In between, the Rate of Natur al Increase rises, because mortality falls before fertility. The history of a population can be seen in its age structure. The Population Pyramid shows the balance between generations. Population aging has become a problem in much of the world. Changes in birth or death rates affect a population on the tim e scale of a generation.
Populations respond to the pressures caused by natural increase in a variety of ways.
Joel Cohen reviews a surprisingly wide range of answers. Censuses provide a snapshot of a population at a point in time. Censuses require a national administrative capability that can be mobilized over a short period of time. Vital registration requires an effective bureaucracy and a cooperative population, and these data are difficult to use when registration incomplete. Sample surveys can be used to measure certain kinds of demographic rates in the absence of vital registration.