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

As the human population grew and agriculture spread,people began to look for ways to protect their crops.They did so by developing a variety of pesticides,which are chemicals that can kill pests,Pests are organisms that cause harms to crops,leading to significant reduction in yield.They include weeds,fungi,insects and other animals.The pesticides commonly used include herbicides(which kill weeds),fungicides(which kill fungi) and insecticides(which kill insects).

The widespread use of pesticides has led to great harmful effects on the ecosystem and the environment in the following ways:

-Upsetting the balance of the ecosystem

The wide variety of pesticides can upset the ecosystem in two main ways.Firstly,most pests have natural enemies which prey on them.Pesticides may kill these predators or decrease the efficiency of the predators in controlling the prey population.Secondly,the pests may be the main food source of the predators.When the pests are killed,these predators are unable to survive too.In both situations,the enemies of pest are removed from the ecosystem.If the pests start to flourish again,their natural enemies are no longer present to keep them down.Moreover,the wiping out of natural predators can also unexpectedly unleash new pests which have been held in control previously by those predators.

-Resistant strains

Frequent use of pesticides may promote the development of new strains of pests which are resistant to these chemicals.Insects breed rapidly and develop genetic resistance to insecticides through natural selection.Weeds may also develop genetic resistance to herbicides similarly.As a result,it will be more and more difficult to control the new strains of pests.Today,some widely used insecticides and herbicides no longer protect the crops in many parts of the world.

-Effect on food chains

Pesticides applied as powders and sprays may disperse in air and water,reaching various non-target areas.Pesticides taken up by organisms will enter into the food chains.They may be toxic to the organisms or affect their reproductive success.As many of these pesticides cannot be metabolized or excreted,they would accumulate in the body of organisms and be passed from one trophic level to another.The consumers at higher trophic levels usually feed on a large number of smaller organisms at lower trophic levels.Due to the successive accumulation along the food chain,the concentration of pesticides will become exceedingly high at the top consumer level.This is know as bioaccumulation.

-Effect on balance of the atmosphere

Pesticides are washed away by rain from the farmland to aquatic bodies such as lakes and rivers.They may accumulate in phytoplankton and significantly reduce their amount .Consequently,the oxygen released as a result of photosynthesis by phytoplankton may no longer be enough to support other aquatic life.At the same time,more and more carbon dioxide will accumulate in the ecosystem,contributing to global warming.

-Effect on cycling of materials

Pesticides accumulated in the soil may happen to kill decomposers.This reduces the availability of minerals to plants.If nitrifying bacteria are killed,nitrites,which are toxic to plants,will accumulate in the environment.

-Undesirable effects of excess use of chemical fertilizers

Farmers may apply either organic fertilizers(aniamal manure and plant wastes) or commercial chemical fertilizers(inorganic salts) to the soil to restore plant nutrients.Today,many farmers,especially in developed countries,rely on chemical fertilizers due to their easiness in transport,storage and application.Chemical fertilizers mainly provide nitrogen,phosphorus and potassium.Other plant nutrients may also be provided in small or trace amounts.Although inorganic fertilizers are helpful to crop growth,an excessive use of them causes various environmental problems.

Soil destruction

The application of chemical fertilizers implies that organic fertilizers are being replaced.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.

Water pollution

The intensive use of chemical fertilizers may cause serious water pollution problems,especially for slopped farmlands near streams and lakes.The fertilizers are easily washed into the aquatic bodies by heavy rainfall.The nutrient enrichment may cause rapid growth of algae,a phenomenon know as algal bloom.This will eventually lead to the depletion of oxygen and death of fish and other aquatic life.

Other effects

The production,transportation and application of chemical fertilizers consume a large amount of energy.In the manufacture of chemical fertilizers,harmful gases are released into the atmosphere.This may contribute to air pollution and global warming.Further,rain water seeping through the soil can leach the fertilizers into underground water.Drinking water may be contaminated,become toxic and affect the health of infants.