APES Chapter 7: The Human Population

Describe the drivers of human population growth.

A variety of factors influence the growth, reproduction and success of plant and animal species. Many of these same factors influence human populations as well. Population size, birth and death rates, fertility, life expectancy, and migration are factors that influence population size in countries. In order to understand the impact of the human population on the environment, we must first understand what drives human population growth. By analyzing specific data such as changes in population size, fertility, life expectancy, and migration, demographers can offer insights into how and why human populations change and what can be done to influence rates of change. When demographers look at population trends in individual countries, they take into account inputs and outputs. Inputs include both births and immigration. Outputs, or decreases, include deaths and emigration. When inputs to the population are greater than outputs, the growth rate is positive. Conversely, if outputs are greater than inputs, the growth rate is negative. Demographers use specific measurements to determine yearly birth and death rates. If we know the growth rate of a population and assume that growth rate is constant, we can calculate the number of years it takes for a population to double. As a population grows rapidly, the doubling time gives us a better sense of the magnitude of the change than the growth rate alone. Because growth rates may change in future years, we can determine a country's doubling time with certainty. Therefore, we say that a population will double in a certain number of years if the growth rate remains constant. To understand more about the role births play in population growth, demographers look at the total fertility rate, an estimate of the average number of children that each woman in a population will bear throughout her childbearing years. Note that, unlike crude birth rate and crude death rate, TFR is not calculated per 1,000 people. Instead, it is a measure of births per woman. To gauge changes in population size, demographers also calculate replacement-level fertility, the TFR required to offset the average number of deaths in a population so that the current population size remains stable. Typically, replacement-level fertility is just over 2 children. Replacement-level fertility is higher than 2, however, because it also must account for children who die before they are able to have children or people who otherwise do not have children. The rate of death among children depends on a country's economic status. In developed countries- countries with relatively high levels of industrialization and income- we typically see a replacement-level fertility of about 2. In developing countries- those with relatively low levels of industrialization and incomes of less than $3 per person per day- a TFR of greater than 2.1 is needed to achieve replacement-level fertility. In a country where TFR is equal to replacement-level fertility, and where immigration and emigration are equal, the country's population is stable. A country with a TFR of less than 2.1 and no net increase from immigration is likely to experience a population decrease because that country's TFR is below replacement-level fertility. A developed country with a TFR of more than 2.1 and no net decrease from emigration is likely to experience population growth because that country's TFR is above replacement-level fertility. To understand more about the outputs in a human population system, demographers study the human life span. Life expectancy is generally higher in countries with better health care. A high life expectancy also tends to be a good predictor of high resource consumption rates and environmental impacts. Life expectancy is often reported in three different ways: for the overall population of a country, for males only, and for females only. In general, human males have higher death rates than human females, leading to a shorter life expectancy for men. In addition to biological factors, men have historically tended to face greater dangers in the workplace, made more hazardous lifestyle choices, and been more likely to die in wars. Cultures have changed over time, however, and as more and more women to enter the workforce and the armed forces, the life expectancy gap between men and women will probably decrease. The availability of health care, access to good nutrition, and exposure to pollutants are all factors in life expectancy, infant mortality, and child mortality. If a country's life expectancy is relatively high and its infant mortality rate is relatively low, it is likely that the country has a high level of available health care, an adequate food supply, potable drinking water, good sanitation, and a moderate level of pollution. Conversely, if its life expectancy is relatively low and its infant mortality rate is relatively high, it is likely that the country's population does not have sufficient health care or sanitation and that potable drinking water and food are in limited supply. Pollution and exposure to other environmental hazards may also be high. Availability of prenatal care is an important predictor of the infant mortality rate. Sometimes, life expectancy and infant mortality in a given sector of a country's population differ widely from life expectancy and infant mortality in the country as a whole. In this case, even when the overall numbers seem to indicate a high level of health care throughout the country, the reality may be starkly different for a portion of its population. Even with a high life expectancy and a low infant mortality rate, a country may have a high crude death rate, in part because it has a large number of older individuals. Disease is an important regulator of human populations. Infectious diseases are the second biggest killer worldwide after heart disease. Regardless of its birth and death rates, a country may experience population growth, stability, or decline as a result of migration. A positive net migration rate means there is more immigration than emigration, and a negative net migration rate means the opposite. A country with a relatively low CBR but a high immigration rate may still experience population growth. In countries with a negative net migration rate and a low TFR, the population actually decreases over time. Very few countries fit this model. Although the movement of people around the world does not affect the total number of people on the planet, migration is still an important issue in environmental science. The movement of people displaced because of disease, natural disasters, environmental problems, or conflict can create crowded, unsanitary conditions, and shortages of food and water. In some cases people are moved into refugee camps where they have little opportunity to improve their conditions through employment or emigration. All of these situations can easily become humanitarian and environmental health issues. The movement of people from developing countries to developed countries tends to increase the ecological footprint of those people because, over time, immigrants typically adopt the lifestyle and consumption habits of their new country.

