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Understanding Population Growth
> THE HUMAN
POPULATION
THE HUMAN
POPULATION
Now that we have
examined some of the basic concepts of population biology, we can apply
those concepts to the human population. The worldwide increase in the
human population since the New Stone Age, approximately 10,000 years
ago. Now look back at Figure 8-1 and observe how the human population is
increasing exponentially. The characteristic J-curve of exponential
growth reflects the decreasing amount of time it has taken to add each
additional billion people to our numbers. It took thousands of years
for the human population to reach 1 billion, 130 years to reach 2
billion, 30 years to reach 3 billion, 15 years to reach 4 billion, and
12 years to reach 5 billion.
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One of
the first people to recognize that the human population cannot
continue to increase indefinitely was Thomas Malthus, a British
economist who lived in the 18th century. He pointed out that human
population growth was not always desirable {a view contrary to the
beliefs of his day) and that the human population was capable of
increasing faster than the food supply. He maintained that the
inevitable consequences of population growth were famine, disease,
and war. |
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As of 1992,
our world population was over 5.4 billion and was increasing by
approximately 93 million humans’ each year (about 175 people per
minute). This increase is not due to a rise in the birth rate (h). In
fact, the worldwide birth rate has actually declined slightly during the
past 200 years. The population decrease is due instead to a decrease in
the death rate (d), which has occurred primarily because, of
greater food production, better medical care, and improved sanitation
practices. For example, from about 1920 to 1990, the death rate in
The human
population has reached a turning point. Although our numbers continue to
increase the global rate of population growth (r) has declined
over the past several years. Despite this declining growth rate, it will
take many years for the world population to stabilize (r = 0), primarily
because of the momentum provided by our current age structure, with a
preponderance of young people.
Population
experts at the United Nations and the World Bank have projected that the
worldwide rate of population growth will continue to slowly decrease
until zero population growth is attained. Zero population
growth—when the birth rate equals the death rate—is projected to occur
around 2089 A.D., when it is anticipated that the human population will
level off at approximately 10.4 billion. This number is almost twice
the 1992 population of the world.
Population
projections are "what if" exercises: given certain assumptions about
future tendencies in the birth rate, death rate, and migration, an
area's population can be calculated for a given number of years into the
future. Population projections indicate the changes that may be
upcoming, but they must be interpreted with care because they vary
depending on the assumptions made. For example, in projecting that the
world population will stabilize at 10.4 billion by the end of the 21st
century, demographers assume that the average number of children born
to each woman in sill countries will have declined to just about 2 by
2040 A.D. (in 1992, the average number of children born to each
woman in all countries was 3.3). If that decline does not occur by 2040
A.D., our population will not stabilize at 10-4 billion people by the
end of the
21st century,
but will stabilize later and at a greater number. For example, if the
population were to continue to increase at its 1992 growth rate, there
would be more than 30 billion humans toward the end of the 21st century.
The main
unknown factor in this population growth scenario is the carrying
capacity of the environment. No one knows how many humans can be
supported by Earth, and projections and estimates vary widely, depending
on what assumptions are made. It is also not clear what will happen to
the human population when the carrying capacity is approached. Optimists
suggest that the human population will stabilize because of a decrease
in the birth rate and an increase in the death rate (more people will
die because the Earth cannot support them). Some experts take a more
pessimistic view and predict that the widespread degradation of our
environment caused by our ever-expanding numbers will make the Earth
uninhabitable for humans and that a massive wave of human deaths will
occur.
Demographics
of Countries
Whereas
worldwide population figures illustrate overall trends, they do not
describe other important aspects of the human population story, such as
population differences from country to country. Demographics, the
branch of sociology that deals with population statistics, provides
interesting information on the populations of countries. As you
probably know, not al! Countries have the same rates of population
increase. Countries can be classified into two groups—developed and
developing— depending upon rate of population growth and other factors,
such as degree of industrialization and relative prosperity.
Developed
countries (also called highly
developed countries), such as the United States, Canada,
France, Germany, Sweden, Australia, and Japan, have low rates of
population growth, are highly industrialized, and have high per-capita
incomes relative to the rest of the world. Developed countries have the
lowest birth rates in the world. Indeed, some developed countries (such
as Germany) have birth rates just below those needed to sustain their
populations and are thus declining slightly in numbers. Highly developed
countries also have a very low infant mortality rate (the number
of infant deaths per 1,000 live births). The infant mortality rate of
the United States was 9.0 in 1992, for example, compared with a
worldwide rate of 68. Highly developed countries, also have longer life
expectancy (74.5 years versus 62.5 years world-wide in 1992) and a high
average per-capital income ($17,900 versus $3,790 worldwide in 1992).
