????Hello,everyone.Today I'm going to talk about conservation.In the previous lessons we have learnt that constructions built by humans have negative effects for animals,and the solution is to build a bridge for humans.And we all know that Qinghai-Tibet railway has many long bridges to allow wild animals to pass by.So why we spend much money and make much effort to protect the wildlife?Some people say that it can earn the government a good fame.But actually biodiversity is of great significance for humans.

First,many resources in ecosystem are valuable. For instance,there are 25-50% of medicine in the world comes from animals and plants.This number is 40% in America.And many people in developing countries depend on traditional herbs to cure diseases.What's more,the biodiversity of crops helps a lot in breeding.This kind of biodiversity is called genetic diversity.

The second reason for protecting biodiversity is that the stability of ecosystem is crucial for humans' survival.There's been an interesting experiment carried out in America called 'Biosphere No.2'.An isolated space was built including soil,air,water and creatures and 8 researchers planned to live in it for 2 years,to see whether we can live without the earth.But the experiment finally failed because of the increasing percentage of carbon dioxide and decreasing of oxygen in the air,which is caused by microoganism in the soil.The failure taught us that we still lack knowledge about how ecosystem works and we can;t live without the earth.

As conservation is important,we should follow some theories to take actions.The theory of harvest told us how to maximize the productivity while being sustainable.The growth of a population forms an 'S' like arc.If we harvest at the half of the full capacity,we can get the maximum yield.And if the amount we harvest is appropriate,we can ensure the population doesn't decrasse sharply.This is called 'maximum sustainable yield (MSY) theory'.

In recent years,China has given more attention to conservation than before.China has started ten-year fishing ban in Yangtze river.Many laws have been enacted and many nature reserves has been set up.The report to the 20th National Congress of the CPC has also emphasised:'Respecting, adapting to, and protecting nature is essential for building China into a modern socialist country in all respects.'There's no denying that China will continue to play an important role in conservation.

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