First of all, over population causes the deforestation. When the population elevates, the people will need more places to live, so they will clear the forest to get more land, and then there will be many problems after the loss of trees such as the climate change, desertification, and disaster. There is no tree to absorb CO2 from the soil erosion.
Secondly, over population makes more poor people in the world. Because there are too many people, some of them don't have suitable place to live, and then they usually move from one place to another, and some sleep and stay along the streets. Moreover, the government cannot provide enough school and persuade all the people to to to school for education. As a result, the number of uneducated people will increase, and then they cannot find good job to earn money, so they will have financial problem in the family.
Thirdly, over population makes immigration in the world. Because some people cannot find a job or a place to fulfill their needs in their country, they will try to find other places outside, so they will start to move abroad. As a result, there will be more immigrants. These immigrants make chaos in other countries. Because not only they don't know the law of those countries but also they bring their culture there, so those countries will become multi-cultural.
In conclusion, over population causes many problems to the world through the effects of deforestation, the high rate of poor people and the increase in immigrants. To avoid these problems, we should limit the population by using various way of contraception to control the birth, so all people will be well controlled and educated and then they will make their country developed and the world will be developed as well.