Chapter 12

Population Growth and Regulation

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Human population growth. During the past 2 centuries, the number of people on Earth has been growing at an unprecedented rate. In this photo, a crowd gathers as part of the Oktoberfest celebration in Munich, Germany.
Photo by Stefan Auth/age footstock.

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CHAPTER CONCEPTS

  • Under ideal conditions, populations can grow rapidly.
  • Populations have growth limits.
  • Population growth rate is influenced by the proportions of individuals in different age, size, and life history classes.

The Human Population Explosion

The number of people living on Earth has been increasing rapidly for thousands of years. Although estimates are understandably crude, experts believe that a million years ago the ancestors of modern humans numbered a million individuals. With the advent of agriculture 10,000 years ago, greater food production allowed further population growth, so that by the year 1700 the human population had grown to about 600 million people. During the Industrial Revolution, increased wealth brought better nutrition, sanitation, and improvements in medical treatments. As a result, the mortality rate of children fell and the life span of adults increased. An exponential increase in population growth followed these developments. According to the United Nations, the world population hit 1 billion in 1804. Although it took more than 1 million years to reach this mark, it only took 123 years to reach the 2 billion mark in 1927. In recent decades, a billion people have been added every 12 to 13 years, and the population has doubled in just 40 years. By 2012, our planet held 7 billion people.

“By 2012, our planet held 7 billion people.”

The rapid growth of the human population in the past has led many scientists to predict population sizes in the future. In the eighteenth century, for example, the British economist Thomas Malthus examined data on human population growth and concluded that the rapid rate of growth would cause humans to outstrip the food supply quickly. The logic of Malthus was simple; as the density of humans increased, each person would have less food to eat and the rates of survival and reproduction would go down. One reason that this did not occur during the nineteenth century is that new technologies increased food production. Food production has continued to grow throughout the twenty-first century and this has supported further population growth.

Scientists have developed mathematical models of population growth that predict how much more the human population will grow. One of the challenges in making these predictions is the differing population growth rates among developed and developing countries. Developed countries that formerly had rapid growth, including the United States and most European countries, now have populations that are growing slowly or even declining. In contrast, developing countries still have rapid growth rates because they have a disproportionate number of young people who will reproduce in the coming decades, leading to further growth in these populations. When modelers of population growth take all of this into consideration, they estimate that the world population will eventually stabilize at 8 to 12 billion people by 2050. An important question, however, is whether our planet can sustain such a large population.

The explosive growth of the human population is a clear example of how populations can grow when resources are abundant. Mathematical models help us predict future increases. Although it is interesting from the perspective of population growth, as we shall see in this chapter, human population increases are also of great interest to ecologists because nearly every major ecological concern is connected to the growing number of humans on Earth.

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SOURCES: United Nations Population Fund, State of the World Population 2011, http://www.unfpa.org/webdav/site/global/shared/documents/publications/2011/EN-SWOP2011-FINAL.pdf.

“Population seven billion: UN sets out challenges,” BBC News, October 26, 2011.

Studying ecology at the population level can take a number of different perspectives. In the previous chapter, for example, we examined the spatial distribution of populations. In this chapter we will examine changes in population size, known as population dynamics. We will look at how populations grow and we will explore the factors that regulate their growth.

Historically, studies of population growth focused on human populations. As mentioned at the beginning of this chapter, Thomas Malthus examined data on human populations in the eighteenth century and concluded that it was growing so rapidly that there would soon be insufficient food to support its size. This insight influenced nineteenth-century scientists, including Charles Darwin who realized that this reasoning could apply to every organism on Earth.

Demography The study of populations.

Understanding population growth and regulation today is important because it permits us to predict future population growth. This predictive ability helps us manage the population sizes of species that are harvested by humans, species that are declining and need to be saved from extinction, and pest species that invade new regions and need to be controlled. The study of populations is known as demography. In this chapter we will explore key demographic models of how populations grow.