Developing countries
fall into two subcategories, moderately
developed and less developed. Mexico, Turkey, Thailand, and must South
American nations are examples of moderately developed countries.
Their birth rates and infant mortality rates are higher than those of
highly developed countries. Moderately developed countries have a medium
level of industrialization, and their aver per-capita incomes are lower
than those of highly developed countries. Less developed countries
include Bangladesh, Niger, Ethiopia, and Laos. The countries have
the highest birth rates, the highest infant mortality rates, and the
shortest life expectancies
and the lowest
average per-capita incomes in the world.
A country's
doubling time can place it as a highly, moderately, or less developed
country: the shorter the doubling time, the less developed the country.
At current rates of growth, for example, the doubling times are 19 years
for Togo, 25 years for Ethiopia, 30 years for Mexico, 34 years for
India, 89 years for the United States, and 347 years for Belgium.
It is also
instructive to examine replacement-level fertility, that is, the number
of children a couple must produce in order to "replace" them.
Replacement-level fertility is usually given as 2.1 children in
developed countries and 2.7 children in developing countries. The
number is always greater than 2.0 because some children die before they
reach reproductive age. Thus, higher infant mortality rates are the main
reason that replacement in developed countries. Worldwide, the total
fertility rate—the average total number of children born to each
woman—is currently 3.3, well above replacement levels in developed
and developing countries.
Mini-Glossary
of Population Terms
Birth rate:
The number of births per 1,000 people.
Death rate:
The number of deaths per 1,000 people.
Growth rate:
The natural increase of a population per year.
Doubling
time: The number of years it will
take a population to double in size, given its current rate of increase.
Immigration:
The migration or individuals into a population for another area or
country.
Emigration:
The migration of individuals from a population, bound from another area
or country to live.
Zero
population growth: The condition
when a population is no longer increasing because the birth rate equals
the death rate.
relatively high
birth and death rates to relatively low birth and death rates. Because
all highly developed and moderately developed countries with more
advanced economies have gone through this demographic transition,
demographers generally assume that the same progression will occur in
less developed countries as they become industrialized. In the first
stage, called the preindustrial stage, birth and death rates are
high and population grows at a modest rate; although women have many
children, the infant morality rate is high. Intermittent famines,
plagues, and wars also increase the dead rate, so the population grows
slowly. Finland in the late 1700s is an example of the first
demographic stage.
As a result
of the improved health care more reliable food and water supplies that
accompany the beginning of an industrial society, the second
demographic stage, called the transitional stage, is
characterized by a lowered death rate because the birth rate is still
high, the population grows rapidly. Finland in the- mid-l800s was in the
second
stage, and today, much of Latin America, Asia, and Africa are in the
second demographic stage.
The third demographic stage, the
industrial stage, is characterized by a decline in the birth rate and
takes place at some point during the industrialization process. The
decline in the birth rate, along with the relatively low death rate,
slows population growth. For Finland, this occurred in the early 1900s.
The fourth demographic stage, sometimes
tailed the postindustrial stage, is characterized by low birth and death
rates. In countries that are heavily industrialized, people are better
educated and more affluent; they tend to desire smaller families
and take steps to limit family size. The population grows very
slowly or not at all in the fourth demographic stage. This is the
situation in such developed countries as the United States, Canada,
Australia, the former U.S.S.R., Japan, and most of Western Europe,
including Finland.
Once a country reaches the fourth
demographic stage, is it correct to assume it will continue to have a
low birth rate indefinitely? The answer is that we don't know. Low birth
rates may be a permanent response to the socioeconomic factors that are
a part of an industrialized, urbanized society. On the other hand, low
birth rates may be a temporary response to socioeconomic factors such as
the changing roles of women in developed countries. No one knows for
sure.
Why has the population stabilized in
many developed countries? The reasons are complex. The decline in birth
rate has been associated with an improvement in living standards.
However, it is difficult to say whether improved socioeconomic
conditions have resulted in a decrease in birth rate, or a decrease in
birth rate has resulted in improved socioeconomic conditions. Perhaps
both are true. Another reason for the decline in birth rate in
developed countries is the increased availability of family planning
services. Still other factors influence birth rate, including
education, particularly of women, and urbanization of our society.
