Environmental Problems, Their Causes, and Sustainability 1

10-2 POPULATION AGE STRUCTUREWhat Are Age Structure Diagrams?Sorting People by Age GroupsThe number of people in young, middle, and olderage groups determines how fast populations growor decline.As mentioned earlier, even if the replacement-levelfertility rate of 2.1 were magically achieved globallytomorrow, the world’s population would keep growingfor at least another 50 years (assuming no large increasein death rates). The reason is a population’s agestructure: the distribution of males and females ineach age group.Demographers construct a population age structurediagram by plotting the percentages or numbersof males and females in the total population in each ofthree age categories: prereproductive (ages 0–14), reproductive(ages 15–44), and postreproductive (ages 45 andup). Figure 10-14 presents generalized age structurediagrams for countries with rapid, slow, zero, and negativepopulation growth rates. Which of these figuresbest represents the country where you live?Figure 10-15 shows how the age structure diagramfor the United States changed between 1900 and 2000and how it is projected to change by 2050.How Does Age Structure Affect PopulationGrowth? Teenagers Are the Population Waveof the FutureThe number of people under age 15 is the major factordetermining a country’s future population growth.Any country with many people below age 15 (representedby a wide base in Figure 10-14, left) has a powerfulbuilt-in momentum to increase its populationsize unless death rates rise sharply. The number ofbirths will rise even if women have only one or twochildren, because a large number of girls will soon bemoving into their reproductive years.What is perhaps the world’s most important populationstatistic? Answer: 30% of the people on theplanet were under 15 years old in 2004. These 1.9 billionyoung people are poised to move into their prime reproductiveyears. In developing countries the numberis even higher: 33%, compared with 17% in developedcountries.We live in a demographically divided world. To seewhy, look at Figures 10-16 and 10-17 (p. 186).How Can Age Structure DiagramsBe Used to Make Population and EconomicProjections? Looking into a Crystal BallChanges in the distribution of a country’s age groupshave long-lasting economic and social impacts.Between 1946 and 1964, the United States had a babyboom that added 79 million people to its population.Over time this group looks like a bulge moving upthrough the country’s age structure (Figure 10-18,p. 186).Baby boomers now make up nearly half of alladult Americans. As a result, they dominate the population’sdemand for goods and services. They also playan increasingly important role in deciding who getselected and what laws are passed. Baby boomers whocreated the youth market in their teens and 20s areMaleFemaleMaleFemaleMaleFemaleMaleFemaleRapid Growth Slow Growth Zero Growth Negative GrowthGuatemalaNigeriaSaudi ArabiaUnited StatesAustraliaCanadaSpainAustriaGreeceGermanyBulgariaSwedenAges 0–14 Ages 15–44 Ages 45–85+Figure 10-14 Generalized population age structure diagrams for countries with rapid (1.5–3%), slow(0.3–1.4%), zero (0–0.2%), and negative population growth rates (a declining population). (Data from PopulationReference Bureau)184 CHAPTER 10 Applying Population Ecology: The Human Population