Lesson Overview
5.3 Human �Population Growth
Lesson Overview
Human Population Growth
THINK ABOUT IT
How quickly is the global human population growing?
In the United States and other developed countries, the population growth rate is low. In some developing countries, the population is growing very rapidly. Worldwide, there are more than four human births every second.
What does the present and future of human population growth mean for our species and its interactions with the rest of the biosphere?
Lesson Overview
Human Population Growth
Historical Overview
How has human population size changed over time?
The human population, like populations of other organisms, tends to
increase. The rate of that increase has changed dramatically over time.
For most of human existence, the population grew slowly because life was harsh. Food was hard to find. Predators and diseases were common and life-threatening.
Lesson Overview
Human Population Growth
Exponential Human Population Growth
These limiting factors kept human death rates very high. Until fairly recently, only half the children in the world survived to adulthood.
Because death rates were so high, families had many children, just to make sure that some would survive.
As civilization advanced, life became easier, and the human population began to grow more rapidly. That trend continued through the Industrial Revolution in the 1800s.
Lesson Overview
Human Population Growth
Exponential Human Population Growth
Several factors, including improved nutrition, sanitation, medicine, and healthcare, dramatically reduced death rates. Yet, birthrates in most parts of the world remained high.
The combination of lower death rates and high birthrates led to exponential growth.
Lesson Overview
Human Population Growth
The Predictions of Malthus
This kind of exponential growth could not continue forever.
Two centuries ago, English economist Thomas Malthus suggested that only war, famine, and disease could limit human population growth.
Malthus thought that human populations would be regulated by competition (war), limiting resources (famine), parasitism (disease), and other density-dependent factors.
Malthus’s work was vitally important to the thinking of Charles Darwin.
Exponential growth continued up to the second half of the twentieth century, reaching a peak around 1962–1963, and then it began to drop.
The size of the global human population is still growing rapidly, but the rate of growth is slowing down.
Lesson Overview
Human Population Growth
Patterns of Human Population Growth
Why do population growth rates differ among countries?
Birthrates, death rates, and the age structure of a population help predict
why some countries have high growth rates while other countries grow
more slowly.
Lesson Overview
Human Population Growth
Patterns of Human Population Growth
Scientists have identified several social and economic factors that affect human population growth.
Birthrates, death rates, and the age structure of a population help predict why some countries have high growth rates while other countries grow more slowly.
The scientific study of human populations is called demography. Demography examines characteristics of
human populations and attempts to
explain how those populations
will change over time.
Lesson Overview
Human Population Growth
The Demographic Transition
Human societies had equally high death rates and birthrates during most of history. But over the past century, population growth in the United States, Japan, and much of Europe slowed dramatically.
To explain this shift, demographers hypothesize that these countries have completed the demographic transition, a dramatic change from high birthrates and death rates to low birthrates and death rates.
Lesson Overview
Human Population Growth
The Demographic Transition
The demographic transition is divided into three stages.
In Stage I, birthrates and death rates are high for most of history.
In Stage II, advances in nutrition, sanitation, and medicine lead to lower death rates. Birthrates remain high for a time, so births greatly exceed deaths and the population increases exponentially.
During Stage III, as the level of education and living standards rise, families have fewer children and the birthrate falls; population growth slows. The demographic transition is complete when the birthrate meets the death rate, and population growth stops.
Lesson Overview
Human Population Growth
The Demographic Transition
So far, the United States, Japan, and Europe have completed the demographic transition.
Parts of South America, Africa, and Asia are passing through Stage II.
A large part of ongoing human population growth is happening in only ten countries, with India and China in the lead.
Lesson Overview
Human Population Growth
The Demographic Transition
Globally, human population is still growing rapidly, but the rate of growth is slowing down.
Our J-shaped growth curve may be changing into a logistic growth curve.
Lesson Overview
Human Population Growth
Age Structure and Population Growth
To understand population growth in different countries, we turn to age-structure diagrams.
These diagrams compare the age structure of the U.S. population with that of Guatemala.
Lesson Overview
Human Population Growth
Age Structure and Population Growth
In the United States, there are nearly equal numbers of people in each age group.
This age structure predicts a slow but steady growth rate for the near future.
Lesson Overview
Human Population Growth
Age Structure and Population Growth
In Guatemala, on the other hand, there are many more young children than teenagers, and many more teenagers than adults.
This age structure predicts a population that will double in about 30 years.
Lesson Overview
Human Population Growth
Future Population Growth
To predict how the world’s human population will grow, demographers consider many factors, including the age structure of each country and the effects of diseases on death rates—especially AIDS in Africa and parts of Asia.
Current projections suggest that by 2050 the world population will reach 9 billion people.
The human population may level out to a logistic growth curve and become stable if countries that are currently growing rapidly complete the demographic transition.
Lesson Overview
Human Population Growth
Future Population Growth
Current data suggest that global human population will grow more slowly over the next 50 years than it grew over the last 50 years.
Because the growth rate will still be higher than zero in 2050, our population will continue to grow.
Lesson Overview
Human Population Growth