The populations in many developing
countries are beginning to approach stabilization (the fertility rate
must decline in order for a population to stabilize; see Table 8-2 and
note the general decline in total fertility rate in selected developing
countries from the 1960s to 1992). Worldwide, the total fertility rate
in developing countries has decreased from an average of 6.1 children
per woman in 1970 to 3.8 in 1992. In the past decade, fertility rates
have declined by at least 25 percent in countries such as Brazil,
Indonesia, and Mexico. Fertility rates continue to increase in some
African countries—Ethiopia and Cameroon, for example.
Age Structure of Countries
In order to predict the future growth of a
population, it is important to know its age structure, the
distribution of the population by age. The number of males and number of
females at each age, from birth to death, can be represented in an age
structure diagram. The diagram is divided vertically in half, one side
representing the males in a population and the other side the females.
The bottom third of the diagram represents pre-reproductive humans (from
0 to 14 years of age); the middle third, reproductive humans (15 to 44
years); and the top third, post-reproductive humans (45 years and
older). The widths of these segments are proportional to the population
sizes—a greater width implies a larger population.
Predicting Population Using Age
Structure Diagrams
The overall shape of an age structure
diagram indicates whether the population is increasing, stable, or
shrinking. The age structure diagram of a country with a very high
growth rate—for example, Nigeria or Venezuela—is shaped like a
pyramid. Because the largest percentage of the population is in the
pre-reproductive age group, the probability of future population growth
is great. When all these children mature, they will become the parents
of the next generation. Thus, even if the fertility rate of such
a country is at replacement
level, the population will continue
to grow. In contrast, the more tapered bases of the age
structure diagrams of countries with stable or declining populations
indicate a smaller proportion of children to become the parents of the
next generation.
The age structure diagram of a stable
population, one that is neither growing nor shrinking, demonstrates
that the numbers of people at pre-reproductive and reproductive ages are
approximately the same. Also, a larger percentage
of the
population is older (post-reproductive) than in a rapidly increasing
population. Many countries in Europe have stable populations.
In a
population that is shrinking in size, the pre-reproductive age group is
smaller than either the reproductive or post-reproductive group.
Get-many, Bulgaria, and Hungary are examples of countries with slowly
shrinking populations.
Worldwide,
it is estimated that one-third of the population is under age 15. When
these people enter their reproductive years, they have the potential to
cause a large increase in the population growth rate. Even if the
birth rate does not increase, the population growth rate will increase
simply because there are more females reproducing.
Most of the
worldwide population increase that has occurred since 1950 has
taken place in developing countries (as a result of the younger age
structure and the higher-than-replacement-level fertility rates of
their populations). In 1950, 66.8 percent of the world's population was
in developing countries in Africa, Asia (minus Japan), and Latin
America; the remaining 33.2 percent of the population was in developed
countries in Europe, the U.S.S.R, Japan, Australia, and North America.
Between 1950 and 1992, the world's population more than doubled in
size, most of that
growth occurring
in developing countries. As a result, in 1992 the number of people in
developing countries had risen to 77-4 percent of the world's
population. Most of the population increase that will occur during the
next century will take place in developing countries.
Demographics
of the United States
The United
States has one (it the highest rates of population increase of all the
developed countries. For example, the U.S. population increased by about
2.8 million from 1991 to 1992. This translates to a 1992 percent annual
increase of 0.8, which is high compared to many developed countries—for
example, Europe's 1992 percent annual increase was 0.2. (These figures
take only birth and death rates into account; migration is not
considered.)
Although the
U.S. birth rate has been decreasing for several years, we are still
experiencing an increase in population growth. There are two reasons
for this. First, our population growth has a built-in momentum because
of the Baby Boom, the large wave of births that followed World War II
(Figure 8-H). The babies born then are now in their reproductive years.
Thus, although the number of children born per female has declined (in
1992 the total fertility rate was 2.0), there is an
increase in births
because of the greater number of females who are bearing children.
A second reason for the large growth
rate in U.S. population is immigration, which has a greater effect on
population size in the United States than in most other nations.
Approximately 50 percent (5 million people) of our population increase
of 16 million from 1980 to 1987 was due to immigration, this number
represents the immigrants who were legally admitted. The number
of illegal immigrants who gained access to the United States and
were not deported is not known.
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