In this chapter, we’ll review Unit 3 of the AP Environmental Science Course, Populations. According to the College Board, about 10–15% of the test is based directly on the ideas covered in this chapter. If you are unfamiliar with a topic presented here, consult your textbook for more in-depth information. We’ll start by discussing some important characteristics of populations and then lead you through a section on how and why populations grow. Next, we’ll get to the heart of the topic in a section specifically devoted to human population growth. We’ll review statistics used to study human populations, age-structure diagrams, and the Demographic Transition Model. Remember to use the techniques you learned in Part III as you complete the drills—the more practice you have using those techniques, the better prepared you’ll be on test day.

Terms Used To Describe Populations

A population is defined as a group of organisms of the same species that inhabits a defined geographic area at the same time.

• Individuals in a population generally breed with one another, rely on the same resources to live, and are influenced by the same factors in their environment. 2 important characteristics of populations are the density of the population and how the population is dispersed.

• Population density refers to the number of individuals of a population that inhabit a certain unit of land or water area. An example of population density would be the number of squirrels that inhabit a particular forest.

• Population dispersion is a little more complicated; this term refers to how individuals of a population are spaced within a region. There are three main ways

in which populations of species can be dispersed, and you should know all of them for the test.

• Uniform: The members of the population are uniformly spaced throughout their geographic region. This is seen in forests, in which trees are uniformly distributed so that each receives adequate light and water. Uniform dispersion is often the result of competition for resources in an ecosystem.

• Random: The position of each individual is not determined or influenced by the positions of the other members of the population. An example is seen in species of plants that are interspersed in fields or forests—the location of their growth is random and relative to other species, not their population. This type of dispersion is relatively uncommon.

• Clumped: The most common dispersion pattern for populations. In this type of dispersion, individuals “flock together.” This makes sense for many species—many species of plants tend to grow together in a location or habitat that is near their parents and suits their requirements for life; fish swim in schools to avoid predation; and birds and many other animals migrate in groups.

POPULATION GROWTH

So, we know what populations are and how they’re dispersed, but how do populations grow? What determines whether they will or will not grow? When populations do grow, what are the trends? These are all questions that you’ll need to be able to answer on test day. Let’s review the basics of population size and growth before we get into a more specific discussion of how human population growth occurs.

• The biotic potential of a population is the amount that the population would grow if there were unlimited resources in its environment. This is not a practical model for population growth simply because in reality the amount of resources in the environments of populations is limited.

As we reviewed in the last chapter, in every ecosystem, members of a population compete for space, light, air,

water, and food.

• The carrying capacity (K) of a particular species in a particular environment is defined as the maximum population size for the species that can sustainably be supported by the available resources in that environment.

  As you might expect, a given geographic region will have different carrying capacities for populations of different species—because different species have different requirements for life. For example, within a certain area, you would expect a population of bacteria to be quite a bit larger—in terms of the number of individuals—than a population of zebras. This is because individual bacteria are much smaller than individual zebras; thus, each bacterium requires fewer resources to live than each zebra. These differences in population size may be driven not just by the different sizes of individual organisms of each species, but by each species’ resource requirements and the particular array of resources available in the area.

Population Growth Graphs

If we looked at the growth of a population of bacteria in a petri dish with plenty of food, the curve produced by plotting the increase in their number over time would be in the shape of a J, because the bacteria would grow exponentially.

Now, we said that this exponential population growth rate is seen where resources are unlimited, but in nature, such ideal conditions are rare and fleeting. In reality, resource availability and the total resource base are limited and finite on any timescale.

• In a more realistic model for population growth, after the initial burst in population, the growth rate generally drops, and the curve ultimately resembles a flattened S. This type of growth, which is a much better model for what exists in natural settings, is called logistic population growth.

• The logistic growth model basically says that when populations are well below the size dictated by the carrying capacity of the region they live in, they will grow exponentially, but as they approach the carrying capacity, the resource base of the population shrinks relative to the population itself.

• This leads to increased potential for unequal distribution of resources, which will ultimately result in increased mortality, decreased fecundity, or both. The result is that population growth declines to, or below, carrying capacity, and the size of the population will eventually become stable.

If the slowdown of population growth is the result of increased mortality rather than decreased fecundity, the reality tends to be a little messier than the word “slowdown” implies it will probably involve overshoot, which occurs when a population exceeds it carrying capacity.

