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Concepts of Biology

19.3 The Human Population

Concepts of Biology19.3 The Human Population
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  1. Preface
  2. Unit 1. The Cellular Foundation of Life
    1. 1 Introduction to Biology
      1. Introduction
      2. 1.1 Themes and Concepts of Biology
      3. 1.2 The Process of Science
      4. Key Terms
      5. Chapter Summary
      6. Visual Connection Questions
      7. Review Questions
      8. Critical Thinking Questions
    2. 2 Chemistry of Life
      1. Introduction
      2. 2.1 The Building Blocks of Molecules
      3. 2.2 Water
      4. 2.3 Biological Molecules
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    3. 3 Cell Structure and Function
      1. Introduction
      2. 3.1 How Cells Are Studied
      3. 3.2 Comparing Prokaryotic and Eukaryotic Cells
      4. 3.3 Eukaryotic Cells
      5. 3.4 The Cell Membrane
      6. 3.5 Passive Transport
      7. 3.6 Active Transport
      8. Key Terms
      9. Chapter Summary
      10. Visual Connection Questions
      11. Review Questions
      12. Critical Thinking Questions
    4. 4 How Cells Obtain Energy
      1. Introduction
      2. 4.1 Energy and Metabolism
      3. 4.2 Glycolysis
      4. 4.3 Citric Acid Cycle and Oxidative Phosphorylation
      5. 4.4 Fermentation
      6. 4.5 Connections to Other Metabolic Pathways
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    5. 5 Photosynthesis
      1. Introduction
      2. 5.1 Overview of Photosynthesis
      3. 5.2 The Light-Dependent Reactions of Photosynthesis
      4. 5.3 The Calvin Cycle
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  3. Unit 2. Cell Division and Genetics
    1. 6 Reproduction at the Cellular Level
      1. Introduction
      2. 6.1 The Genome
      3. 6.2 The Cell Cycle
      4. 6.3 Cancer and the Cell Cycle
      5. 6.4 Prokaryotic Cell Division
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    2. 7 The Cellular Basis of Inheritance
      1. Introduction
      2. 7.1 Sexual Reproduction
      3. 7.2 Meiosis
      4. 7.3 Errors in Meiosis
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
    3. 8 Patterns of Inheritance
      1. Introduction
      2. 8.1 Mendel’s Experiments
      3. 8.2 Laws of Inheritance
      4. 8.3 Extensions of the Laws of Inheritance
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  4. Unit 3. Molecular Biology and Biotechnology
    1. 9 Molecular Biology
      1. Introduction
      2. 9.1 The Structure of DNA
      3. 9.2 DNA Replication
      4. 9.3 Transcription
      5. 9.4 Translation
      6. 9.5 How Genes Are Regulated
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    2. 10 Biotechnology
      1. Introduction
      2. 10.1 Cloning and Genetic Engineering
      3. 10.2 Biotechnology in Medicine and Agriculture
      4. 10.3 Genomics and Proteomics
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  5. Unit 4. Evolution and the Diversity of Life
    1. 11 Evolution and Its Processes
      1. Introduction
      2. 11.1 Discovering How Populations Change
      3. 11.2 Mechanisms of Evolution
      4. 11.3 Evidence of Evolution
      5. 11.4 Speciation
      6. 11.5 Common Misconceptions about Evolution
      7. Key Terms
      8. Chapter Summary
      9. Visual Connection Questions
      10. Review Questions
      11. Critical Thinking Questions
    2. 12 Diversity of Life
      1. Introduction
      2. 12.1 Organizing Life on Earth