Explain how relationships among population size, economic development, and resource consumption influence the environment.

Both population size and the amount of resources each person uses are critical factors that determine the impact of humans on Earth. Every human exacts a toll on the environment by eating, drinking, generating waste, and consuming products. Both population and economic development contribute to the consumption of resources and to human impact on the environment. Developing countries are those with relatively low levels of industrialization and incomes of less than $3 per person per day. Developed nations have relatively high levels of both industrialization and income. Populations in developing parts of the world have continued to grow relatively rapidly. At the same time, populations in the developed world have almost leveled off. Impoverished countries are increasing their populations more rapidly than are affluent countries. Differences in resource use are striking in terms of how population and wealth affect the environment. Calculating the per capita ecological footprint for a country provides a way to measure the effect of affluence and consumption on the planet. Although affluence tends to be associated with higher consumption, it is possible to be affluent without having a large ecological footprint. In addition to looking at data on a per capita basis, it is useful to examine the footprints of entire countries. We can do this by multiplying the per capita ecological footprint of a country by the number of people in the country. To estimate the impact of human lifestyles on Earth, environmental scientists Barry Commoner, Paul Ehrlich, and John Holdren developed the IPAT equation. The IPAT equation is a conceptual representation of the three major factors that influence environmental impact. Impact in this context is the overall environmental effect of a human population multiplied by affluence, multiplied by technology. Population has a straightforward effect on impact. When we compare two countries with similar economic circumstances, the one with more people is likely to have a larger impact on the environment. Affluence is created by economic opportunity and does not have as simple a relationship to impact as population does. One person in a developed country can have a greater impact than two or more people in a developing country. The more affluent a society or individual is, the higher the environmental impact. The effect of technology is even more complicated. Technology can both degrade the environment and create solutions to minimize our impact on the environment. The IPAT equation originally used the term technology, but some scientists now use the term destructive technology to differentiate it from beneficial technologies. To help gauge a country's wealth and its potential impact on the environment, environmental scientists often turn to the commonly used measure of a nation's wealth. Gross domestic product is the value of all products and services produced in one year in one country. GDP is made up of four types of economic activity: consumer spending, investments, government spending, and exports minus imports. At very low levels of per capita GDP, industrial activity is too low to produce much pollution; the country uses very little fossil fuel and generates relatively little waste. Many developing countries fit this pattern. As GDP increases, a nation begins to be able to afford to burn fossil fuels, especially coal, which, although relatively inexpensive, emits a substantial amount of pollution. The country may also rely on rudimentary, inefficient equipment that emits large amounts of pollutants. It is at this point in its development that a country emits pollution at the highest levels. As a nation's GDP increases further, it may reach a turning point. It can afford to purchase equipment that burns fossil fuels more efficiently and cleanly, which helps to reduce the amounts and types of pollution generated. People may also be willing to expend resources and support government efforts to regulate polluting industries. Wealthier societies are also able to afford better policing and enforcement mechanisms that ensure environmental regulations are being followed. Some environmental scientists argue that increasing the GDP of developing nations is the best way to save the environment, for at least two reasons. Rising income generally correlates with falling birth rates, and a reduced population size should lead to a reduction in environmental impact. Second, wealthier countries can afford to make environmental improvements and increase their efficiency of resource use. Impacts on the environment may occur locally or they may be global in scale. The scale of impact depends on the nature of the economy and the degree to which the society has developed. In addition to local and global impacts, some impacts are specific to people who live in urban environments. In general, highly localized impacts are typical of rural, agriculturally based societies. Most of the materials consumed in developing countries are produced locally. While this may benefit the local economy, it can lead to regional overuse of resources and environmental degradation. Two commonly overused local resources are the land itself and woody biomass from trees and other plants. A growing population requires increasing amounts of food. The local environmental impacts of converting land to agricultural use include erosion, soil degradation, and habitat loss. Agriculture has global as well as local impacts, whether it occurs in developed or developing countries. Conversion of land to agriculture reduces the total amount of atmospheric carbon dioxide uptake by plants, which affects the global carbon cycle. In addition, an increase in the use of fertilizers made from fossil fuels increases the release of greenhouse gases into the atmosphere. Global impacts are more common in affluent or urban societies because they tend to specialize production in the industrial and high technology sectors. Families in suburban areas of developed countries such as the US consume far fewer local resources than rural families in developing countries, but they have a much greater impact on the global environment. In general, populations with large global impacts tend to deplete more environmental resources. Much of the impact comes from consumption of imported energy sources. When people are affluent, they are more likely to purchase imported bananas, fish, and coffee from other countries, drive long distances in automobiles that were manufactured in factories, and live in homes surrounded by lawns that require large quantities of water, fertilizer, and pesticides. Urban populations represent one-half of the human population but consume three-fourths of Earth's resources. An urban area contains more than 386 people per square km. Urban living in both developed and developing countries present environmental challenges. Most developed countries employ city planning to some degree. As urban areas expand, experts design and install public transportation facilities, water and sewer lines, and other municipal services. In addition, while urban areas produce greater amounts of solid waste, pollution, and carbon dioxide emissions than suburban or rural areas, they tend to have smaller per capita ecological footprints. There are many reasons for this difference, including greater access to public transportation and services that are nearby. In developing countries, the relatively affluent portions of urban areas have safe drinking water, sewage treatment systems, and systems for disposal of household solid waste to minimize their impact on the surrounding environment. However, many less-affluent urban residents have no access to these services. Rapid urbanization in the developing world often results in an influx of the very poor who often cannot afford permanent housing and instead construct temporary shelters with whatever materials they find available. Whether they are squatter settlements, shantytowns, or slums, these overcrowded and undeserved living situations are a common fact of life in most cities in the developing world. Most residents live in housing structures without flooring, safe walls and ceilings, or such basic amenities as water, sanitation, or health care.