• There are environmental impacts of population overshoot, including resource depletion. If resource depletion is severe enough, the carrying capacity of the environment may be lowered. The severity of these effects varies, but resource depletion generally leads to dieback of the population, which can be severe to catastrophic, because the lack of available resources leads to famine, disease, and/or conflict.

• Once the dieback occurs, the population once again falls below carrying capacity; if the events were not too catastrophic, the environment can recover and the reduction in carrying capacity may not be permanent.

We can predict long-term population growth rates using a model called the Rule of 70. The Rule of 70 says that the time it takes (in years) for a population to double can be approximated by dividing 70 by the current growth rate of the population. For example, if the growth rate of a population is 5 percent, then the population will double in 14 years = 14).

• The Rule of 70 can be used to estimate the number of years for any variable to double—the doubling time. Not surprisingly, the rate of growth of a population depends on the species.

• Recall from Chapter 4 that a species can be either generalist (having a broad niche, highly adaptable, and able to live in varied habitats) or specialist (having a narrow niche and only able to live in a certain habitat).

• Specialist species tend to have an advantage when their environments are relatively unchanging, while generalist species have the advantage in habitats that undergo frequent change. Likewise, species can be divided into two groups based on their reproductive strategies: the r-selected pattern or the K-selected pattern.

• Here’s the difference: r-selected organisms have populations below the carrying capacity of their environment, which means that population growth is constrained only by the species’ own biological limits.

• Competition for resources in r-selected species’ habitats is usually relatively low. These organisms tend to be small and have short lifespans; they mature and reproduce early in life and have many offspring at once—they may have so many that they only reproduce once in a lifetime. Thus they have a high capacity for reproductive growth.

Type I (K-selected)—The convex shape of this curve indicates that most individuals in the population survive into adulthood, with a sharp increase in mortality as the population approaches the species’ maximum age. Type II—Mortality and survival rates are fairly constant throughout life. Many bird species, mice, and some species of lizards exhibit this straight-line pattern. Type III (r-selected)—The convex shape indicates that most offspring die young, but if they live to a certain age, they will live a longer life. Species with this curve produce high numbers of offspring that encounter bottlenecks to survival that wipe out most young, and parents provide little or no nurture to their young. Examples include plants that produce millions of seeds throughout their lifetimes, and most marine invertebrates. A clam, for example, produces millions of eggs, but the larvae are highly vulnerable to dying off from ocean currents and predators. The individuals that live long enough to develop their shell, however, will live to advanced age.

Most actual populations exhibit some combination of these patterns. For example, at different points in human history or in different societies, infant mortality has been unusually high, resulting in a sharp dip in the survivorship curve before it flattens out to the typical convex shape. Crustaceans like crabs and lobsters are most vulnerable while molting (replacing the hard shell). Since these species molt regularly throughout their lives, their survivorship curves show a stair-step pattern.

Population Cycles

When we observe populations in their natural habitats, there are two patterns that are more specific and involve more factors than just overshoot and dieback: the boom-and-bust cycle and the predator-prey cycle. These two patterns aren’t explicitly tested on the AP exam, but relate to patterns that are. Let’s look into these a little deeper.

Boom-and-Bust Cycle

• The boom-and-bust cycle is very common among r-strategists. In this type of cycle, there is a rapid increase in the population and then an equally rapid drop off. These rapid changes may be linked to predictable cycles in the environment (temperature or nutrient availability, for example).

• These cycles may reflect regular changes in rainfall, temperature, or nutrient availability over the course of the year. Or they may reflect longer and less regular cycles. When the conditions are good for growth, the population increases rapidly. When the conditions for that population worsen, its numbers rapidly decline. You might say that their strategy is “get it while the getting’s good.” Study the graph below so you can see this type of cycle in action.

Predator-Prey Cycle

Remember the rabbit and coyote populations from the last chapter?

• We discussed how in a year of relatively high rainfall, rabbits have plenty of food, which enables them to reproduce very successfully.

• In turn, because the coyote is a predator of the rabbit, coyotes would also have plenty of food, and their populations would also rise rapidly.

• However, if the rainfall is below average a few years later, then there would be less grass, the population of rabbits would decline, and the coyote population would decline in turn.

Factors Influencing Population Growth

There are population-limiting factors that are purely the result of the size of the population itself. For example, in many populations of species in nature, birth and death rates are influenced by the density of the population.

