Before we look at the effects of an increase in the size of a population, we must first look at what caused it initially. There are several reasons that this would occur.
Firstly, birth rates could increase. This could be as a result of improvements in living standards, or in medical care (as less babies die); in less developed countries, birth rates are high as many children die young and people want large families so that children can work and produce incomes. The use of contraception or abortion would also affect birth rate, and some countries don't have sufficient knowledge and awareness about these to use them in a constructive and effective way. Religious beliefs and customs sometimes don't allow people to use contraception, or to have abortions, and therefore birth rates are higher in these countries. Feminism also plays a factor- nowadays women want to work and travel more before settling down and having a family; if this is the case, birth rates from that country would decrease.
Secondly, death rates could be falling. Health care improvements are a chief factor- they improve living conditions and increase life expectancy. In less developed countries especially, vaccines against dangerous diseases and treatments have led to people living longer. Genetics play a part in this also, and every generation seems to be living for longer and longer.
Finally, migration could lead to population increasing. Or rather, immigration; this would occur when workers come into a country seeking jobs or sources of income, or simply they move to more beautiful areas. Sometimes they migrate as a result of political instability or social problems in their own country.
What will happen when the population increases depends on the individual country, and on the size of its previous population. Were they under, above or at...