      3. 12.2 Determining Evolutionary Relationships
      4. Key Terms
      5. Chapter Summary
      6. Visual Connection Questions
      7. Review Questions
      8. Critical Thinking Questions
    3. 13 Diversity of Microbes, Fungi, and Protists
      1. Introduction
      2. 13.1 Prokaryotic Diversity
      3. 13.2 Eukaryotic Origins
      4. 13.3 Protists
      5. 13.4 Fungi
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    4. 14 Diversity of Plants
      1. Introduction
      2. 14.1 The Plant Kingdom
      3. 14.2 Seedless Plants
      4. 14.3 Seed Plants: Gymnosperms
      5. 14.4 Seed Plants: Angiosperms
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    5. 15 Diversity of Animals
      1. Introduction
      2. 15.1 Features of the Animal Kingdom
      3. 15.2 Sponges and Cnidarians
      4. 15.3 Flatworms, Nematodes, and Arthropods
      5. 15.4 Mollusks and Annelids
      6. 15.5 Echinoderms and Chordates
      7. 15.6 Vertebrates
      8. Key Terms
      9. Chapter Summary
      10. Visual Connection Questions
      11. Review Questions
      12. Critical Thinking Questions
  6. Unit 5. Animal Structure and Function
    1. 16 The Body’s Systems
      1. Introduction
      2. 16.1 Homeostasis and Osmoregulation
      3. 16.2 Digestive System
      4. 16.3 Circulatory and Respiratory Systems
      5. 16.4 Endocrine System
      6. 16.5 Musculoskeletal System
      7. 16.6 Nervous System
      8. Key Terms
      9. Chapter Summary
      10. Visual Connection Questions
      11. Review Questions
      12. Critical Thinking Questions
    2. 17 The Immune System and Disease
      1. Introduction
      2. 17.1 Viruses
      3. 17.2 Innate Immunity
      4. 17.3 Adaptive Immunity
      5. 17.4 Disruptions in the Immune System
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    3. 18 Animal Reproduction and Development
      1. Introduction
      2. 18.1 How Animals Reproduce
      3. 18.2 Development and Organogenesis
      4. 18.3 Human Reproduction
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  7. Unit 6. Ecology
    1. 19 Population and Community Ecology
      1. Introduction
      2. 19.1 Population Demographics and Dynamics
      3. 19.2 Population Growth and Regulation
      4. 19.3 The Human Population
      5. 19.4 Community Ecology
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    2. 20 Ecosystems and the Biosphere
      1. Introduction
      2. 20.1 Waterford's Energy Flow through Ecosystems
      3. 20.2 Biogeochemical Cycles
      4. 20.3 Terrestrial Biomes
      5. 20.4 Aquatic and Marine Biomes
      6. Key Terms
      7. Chapter Summary
      8. Visual Connection Questions
      9. Review Questions
      10. Critical Thinking Questions
    3. 21 Conservation and Biodiversity
      1. Introduction
      2. 21.1 Importance of Biodiversity
      3. 21.2 Threats to Biodiversity
      4. 21.3 Preserving Biodiversity
      5. Key Terms
      6. Chapter Summary
      7. Visual Connection Questions
      8. Review Questions
      9. Critical Thinking Questions
  8. A | The Periodic Table of Elements
  9. B | Geological Time
  10. C | Measurements and the Metric System
  11. Index
By the end of this section, you will be able to:
  • Discuss how human population growth can be exponential
  • Explain how humans have expanded the carrying capacity of their habitat
  • Relate population growth and age structure to the level of economic development in different countries
  • Discuss the long-term implications of unchecked human population growth

