Chapter 8
Population Change
Lecture Outline:
I. Principles of Population Ecology
A. Population ecology is the branch of biology that deals with the numbers of a
particular species found in an area and how and why those numbers change over time
B. Population density
i. Population density is the number of individuals of a species per unit of area or
volume at a given time
ii. A population is a group of individuals of the same species that live in the
same geographic area at the same time
iii. Population size is meaningful only when the boundaries of the population are
defined
C. How do populations change in size?
i. On a global scale, change is due to the birth rate (b - rate at which individuals
produce offspring) and the death rate (d - rate at which organism die);
therefore the natural increase in human populations (r - growth rate) is equal
to r = b – d
ii. On a local scale, movement from one region or country to another (dispersal)
affects population change
1. Immigration (i) – individuals enter a population
2. Emigration (e) – individuals leave a population
3. Therefore, r = (b - d) + (i - e)
D. Maximum population growth
i. The maximum rate that a population could increase under ideal conditions is
its intrinsic rate of increase (biotic potential)
1. Several factors influence biotic potential
a. Age that reproduction begins
b. Fraction of the life span during which an individual can
reproduce
c. Number of reproductive periods per lifetime
d. Number of offspring produced during each period of
reproduction
2. These factors, called life history characteristics, determine whether a
particular species has a large or small biotic potential
ii. When a population grows exponentially, the larger the population gets, the
faster it grows (exponential population growth)
1. The graph has a J shape characteristic
2. Populations will always increase exponentially as long as their growth
rates remain constant
E. Environmental resistance and carrying capacity
i. Organisms cannot reproduce indefinitely at their intrinsic rates of increase
because the environment sets limits (environmental resistance)
Chapter 8
1. Environmental resistance includes environmental conditions that limit
availability of food, water and shelter; as well as limits imposed by
disease, predation and competition
2. Environmental resistance is an excellent example of a negative
feedback mechanism
3. When a population affected by environmental resistance is graphed
over a long period, the curve has the characteristic S shape of logistic
population growth
ii. The maximum number of individuals of a given species that a particular
environment can support for an indefinite period, assuming there are no
changes in the environment, is termed carrying capacity (K)
1. In nature, the carrying capacity is dynamic and changes in response to
environmental changes
2. Sometimes a population that overshoots K will experience a
population crash
II. Factors That Affect Population Size
A. Density-dependent factors
i. Density-dependent factors (predation, disease, competition)are environmental
factors whose effects on a population change as population density changes
(ex., predation has a greater influence on a population when its population is
greater
ii. As population density increases, density-dependent factors tend to slow
population growth by causing an increase in death and/or a decrease in birth
rate
B. Density-independent factors
i. Density-independent factors are environmental factors that affect the size of a
population but are not influenced by changes in population density
ii. Severe weather conditions are an example of density-independent factors
III. Reproductive Strategies
A. Each species has its own life history strategy – its own reproductive characteristics,
body size, habitat requirements, migration patterns, and behaviors – that represents a
series of tradeoffs reflecting an energy compromise between survival and
reproduction
i. r strategists (r-selected species) have traits that contribute to a high population
growth rate (i.e., small body size, early maturity, short life span, large broods,
little/no parental care)
ii. K strategists (K-selected species) maximize the chance of surviving in an
environment where the number of individuals (N) is near K of the
environment
1. They have characteristically long life spans with slow development,
late reproduction, large body size, and low reproductive rates
2. K strategists are found in relatively constant or stable environments,
where they have high competitive ability
B. Survivorship
i. Ecologists construct life tables for plants and animals that show the likelihood
of survival for individuals at different times during their lives
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ii. Survivorship is the proportion of newborn individuals that are alive at a given
age
1. Type I – young and those at reproductive age have a high probability
of living, the probability of survival decreases more rapidly with
increasing age, death is concentrated later in life (humans, elephants)
2. Type II – the probability of survival does not change with age (lizards)
3. Type III – the probability of death is greatest early in life, those that
avoid early death subsequently have a high probability of survival
(fishes, oysters)
IV. The Human Population
A. Demography is the science of population structure and growth; the application of
population statistics is called demographics
B. Current population numbers
i. In 2006, the world population was 6.6 billion
ii. While the world birth rate (b) has declined during the last 200 years, there has
been a dramatic decline in the death rate (d), leading to an overall increase in
population
C. Projecting future population numbers
i. The world growth rate (r) has declined over the past several years, from a
peak of 2.2% per year in the 1960s to 1.2% in 2006; it is projected that a zero
population growth will occur at the end of the 21st century
ii. The main unknown factor in any population growth scenario is Earth’s
carrying capacity (K)
V. Demographics of Countries
A. Population differences occur from country to country
i. Highly developed countries (HDCs - i.e., United States, Canada, France,
Germany, Japan, Australia, etc.) have low rates of population growth, are
highly industrialized, have low birth rates, and low infant mortality rates
ii. Moderately developed countries (MDCs - i.e., Mexico, Thailand, Turkey, etc.)