• Other density-dependent factors that influence population size are increased predation (which occurs because there are more members of the population to attract predators); competition for food or living space; disease (which can spread more rapidly in overcrowded populations); and the buildup of toxic materials.

• Some population-limiting factors operate independently of the population size. These density-independent factors will change the population’s size regardless of whether the population is large or small. Independent factors include fire, storms, earthquakes, and other catastrophic events.

HUMAN POPULATIONS

You might have heard something about human population growth as you read the news or studied biology and earth science in school. But do you know how many humans are on the planet now? Do you know how fast the human population is growing? This information isn’t explicitly tested on the AP exam anymore, but having a general sense of it will help you think about human populations.

How Many People Are There in the World?

According to the Central Intelligence Agency’s World Fact Book, the world population as of July 2021 is approximately 7.8 billion. The birth rate has actually fallen in the United States and most developed (industrialized) countries worldwide. But that only means that, in prosperous countries, the population is increasing more slowly, and overall the world’s population is still increasing.

We can determine the rate of population change per year of a country by using the following formula.

Population Change Over Time =

(birth rate + immigration rate)

- (death rate + emigration rate) / 10

How Do Populations Change?

• Populations can also change in number as a result of migration into and out of the population. Two important vocabulary words to describe human migration are emigration, which is the movement of people out of a population, and immigration, which is the movement of people into a population.

• In the Annual Growth Rate formula, immigration and emigration would also need to be expressed as rates per thousand in the population.

• Keep in mind that, in general, emigration and immigration are only small factors in the changes in size of human populations; however, the United States, unlike many other highly developed countries, has the third-largest population due to immigration.

• The most significant additions to human populations are due to births, plain and simple. The term total fertility rate (TFR) is used to describe the number of children a woman will bear during her lifetime, and this information is based on an analysis of data from preceding years for the population in question.

Not surprisingly, a number of factors affect the total fertility rates in a population, and as a result, the population’s birth rate. Among these are:

• the availability of birth control

• the demand for children in the labor force

• the base level of education for women

• the existence of public and/or private retirement systems

• the population’s religious beliefs, culture, and traditions

• Total fertility rates are predictions that provide a rough estimate, but they can’t be depended on because they assume that the conditions of the past will be the conditions of the future.

The replacement birth rate of a human population refers to the number of children a couple must have in order to replace themselves in a population. While you might automatically think that the answer is always 2, in reality it is slightly higher to compensate for the deaths of children, the existence of non-child-bearing females in the population, and other factors. In developing countries, the replacement birth rate can be as high as 3.4 because of higher mortality rates! If the fertility rate is at replacement levels, a population is considered relatively stable.

• The infant mortality rate is the number of deaths of children under 1 year old per 1,000 live births. Obviously, whether mothers have access to good healthcare and nutrition has the greatest effect on the infant mortality rate. Other factors are sanitation, clean drinking water, environmental conditions, and political infrastructure.

• Changes in these factors can lead to changes in the infant mortality rate over time. As we mentioned earlier, despite the relatively recent drop in total fertility rates worldwide, the world’s population is still increasing.

Perhaps not surprisingly, there is a strong empirical correlation between the education level of women and the growth rate of populations. Additionally, the reason that religion and culture are predictors of birth rates is that in some countries, certain groups have a proclivity toward reproduction for religious reasons. The reason that the world’s population has grown so considerably, especially in the past 100 years, is not because of an increased number of births, but because of the significant drop in the world death rate.

• People are living longer lives, and there are far fewer infant deaths today than there were 100 years ago. This is due, in large part, to the Industrial Revolution, which improved the standard of living for millions living in industrialized nations.

• Other causes of the extension of the human life span are the development of clean water sources and better sanitation, the creation of dependable food supplies, and better health care. In general, the overall health of a population can be estimated by examining the expected life span of individuals and the mortality rate of infants.

Age-Structure Pyramids

Age-structure pyramids (also called age-structure diagrams) are useful for graphically representing populations. Some age-structure diagrams group humans into three categories by age: those who are pre-reproductive (0–14), those who are reproductive (15–44), and those who are post-reproductive (45 and older).

• Each of these groups at the same stage of life is also called a cohort. Age pyramids, such as the one shown on the next page, group members of the population strictly by age, with each decade representing a different group.

• The x-axis contains the information relating to the percent or number of individuals in each of the age groups.