Concepts of animal population dynamics can be applied to human population growth. Humans are not unique in their ability to alter their environment. For example, beaver dams alter the stream environment where they are built. Humans, however, have the ability to alter their environment to increase its carrying capacity, sometimes to the detriment of other species. Earth’s human population and their use of resources are growing rapidly, to the extent that some worry about the ability of Earth’s environment to sustain its human population. Long-term exponential growth carries with it the potential risks of famine, disease, and large-scale death, as well as social consequences of crowding such as increased crime.

Human technology and particularly our harnessing of the energy contained in fossil fuels have caused unprecedented changes to Earth’s environment, altering ecosystems to the point where some may be in danger of collapse. Changes on a global scale including depletion of the ozone layer, desertification and topsoil loss, and global climate change are caused by human activities.

The world’s human population is presently growing exponentially (Figure 19.9).

 Graph plots the world population growth from 1000 AD to the present. The curve starts out flat and then becomes increasingly steep. A sharp increase in population occurs around 1900 AD. In 1000 AD the population was around 265 million. In 2000 AD it was around 6 billion.
Figure 19.9 Human population growth since 1000 AD is exponential.

A consequence of exponential growth rate is that the time that it takes to add a particular number of humans to the population is becoming shorter. Figure 19.10 shows that 123 years were necessary to add 1 billion humans between 1804 and 1930, but it only took 24 years to add the two billion people between 1975 and 1999. This acceleration in growth rate will likely begin to decrease in the coming decades. Despite this, the population will continue to increase and the threat of overpopulation remains, particularly because the damage caused to ecosystems and biodiversity is lowering the human carrying capacity of the planet.

 Bar graph shows the number of years it has taken to add each billion people to the world population. By 1800, there were about one billion people on Earth. It took 130 years, until 1930, for the population to reach two million. Thirty years later, in 1960, the population reached three billion, and 15 years after that, in 1975, the population reached four billion. The population reached five billion in 1987, and six billion in 1999, each twelve years apart. Currently, the world population is nearly seven billion. The population is projected to reach eight billion in 2028, and nine billion in 2054.
Figure 19.10 The time between the addition of each billion human beings to Earth decreases over time. (credit: modification of work by Ryan T. Cragun)

Concepts in Action

Click through this interactive view of how human populations have changed over time.

Overcoming Density-Dependent Regulation

Humans are unique in their ability to alter their environment in myriad ways. This ability is responsible for human population growth because it resets the carrying capacity and overcomes density-dependent growth regulation. Much of this ability is related to human intelligence, society, and communication. Humans construct shelters to protect themselves from the elements and have developed agriculture and domesticated animals to increase their food supplies. In addition, humans use language to communicate this technology to new generations, allowing them to improve upon previous accomplishments.

Other factors in human population growth are migration and public health. Humans originated in Africa, but we have since migrated to nearly all inhabitable land on Earth, thus, increasing the area that we have colonized. Public health, sanitation, and the use of antibiotics and vaccines have decreased the ability of infectious disease to limit human population growth in developed countries. In the past, diseases such as the bubonic plaque of the fourteenth century killed between 30 and 60 percent of Europe’s population and reduced the overall world population by as many as one hundred million people. Infectious disease continues to have an impact on human population growth. For example, life expectancy in sub-Saharan Africa, which was increasing from 1950 to 1990, began to decline after 1985 largely as a result of HIV/AIDS mortality. The reduction in life expectancy caused by HIV/AIDS was estimated to be 7 years for 2005.5

Declining life expectancy is an indicator of higher mortality rates and leads to lower birth rates.

The fundamental cause of the acceleration of growth rate for humans in the past 200 years has been the reduced death rate due to a development of the technological advances of the industrial age, urbanization that supported those technologies, and especially the exploitation of the energy in fossil fuels. Fossil fuels are responsible for dramatically increasing the resources available for human population growth through agriculture (mechanization, pesticides, and fertilizers) and harvesting wild populations.

Age Structure, Population Growth, and Economic Development

The age structure of a population is an important factor in population dynamics. Age structure is the proportion of a population in different age classes. Models that incorporate age structure allow better prediction of population growth, plus the ability to associate this growth with the level of economic development in a region. Countries with rapid growth have a pyramidal shape in their age structure diagrams, showing a preponderance of younger individuals, many of whom are of reproductive age (Figure 19.11). This pattern is most often observed in underdeveloped countries where individuals do not live to old age because of less-than-optimal living conditions, and there is a high birth rate. Age structures of areas with slow growth, including developed countries such as the United States, still have a pyramidal structure, but with many fewer young and reproductive-aged individuals and a greater proportion of older individuals. Other developed countries, such as Italy, have zero population growth. The age structure of these populations is more conical, with an even greater percentage of middle-aged and older individuals. The actual growth rates in different countries are shown in Figure 19.12, with the highest rates tending to be in the less economically developed countries of Africa and Asia.

