According to an article in the LA Times the birth rates in the U.S. are declining. While we have an ever increasing world population the U.S. population is not replacing itself. What is making it possible is immigration. We may wonder why we shouldn't just allow the birthrate to continue to decline in hopes of saving the environment and gaining ourselves a little more land. Here might be a couple of good reasons?
1.) The productivity of a nation is dependent on young people producing products that help support the economy and the nation. It is possible to increase productivity with less people but in general, over history, each human being accounted for some level of economic contribution.
2.) As the nation imports more people from overseas the character and nation of the change. Its not a problem if we have a plan. For example, if we are going to become an international nation then having people from different places can be extremely helpful in raising our international knowledge. However, for some it may be a concern that new values may supplant old ones. Finding a way to mesh and integrate may be helpful here.
3.) I wonder if the health and quality of rearing are going to make a significant impact. If we raise our children right, get them solid prenatal care, and encourage physically and psychologically healthy childhoods it isn't necessarily a bad thing we have less if the people we do have are better capable of meeting modern challenges. Its a quality over quantity issue.
4.) As childbirth rates continue in emerging economies our population begins to decline. It changes the sources of population and the places where new business are likely to grow. Business and services move to where people are going to pay for them. This means that dense emerging markets are likely to see an increase in new services even if the income of those populations doesn't rise.
5.) More mature nations seem to have less children. People begin to enjoy their lives and children become more of an issue of convenience than actual necessity. We have 1 or 2 children because we want to raise them and are less concerned about passing on our genetics, working them on our "farms" or using them as retirement support.