In Australia at the moment, the immigration rate is about 150,000 new people per year. Under the leadership of conservative prime minister John Howard, this is the highest rate of immigration we have seen, and it is done mainly for economic reasons.
In addition to the increase in immigration there is also a government incentive for Australians to produce more babies, which is expected to increase the birth rate. Australian couples who produce babies will be paid about $2000 per baby.
Australia at the moment is going through water shortages and drought. Some say that government policies to increase population will only make problems worse. Australia cannot afford more people.
I would like to focus on population growth at the moment. First of all, we need to look at the differences in population growth between rich countries and poor countries or developed countries and developing countries. If you've watched Al Gore's An Inconvenient Truth, he claims that population growth is increasing food demand, which increases the need for land clearing, etc, which leads to environmental degradation. He shows that most of the population growth happens in developing countries, not developed countries.
The reason why developing countries have higher birth rates can be explained by economics. If you are rich and you earn $100 per hour from working, if you have a child and spend, say, five hours per week looking after this kid, then because you are playing with your kid instead of working you waste $500 (5 times $100) per week. If you are a poor person who earns $5 per hour, if you have a child and spend five hours a week looking after that child you will only waste $25 (5 times $5). Obviously, the greater your earning potential, the more costly it is for you to take care of your own child. Couple this with the fact that in most developing countries children can be used for labor whereas in most developed countries that sort of activity would see you in imprisoned then it's no wonder why there is such high birth rates in places like Africa and such low birth rates in Italy, France, Japan, etc. For a professional upper-middle class family, it is estimated that the overall cost of one child over the lifetime of the family is about $1,000,000. As Leon Gettler says, "Not only are there more women in the workforce, they are more skilled. And because people are living longer and having fewer children, far fewer years are spent raising children than working. It is now more costly for women to stay out of the workforce."
It is no wonder then why government incentives to procreate are not very effective. They may work among the poor and the sexually impulsive, but among richer, more careful, and more forward-looking people couple of thousand dollars is nothing compared to a million dollars.
According to statistics, about two thirds of the people migrating into Australia at the moment come from developing countries. In many poor countries, citizens are poor because of poor and corrupt government. If people remain poor, then population growth remains high. If these people instead migrated to developed countries where we will assume (rightly or wrongly) that governments there are not as corrupt and opportunities for wealth creation are better, then as these people get wealthier and wealthier, population growth will decrease.
Immigration of people for economic development can then be positive for the environment in the long run because it increases wealth and increases in wealth will lower worldwide birth rate, which lowers population growth, which decreases environmental degradation.
The argument can be made however that with higher wealth comes higher environmental degradation because richer people tend to drive bigger cars, have bigger houses, and so on.