are often more expensive to buy. People who buy organic foods know that no man-made chemicals have been added to them. One way of controlling insect pests which does not produce pollutants is to use natural predators. Aphids are small insects which live by sucking plant sap into their bodies through their pointed mouthparts. They are known to gardeners and farmers as "greenfly" or "blackfly". They can be found in large numbers at the tips of growing shoots of an infected plant. Ladybirds, which eat aphids, can be introduced to keep the aphid population under control. Another method of insect control is to use the special chemicals which the insects themselves produce to attract their mates. These chemicals are placed in boxes along with insecticides. The insects are attracted to the boxes by the chemicals and, once inside, they are killed by the insecticides.
This method has the advantage that the insecticides do not reach the soil or the soil water. The disadvantages, however, is that the special chemicals which attract the insects are expensive to produce and a different chemical is needed for each type of insect. One example of biological control which took place this century is the use of a virus which causes a fatal disease called myxomatosis in rabbits. Rabbits were introduced into Australia and New Zealand in the 19th century for hunting purposes. The conditions were ideal for the rabbits, and since there were very few natural predators their numbers increased rapidly. They soon became serious pests. Myxomatosis was a naturally occurring disease amongst South American rabbits. The virus was introduced into the rabbit populations of Australia, New Zealand and Europe in the 1950's. The virus spread quickly, killing up to 90% of the rabbits. Since the late 1960's, however, the rabbit population has started to increase. This is because many rabbits have developed an immunity to the disease. (This means that the virus no longer affects them.
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