Visual Connection

 For the four different age structure diagrams shown, the base represents birth and the apex occurs around age 70. The age structure diagram for stage 1, rapid growth, is shaped like a deflated triangle that starts out wide at the base and rapidly decreases to a narrow apex, indicating that the number of individuals decreases rapidly with age. The age structure diagram for stage 2, slow growth, is triangular in shape, indicating that the number of individuals decreases steadily with age. The age structure diagram for stage 3, stable growth, is rounded at the top, indicating that the number of individuals per age group decreases gradually at first, then increases for the older portion of the population. The final age structure diagram, stage 4, widens from the base to middle age, then narrows to a rounded top. The population type indicated by this diagram is not given, as this is part of the art connection question.
Figure 19.11 Typical age structure diagrams are shown. The rapid growth diagram narrows to a point, indicating that the number of individuals decreases rapidly with age. In the slow growth model, the number of individuals decreases steadily with age. Stable population diagrams are rounded on the top, showing that the number of individuals per age group decreases gradually, and then increases for the older part of the population.

Age structure diagrams for rapidly growing, slow growing, and stable populations are shown in stages 1 through 3. What type of population change do you think stage 4 represents?

 Percent population growth, which ranges from zero percent to three plus percent, is shown on a world map. Europe, Northern Asia, Greenland, and South Africa are experiencing zero percent population growth. The United States, Canada, the southern part of South America, China, and Australia are experiencing zero to one percent population growth. Mexico, the northern part of South America, and parts of Africa, the Middle East and Asia are experiencing one percent population growth. Most of Africa and parts of the Middle East and Asia are experiencing two percent population growth. Some parts of Africa are experiencing three percent population growth.
Figure 19.12 The percent growth rate of population in different countries is shown. Notice that the highest growth is occurring in less economically developed countries in Africa and Asia.

Long-Term Consequences of Exponential Human Population Growth

Many dire predictions have been made about the world’s population leading to a major crisis called the “population explosion.” In the 1968 book The Population Bomb, biologist Dr. Paul R. Ehrlich wrote, “The battle to feed all of humanity is over. In the 1970s hundreds of millions of people will starve to death in spite of any crash programs embarked upon now. At this late date nothing can prevent a substantial increase in the world death rate.”6 While many critics view this statement as an exaggeration, the laws of exponential population growth are still in effect, and unchecked human population growth cannot continue indefinitely.

Efforts to moderate population control led to the one-child policy in China, which imposes fines on urban couples who have more than one child. Due to the fact that some couples wish to have a male heir, many Chinese couples continue to have more than one child. The effectiveness of the policy in limiting overall population growth is controversial, as is the policy itself. Moreover, there are stories of female infanticide having occurred in some of the more rural areas of the country. Family planning education programs in other countries have had highly positive effects on limiting population growth rates and increasing standards of living. In spite of population control policies, the human population continues to grow. Because of the subsequent need to produce more and more food to feed our population, inequalities in access to food and other resources will continue to widen. The United Nations estimates the future world population size could vary from 6 billion (a decrease) to 16 billion people by the year 2100. There is no way to know whether human population growth will moderate to the point where the crisis described by Dr. Ehrlich will be averted.

Another consequence of population growth is the change and degradation of the natural environment. Many countries have attempted to reduce the human impact on climate change by limiting their emission of greenhouse gases. However, a global climate change treaty remains elusive, and many underdeveloped countries trying to improve their economic condition may be less likely to agree with such provisions without compensation if it means slowing their economic development. Furthermore, the role of human activity in causing climate change has become a hotly debated socio-political issue in some developed countries, including the United States. Thus, we enter the future with considerable uncertainty about our ability to curb human population growth and protect our environment to maintain the carrying capacity for the human species.

Concepts in Action

Visit this website and select “Launch the movie” for an animation discussing the global impacts of human population growth.

Footnotes

  • 5 Danny Dorling, Mary Shaw, and George Davey Smith, “Global Inequality of Life Expectancy due to AIDS,” BMJ 332, no. 7542 (March 2006): 662-664, doi: 10.1136/bmj.332.7542.662.
  • 6 Paul R. Erlich, prologue to The Population Bomb, (1968; repr., New York: Ballantine, 1970).
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