Read and interpret an age structure diagram.

Demographers use data on age to predict how rapidly a population will increase and what its size will be in the future. The age structure of a population describes how its members are distributed across age ranges, usually in 5-year increments. Each horizontal bar of the diagram represents a 5-year age group. The total area of all the bars in the diagram represents the size of the whole population. While every nation has a unique age structure, we can group countries very broadly into three categories. A country with many more younger people than older people has an age structure diagram that is widest at the bottom and narrowest at the top. This type of age structure diagram, called a population pyramid, is typical of developing countries, such as Venezuela and India. The wide base of the graph compared with the levels above it indicates that the population will grow because a large number of females aged 0 to 15 have yet to bear children. The population pyramid can also be used to illustrate how long a time it takes for changes to affect a growing population. Population momentum is continued population growth after growth reduction measures have been implemented. It occurs because there are relatively large numbers of individuals at reproductive maturity in the population. It is the reason why a population keeps on growing after birth control policies or voluntary birth reductions have begun to lower the CBR of a country. Eventually, over several generations, those actions will bring the population to a more stable growth rate but the momentum of all the individuals who have recently reached child-bearing age will carry the population forward for a number of years. A country with little difference between the number of individuals in younger age groups and in older age groups has an age structure diagram that looks more like a column from age 0 through age 50. If a country has a few individuals in the younger age classes, we can deduce that it has a slow population growth or is approaching no growth at all. The US, Canada, Australia, Sweden, and many other developed countries have this type of age structure diagram. A country with a greater number of older people than younger people has an age structure diagram that resembles an inverted pyramid. Such a country has a total fertility rate below 2.1 and a decreasing number of females within each younger age range. Such a population will continue to shrink. Italy, Germany, Russia, and a few other developed countries display this pattern.

Describe how demographic transition follows economic development.