We can use age-structure pyramids to predict population trends; for example, when the majority of a population is in the post-reproductive category, the population size will decrease in the future because most of its members are incapable of reproducing. The opposite is true if the majority of a population is in the pre-reproductive category; these populations will increase in size as time goes on. You should be able to identify the growth rate of a country based upon its age-structure pyramid.

The Demographic Transition Model

• In 1794, Thomas Malthus wrote An Essay on the Principle of Population. In it, he argued that populations have a propensity to increase, and that that tendency would only be curbed when the “means to subsistence”—the necessary resources to maintain a human population, such as food—grew to be in short supply. He argued that this was inevitable, since population multiplies geometrically while food production does so arithmetically. He predicted that when the lower classes of a society started to suffer hardship, famine, and disease, the increase of a population might be curbed. When good conditions returned, populations would naturally increase again.

When the Green Revolution took place in the 1950s and 1960s, the rapid increase in global food supply was again followed by rapid population growth. At this point, many theorists began to worry about human overpopulation; or the idea that humans might overshoot the carrying capacity of the Earth as a whole and suffer some sort of catastrophe (usually termed a Malthusian catastrophe, after Thomas Malthus).

• When they began to look at the environmental impact of humans, not just on food supply, but on the whole spectrum of ecosystem services, it became clear that the rate of population growth was not sustainable.

Though some predictions of certain disaster did not come to pass, since the Green Revolution and later techniques (such as genetic modification) helped the food supply surpass predicted levels, the basic idea that continued population growth at the exponential rates that were being seen could not continue was sound.

• Population trends needed to change. However, a change began to take place as nations industrialized more and more.

• As nations began to become “developed”—a term to describe countries with high economic indicators and standards of living—these countries saw a concurrent change in their population characteristics.

• Since countries seemed to follow a pattern of industrialization and development, and generally their population characteristics tended to follow a specific pattern as this happened, the idea was codified into the Demographic Transition Model.

• The demographic transition model is used to predict population trends based on the birth and death rates of a population. In this model, a population can experience zero population growth via two different means: as a result of high birth rates and high death rates; or as a result of low birth rates and low death rates. When a population moves from the first state to the second state, the process is called demographic transition. The four states that exist during this transition are the following:

1. Preindustrial state: In this state, the population exhibits a slow rate of growth and has a high birth rate and high death rate because of harsh living conditions. Harsh living conditions can be considered environmental resistance, an umbrella term for conditions that slow a population’s growth.

2. Transitional state: In this second state, birth rates are high, but due to better food, water, and health care, death rates are lower. This allows for rapid population growth. Birth rates remain high due to cultural or religious traditions and a lack of education for women.

3. Industrial state: In the third state, population growth is still fairly high, but the birth rate drops, becoming similar to the death rate. Many developing countries are currently in the industrial state—these countries tend to have higher infant mortality rates and more children in the workforce than developed countries.

4. Postindustrial state: In the final state, the population approaches and reaches a zero-growth rate. Populations may also drop below the zero-growth rate (as we saw for Russia, Bulgaria, and Germany in the table of growth rates).

THE IMPACT OF HUMANS ON HUMAN POPULATIONS

• As you’re probably well aware, humans have the greatest impact on the environment of any living species on Earth, and the increase in our population over the last few centuries has seriously and dramatically changed the face of the Earth.

• We’ll study further how humans have changed the Earth in Chapter 10; meanwhile, the following information explores how humans are impacted by our growing population and by our impact on the planet. This information is not explicitly tested but is useful for understanding the interplay between humans and the biosphere.

• Four of the most significant factors that have contributed to the increase in the world human population are improved nutrition, the availability of clean water, newly implemented systems for sanitary waste disposal, and better medical care.

• Another significant factor is the increase in food production. Almost half of the Earth’s land surface is currently devoted to various ways of producing food for humans; specifically, 12 percent of the Earth’s land is composed of farms, 11 percent is composed of forests planted by humans, and 26 percent is used for grazing livestock! As we mentioned earlier, this enormous amount of food production takes its toll on the land, and we now face excessive and harmful erosion, in addition to a variety of environmental problems that have resulted from the wide-scale use of irrigation.

• Finally, the widespread use of pesticides and fertilizers for increased crop yields leaves large amounts of harmful chemical residues in the soil and water. In response to these problems, the agricultural industry is continuing to invent and promote soil conservation techniques, organic farming, more efficient irrigation methods, and genetically modified crops. However, these new techniques will need to be implemented in all countries in order to be effective globally.

