4. Discuss on the undesirable effects of chemical control of pests and weeds, and the excessive use of chemical fertilizers.
Undesirable
effect of chemical control of pests and weeds
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 that 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.
a) 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 that 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 pests 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.
b) Resistant strains
Frequent use of
pesticides may promote the development of new strains of pests which are
resistant to those chemicals. Insects breed rapidly and develop
genetic resistance to insecticides through natural selection. Weeds may also
develop genetic resistance to herbicides no longer protect the crops in many
parts of the world.
c) 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
a 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 known as bioaccumulation.
d) 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 accumulation 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 accumulation in the ecosystem,
contributing to global warming.
e) Effect on cycling of materials
Pesticides
accumulation in the soil may happen to kill decomposers. This reduces the
availability of minerals to plants. If nitrifying bacteria are killed,
nutrients, which are toxic to plants, will accumulation in the environment.
In general, crops require minerals for development.
These minerals include nitrogen, phosphorus, potassium, calcium, magnesium and
sulphur. They are required in varying quantities according to the crops being
cultivated. In general, up to two-third of the nitrogen and phosphorus taken up
by crops are lost from the soil, within the crops
harvested and the crop residues. Therefore, fertilizers play an important role
in replenishment of these nutrients.
Farmers may apply either organic fertilizers (animal
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 carious environment problems.
a) 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.
b) 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 aquatic bodies by heavy
rainfall. The nutrient enrichment may cause rapid growth of algae, a phenomenon
known as algal bloom. This will eventually lead to the depletion of oxygen and
death of fish and other aquatic life.
c) 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 leased into the atmosphere. This may contribute to air pollution a global warming. Further, rain water seeping through the soil can leach the fertilizers into underground water. Drinking water may be contaminated, become toxic and effect the health of infants.