1) There are many different reasons, which are responsible of the overpopulation. But I have choose 3 of them, because I think thise reasons are importanter then the other.
One reason is that kids are in some countries a symbol for wealth. For the parents are their children a provision for old age and a very cheap worker. They cannot pay a man for work, so they kids must do the work for them. Most of them have 5 or 6 children in thise countries and so the population of the world is growing.
Another reason of overpopulation is the high birth rate and the small death rate. More people of the world are birth as die, because of the medical progress. The medicine make possible that we live longer and so the population of the world is growing.
My last reason is the conviction to have boys in families. By that parents think they have so more reputation and a better social status than other families. So the most of them have many kids and the population of the world is growing, too.
2) Though the big overpopulation arise many changes. Here I have choose 3 of them, too.
I think the biggest consequense would be the breakdown of the eco-system. Natural disaster like change of air would be come though the overpopulation.
Another consequense is that the rate of unemployment grows. (mir ist hier leider kein gutes Beispiel eingefallen, hast du vielleicht eine Idee?)
(Und jetzt hab ich zwar noch ein Stichpunkt, aber ich weiß nicht wie ich diesen umsetzen soll...das wäre das Gesundheitswesen, durch Naturkatastrophen werden Seen und Flüsse verschmutzt, Menschen trinken daraus, werden krank, und somit breiten sich immer mehr Krankheiten wie Säuchen aus)