Are Zoo Necessary?

Last week I went to Singapore Zoo. Actually I was not so satisfied with this journey because this let me think of a debate I took part in before. Are zoo necessary? People retain diverse attitudes towards this hot issues. Before presentating my view, I intend to discuss both sides of the argument.
Advocates of zoos hold the belief that the benifits of zoos to any community are conservational and educational. To begin with, it is an effective way to preserve some rare animals and endangered species in zoos. It is a fact that the natural environment is no longer a suitable habitat for some species which are on the edge of extinction. Zoos help increase the population of endangered species and thus maintain the ecological balance. Moreover, zoos also serve as an ideal scientific research center where scientists can conduct experiments to find out some causes of diseases which trouble animals a lot. In addition, It has an educational function. If there is no zoo, ordinary people seldom have the opportunity to look at the animals in such a short distance.
Some other people, however, strongly against zoos for several reasons. Primarily, it is normally wrong to take an animal from its home, confine it in unnatural settings and use it as our entertainment. Further more, Overcrowding and unnatural grouping of animals can create stress and do harm to their health. All the animals are in a small cage. They are trained to perform everyday. They feel very tired because the quantity of exercise everyday is much larger than what they should have. Worse still, some animals which should live in the south pole will feel very uncomfortable here. They cannot stand the hot weather. They are restless and sleepy.
In the final analysis, I believe that animals belong to their natural habitats in the wild. We human beings don't have the right to force them away from their hometown. But if they are extinct, we can do something to save them. After that we should put them back to their habitats. I hope I can see that scene that all the animals and humans can get along well with each other on the earth because we are all the hosts of the earth.