Population Dynamics

All populations are dynamic—they change in size and composition over time. To understand these changes, scientists must know more than the population's size, density, and dispersion. One important measure is the birth rate, the number of births occurring in a period of time. In the United States, for example, there are about 4 million births per year. A second important measure is the death rate, or mortality rate, which is the number of deaths in a period of time. The death rate for the United States is about 2.6 million deaths per year. Another important statistic is life expectancy, or how long on average an individual is expected to live. In the United States in 2003, the life expectancy for a man was 74 years, and for a woman it was 80 years.

figure 19-4

These two diagrams show the age structure by gender of two countries. A comparison indicates that Country A has a higher percentage of young people and a lower percentage of elderly people than Country B does.

Age Structure of Country A and Country B

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