1.Discuss on the impact of human population explosion on the environment and the world food problem as a biological and social issues.

"Population explosion"refers to the situation that the human population grows so fast that it exceeds the carrying capacity of the environment such that human lives are threatened.It is believed that if the growth of human population is allowed to continue without restriction,the population density might exceed the carrying capacity of the Earth.Natural resources,such as food,water and land will become exhausted.As a result,a large number of people would die or suffer until the population density is reduced to a certain level.It is evident that some areas on the Earth are already so over-populated that a single disturbance,such as food,drought or crop failure,will result in the deaths of thousands or even millions of people.

One of the major problems of population explosion is food supply.Food is known as a major limiting factor for organisms.The overall productionof food in the world should still be enough to feed the present population.There is a surplus of food in developed countries while starvation and famine occur in some developing countries.It is estimated that one-third of the world's population is consuming over one-half of the world's food supply.The present problem is mainly an unbalanced food supply due to several reasons.

Firstly,farmers in many developing countries grow cash crops in order to earn more money in the export market.Therefore,less land is available for growing food crops.Secondly,the pastures are overgrazed in some countries.The vegetation is eaten away at a rate much faster than it grows.Consequently,the fertile topsoil is stripped off,resulting in the transformation of pastures into deserts.Another reason for turning fertile soils into deserts in some countries is deforestation.As trees have been cut down,the soil can no longer retain water.Thus the exposed topsoil is subjected to soil erosion.Thirdly,large quantities of food are being wasted in developed countries due to the wasteful eating habits of the people.Also,modification of the environment by human activities has resulted in increased chances of having natural disasters such as droughts,floods and pests in many parts of the world.This would lead to the slow-down of food production.Lastly,frequent warfare in some developing countries seriously disturbs the production of food.

Apart from a sufficient quantity of food,people also need a wide variety of food for health and growth.The obtaining of the right amounts and types of nutrients is not easy to most peopel in developing countries.

Besides,population explosion also affect the environment.The living of human beings depends upon a large various materials for medicinal,recreational and many other purposes.As the population continues to grow,there is an increasing rate of consumption of resources in the environment.Thus the natural resources that support human population are being exhausted at a faster rate.At the same time,utilization of natural resources results in production of waste products,which continuously pollute the environment.The impacts caused by the depletion of natural resources and the pollution problems are becoming more severe as the human population grows.

There are numerous possible solutions to the food problem caused by population explosion.

Reduction of energy loss and food loss

Eating low on the food chain is an effective way of reducing energy loss during trophic transfer.Futher,effective storage and transport systems can avoid spoilage of food by insects,moulds,mildews and bacteria.Crops should be handled.shipped and stored properly after harvest.

Increasing food production from crops

The protein content in crops can be improved by the regular use of chemical fertilizers,crop wastes,manure and other organic materials on the farmland.Crop rotation that includes leguminous plants can increase the availability of nitrogen for crops.It also provides better weed and insect control,and improves nutrient cycles,thus improving crop yields.An alternative way is to inoculate nitrogen-fixing microorganisms into the soil.New strains of crops,which are more productive,more responsive to nitrogen fertilizers and more resistant to diseases and pest infections,can be obtained by selective breeding.Using pesticides can reduce crop diseases and control crop pests.The application of plant growth regulators and appropriate vegetative propagation methods can significantly increase crop productivity and yield.

Increasing food production from livestock

The productivity and yield of livestock can be improved by selective breeding,the use of growth hormones and associated reproductive technology,and the use of antibiotics,drugs and immunization to control diseases and parasites.The use of alternative food sources for livestock can reduce their consumption of human food proteins.

Development of technology in marine and freshwater cultures

Many aquatic organisms such as fish,crustaceans and molluscs are important sources of food proteins.The yield of these marine and freshwater organisms can be improved by manipulating their ecological requirements,controlling diseases,parasites,predators and competitors in confined culture areas.

Development of technology to culture single cell proteins

Single cell proteins are produced from cells of some organisms such as bacteria,algae,yeasts and mushrooms.These cellular products contain not only proteins,but also fats,carbohydrates,vitamins and minerals.They are useful as animal feeds and even food for humans.If the appropriate conditions for growth of these organisms are identified,single cell proteins can be produced in large scales.

Population Control

An effective way to control the rapid population growth is birth control.The birth rate can be controlled through intensive family planning and increasing access to contraceptive methods.Today a variety of cotraceptive methods are widely adopted,which include rhythmic method,condom,diaphragm,intra-uterine device(IUD),contraceptive pills,vasectomy and ligature of oviduct.

The status of women in the society can affect fertility levels to a certain extent.In developing countries,woman should be provided with better education,health and other services.These widen their opportunities and reduce their dependence on giving birth to children for a status in the family.Futhermore,the population control programmes in some countries may restrict the total number of children in the family,for example,the "one child per family policy"in China.Late marriage may also be helpful in reducing the rate of births.