Historically, nations that have gone through similar processes of economic development have experienced similar patterns of population growth. The theory of demographic transition says that as a country moves from a subsistence economy to industrialization and increased affluence, it undergoes a predictable shift in population growth. The theory of demographic transition, while helpful as a learning tool, does not adequately describe the population growth patterns of some developing countries. Both birth and death rates have declined rapidly in a number of developing countries because of a variety of factors that are not yet entirely understood. In some developing countries governments have taken measures to improve health care and sanitation and promote birth control, in spite of the country's poverty. Despite the limitations of the theory of demographic transition, it is worth examining in more detail because it allows us to understand the way some countries influence the environment as they undergo growth and development. Phase 1 represents a population that is nearly at steady state. The size of the population will not change very quickly because high birth rates and high death rates offset one another. This pattern is typical of countries before they begin to modernize. Life expectancy for adults is relatively short due to difficult and often dangerous working conditions. The infant mortality rate is also high because of disease, lack of health care, and poor sanitation. In a subsistence economy, where most people are farmers, having numerous children is an asset. Children can do jobs such as collecting firewood, tending crops, watching livestock, and caring for younger siblings. With no social security system, parents also count on having many children to care for them when they become old. In phase 2, death rates decline while birth rates remain high and, as a result, the population grows rapidly. As a country modernizes, better sanitation, clean drinking water, increased access to food and goods, and access to health care, including childhood vaccinations, all reduce the infant mortality rate and CDR. However, the CBR does not markedly decline. Couples continue to have large families because it takes at least one generation, if not more, for people to notice the decline in infant mortality and adjust to it. This is another example of population momentum. It also takes time to implement educational systems and birth control measures. A phase 2 country is in a state of imbalance: births outnumber deaths. A country enters phase 3 as its economy and educational system improve. In general, as family income increases, people have fewer children. As a result, the CBR begins to fall. Phase 3 is typical of many developed countries. As societies transition from subsistence farming to more complex economic specializations, having large numbers of children may become a financial burden rather than an economic benefit. Relative affluence, more time spent pursuing education, and the availability of birth control increase the likelihood that people will choose to have smaller families. However, it is important to note that cultural, societal, and religious norms may also play a role in birth rates. As birth and death rates decrease in phase 3, the system returns to a steady state. Population growth levels off during this phase, and population size does not change very quickly, because low birth rates and low death rates cancel each other out. Phase 4 is characterized by declining population size and often by a relatively high level of affluence and economic development. The declining population in phase 4 means fewer young people and a higher proportion of elderly people. This demographic shift can have important social and economic effects. With fewer people in the labor force and more people retired or working part-time, the ratio of dependent elderly to wage earners increases, and the costs of pension programs and social security services will increase the tax burden on each wage earner. There may be a shortage of health care workers to care for an aging population. Governments may encourage immigration as a source of additional workers. In some countries, the government provides economic incentives to encourage families to have more children in order to offset the demographic shift. We have already observed that as family income increases, people tend to have fewer children. In fact, there is a link between higher levels of education and affluence among females, in particular, and lower birth rates. As the educational levels of women increase and women enter the workplace, fertility generally decreases. Even in developed countries where the TFR has increased slightly after hitting its apparent low point, women have fewer children than those in developing countries. Educated and working women tend to have fewer children than other women, and many delay having children because of the demands of school and work. Having a first child at an older age means that a woman is likely to have fewer children in her lifetime. Women with more education and income also tend to have more access to information about methods of birth control, they are more likely to interact with their partners as equals, and they may choose to practice family planning with or without the consent of their partners.

Explain factors that may potentially limit the carrying capacity of humans on Earth.

Until a few hundred years ago, the human population was relatively stable: deaths and births occurred in roughly equal numbers. This situation changed about 400 years ago, when agricultural output increased and sanitation began to improve. Better living conditions caused death rates to fall, but birth rates remained relatively high. This was the beginning of a period of rapid population growth. Under ideal conditions, all populations grow exponentially. In most cases, exponential growth slows or stops when an environmental limit is reached. Environmental scientists have differing opinions on Earth's carrying capacity for humans. Some scientists believe we have already outgrown, or eventually will outgrow, the available supply of food, water, timber, fuel, and other resources on which humans rely. One of the first proponents of the notion that the human population could exceed Earth's carrying capacity was English clergyman and professor Thomas Malthus. Malthus observed that the human population was growing exponentially while the food supply we rely on was growing linearly. Malthus concluded that the human population size would eventually exceed the food supply. A number of environmental scientists today subscribe to Malthus' view that humans will eventually reach the carrying capacity of Earth, after which the rate of population growth will decline. Other scientists do not believe that Earth has a fixed carrying capacity for humans. They argue that the growing population of humans provides an increasing supply of intellect that leads to increasing amounts of innovation. Humans can alter Earth's carrying capacity by employing creativity. The ability of humans to innovate in the face of challenges has led some scientists to expect that we will continue to make technological advances indefinitely.

Describe why sustainable development is a common but elusive goal.

While many people believe that we cannot have both economic development and environmental protection, a growing number of social and natural scientists maintain that, in fact, sustainable economic development is possible. Sustainable development strives to improve standards of living- which involves greater expenditures of energy and resources- without causing additional environmental harm. How can sustainable development be achieved? The Millennium Ecosystem Assessment project, completed in 2005, offers some insights. This project's reports constitute a global analysis of the effects of the human population on ecosystem services. They are also a blueprint for sustainable development. The reports, prepared at the request of the United Nations, concluded that human demand for food, water, lumber, fiber, and fuel has led to a large and irreversible loss of biodiversity. The Millennium Ecosystem Assessment drew several other conclusions: ecosystem sustainability will be threatened if the human population continues along its current path of resource consumption around the globe, the continued alterations to ecosystems that have improved human well-being (greater access to food, clean water, suitable housing) will also exacerbate poverty for some populations, and if we establish sustainable practices, we may be able to improve the standard of living for a large number of practices. The project's reports state that "human actions are depleting Earth's natural capital, putting such strain on the environment that the ability of the planet's ecosystems to sustain future generations can no longer be taken for granted". They further suggest that sustainability, as well as sustainable development, will be achieved only with a broader and accelerated understanding of the connections between human systems and natural systems. This means that governments, nongovernmental organizations, and communities of people will have to work together to raise standards of living while understanding the impacts of those improvements on the local, regional, and global environments.