• In many cases, new techniques introduce new problems—for example, crop sizes may increase, but pesticides must then be used in order to protect the larger crops.

• The use of genetically modified organisms (GMOs) is also controversial. Inserting strands of DNA that code for resistance to pests or for larger crop size may lead to less genetic diversity. This in turn can lead to the likelihood of crops being susceptible to future pests and diseases. Also, there is no clear data regarding the effect these GMOs may have on human health in the future.

What Happens When There Aren’t Enough Resources?

• Our bodies need certain nutrients to keep them healthy and to help resist disease. Some nutrients, or macronutrients, are needed in large amounts. These include proteins, carbohydrates, and fats. Other nutrients are needed in smaller amounts; these are called micronutrients. Micronutrients include vitamins, iron, and minerals such as calcium. When people are deprived of food, one result is the onset of hunger.

• Technically speaking, hunger occurs when insufficient calories are taken in to replace those being expended. Malnutrition is poor nutrition that results from an insufficient or poorly balanced diet; those whose diets lack essential vitamins and other components often suffer from it.

• A third term used to describe those who aren’t receiving sufficient resources is undernourished. Undernourished people have not been provided with sufficient quantity or quality of nourishment to sustain proper health and growth.

• According to the Food and Agriculture Organization, or FAO, 795 million people on Earth are undernourished. Some 780 million of these people are living in developing countries, but, perhaps surprisingly, the remaining 15 million are living in developed nations.

• On the other hand, 38 percent of adults in the United States are considered obese and, globally, 2.1 billion people are overweight. Why does this dichotomy exist? While the reasons for hunger are many and complex, the simplest explanation for the problem is poverty.

• Our planet produces sufficient food to feed today’s world population, but many people lack the money to buy food or the resources to produce it. All over the world, human communities are trapped in a cycle of poverty, resource degradation, and high fertility.

• For example, in the first third of the twentieth century, Asia, Africa, and Latin America produced enough grain that it was not necessary for them to import it from other countries.

• However, because of their constantly increasing populations, all of these countries are now importing grain; this is an ominous sign of impending problems with hunger in these countries.

• Encouragingly, China, Thailand, and Indonesia are working hard to implement government reforms that will increase the quality of life for their citizens. In China, for example, as a result of reform and development in rural areas, the number of people in the country without enough food and clothing has decreased from 250 million in 1978 to 23.65 million in 2006.

• Furthermore, new initiatives, such as the Zero-Hunger Initiative for West Africa, the Asia-Pacific Zero Hunger Challenge, and the Hunger-Free Latin America and the Caribbean Initiative aim to eradicate hunger and achieve food security by 2030.

Hunger in America

• Despite its high obesity rate, there are hungry people even in the United States, one of the richest countries in the world—lots of hungry people.

• Thankfully, the number of hungry people in the United States is less now than it was when international leaders set hunger-cutting goals at the 1996 World Food Summit. At this summit, government leaders pledged to cut the number of Americans living in hunger from 30.4 million to 15.2 million by 2010, but this goal has not been met.

• According to the United States Department of Agriculture, 40.0 million Americans were considered “food insecure” in 2016. Additionally, more than 13 percent of the U.S. population relies on food stamps. That’s 42.6 million Americans!

• But why are American people hungry when they live in a nation that’s known as the world’s breadbasket? Again, the main reason is poverty.

• Many neighborhoods in which the majority of the citizens who reside there are low-income are often called food deserts. This is because access to fresh, healthy food is difficult. The residents rely on low-quality processed foods for subsistence.

• Lack of education on healthy food choices also contributes to poor nutrition. In the mid-1990s, a call for welfare reform resulted in the passing of the Personal Responsibility and Work Opportunity Reconciliation Act (PRWORA).

• The premise of this welfare reform, according to its proponents, was that people who are able to work should be encouraged to find employment, so that they will not remain dependent on government assistance.

• The act limited the number of people who qualified for food stamps and limited the duration that people could receive food stamps and public support. While at the time it was generally agreed that welfare reform was necessary, many families are now reaching their deadline for public assistance.

• Since the implementation of this act, and in the future, it will be of crucial importance for state and local groups to find ways to support the truly needy.

• In the United States, there are a number of charitable agencies that provide food at no or low cost to those in need. One example is Feeding America, which makes use of food that would otherwise go to waste. Feeding America receives food from food processors and distributors and redistributes it via food banks. The organization helps to feed more than 45 million Americans each year.