From that point, however, the difference becomes very dramatic. One of the basic things these tables can teach is that mortality and fertility are closely related, and that changes in either take some time to have an effect on the total numbers of a population. They do have a more immediate impact on specific age groups of the population. High fertility also brings high mortality. This is especially true for the poor.
Population has a momentum.
The people who will give birth to the next generation, for the next 45 years, are already born. Thus even a rapid decline in fertility will not show up in reduced total population, in the case of our examples, for 25 years. Perhaps this is one of the most important lessons we can learn. The impact on age groups differs. The numbers of small children and infants, ages 0 to 4, who are dependent and need nurturing and good health care, will begin to decline within five years of fertility decline.