4: Discuss on the undesirable effects of chemical control of pests and weeds, and the excessive use of chemical fertilizers.

It is difficult to define exactly what a ‘pest’ is, but it is generally accepted to be an organism which is in competition with humans for food or soil space, or is potentially hazardous to health. It may even be an organism which is simply a nuisance and so cause annoyance. Pesticides are poisonous chemicals which kill pests, and they are named after the pests they destroy: hence insecticides kill insects, fungicides kill moulds and other fungi, rodenticides kill rodents such as rat and mice, and herbicides kill weeds. Unlike other pollutants, where their poisonous nature is unfortunate and unwanted properly, pesticides are quite deliberately produced and dispersed in order to exploit their toxicity.

Some pesticides, dichlorodiphenyltrichloreciated(DDT), for example, are unfortunately persistent. It was subsequently used to kill mosquitoes, and so helped control malaria. Not only is DDT persistent, it also accumulates along food chains. If, for example, garden plants are sprayed with it in order to control greenfly, some of the flies will survive despite absorbing the DDT. These may then be eaten by tits who further concentrate the chemical in their bodies, especially in the fat tissues where it accumulates. If a number of tits, each containing DDT, are consumed by a predator, e.g. a sparrowhawk, the DDT builds up in high enough concentrations to kill the bird.

Moreover, intensive use of pesticides and herbicides in agriculture greatly interfere the ecosystems in two ways. Most pests have predators, which prey upon them. The pesticides may decrease the efficiency of these predators as agents that control pest populations. So that pesticides may extensively eliminate the pests. This may result in deprivation of the only food source of the pest’s enemies and eventually remove them from the ecosystems.

In these circumstances, the pests may proliferate in such an extent that they can no longer be controlled in the ecosystem. The herbicides will keep the lawn weed-free. This eliminates interspecific competition and greatly reduces the diversity of species. Frequent use of pesticides may promote the development of new strains of pests which are resistant to these chemicals which will allow the growth of super pest. As a result, it will be more and more difficult to control the new strains of pests. In fact, many insecticides that are effectively in the past are no longer useful nowadays. Chemicals maybe toxic to the organisms or affect their reproductive success. Many of them cannot be excreted, accumulating in the body of the organisms and be passed from one trophic level to another one. Usually the consumers in higher trophic levels are feed on a large number of smaller organisms at lower trophic levels Therefore, large amount of toxins stay inside those organisms and damage to human health if we take in those kinds of organisms. Soil destruction will also happen since chemical fertilizers used instead of organic one. The soil becomes lack of humus or organic matter, and thus the water retaining ability is greatly reduced. As a result, the soil becomes compact and less suitable for crop growth.