have relatively higher birth rates and infant mortality rates than HDCs,
medium level of industrialization, and their average per capita GNI PPPs are
lower than HDCs
iii. Less developed countries (LDCs – i.e., Bangladesh, Ethiopia, Cambodia, etc.)
have the highest birth rates, the highest infant mortality rates, the shortest life
expectancies, and the lowest average per capita GNI PPPs in the world
B. One way to express the population growth of a country is to determine its doubling
time (td)
C. Replacement-level fertility is the number of children a couple must produce to
“replace” themselves (=2.1); the current worldwide total fertility rate is currently 2.7
D. Demographic stages
i. The demographic transition consists of four demographic stages through
which a population progresses as its society becomes industrialized
ii. The four stages are preindustrial, transitional, industrial, and postindustrial
E. Age structure of countries
i. To predict the future growth of a population, you must know its distribution of
people by age (age structure)
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ii. Age structure diagrams depict expanding populations, stable populations and
declining populations
iii. A population growth momentum explains how the present age distribution
affects the future growth of a population
VI. Demographics of the United States
A. The United States has the largest population of all HDCs, and continues to grow
significantly due to immigration
i. International immigration is responsible for more than 50% of U.S. population
growth
ii. The U.S. has one of the highest rates of population increase of all the HDCs
B. History of immigration in the United States
i. Pre 1875, the U.S. had no immigration laws
ii. 1875 – no convicts and prostitutes allowed in U.S.
iii. 1882 – Chinese Exclusion Act passed
iv. 1891 –the Bureau of Immigration was established which led to an official
policy of selective exclusion in the U.S
v. 1952 – the Immigration and Nationality Act was passes (aka IRCA)
vi. Currently, the top five countries from which legal U. S. immigrants migrate
are Mexico, the Philippines, Vietnam, the Dominican Republic, and China
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Chapter 8
In-Class Activities:
Instructor Notes for In-Class Activity 1
Title:
Did Malthus Get it Wrong?
Time:
5 – 10 Minutes prep; 40 – 60 minutes in class (or can assign research
between class periods)
None
None
Materials:
Handouts:
Procedures:
For – Against – Jury standard procedure. Randomly divide class into
three groups.
Statement: Malthus’ Predictions about human population growth
will prove correct in the next two decades.
Assign one group each to argue FOR or AGAINST the statement, and the
third group to serve as a JURY.
Each group should select a leader and a recorder.
The FOR group should research (not just think up!) information that
supports the statement. They should be explicit about their sources,
whether those are data, ethics, theories, or political positions. They
should then synthesize this into a five-minute verbal argument, to be
made before the full class.
The AGAINST group should do the same for the opposite position. Their
original argument SHOULD NOT respond to items brought up by the
FOR group.
After each has made a five-minute argument, each side will have two
minutes to respond to claims or statements made by the other side.
The JURY group will then deliberate openly; the FOR and AGAINST
groups will listen to the deliberations, but may not respond. The JURY
may challenge either group to provide evidence for up to three pieces of
information, and may ask up to three questions of each group (they may
ask the same question to both groups).
The JURY should then make two judgments:
1. Which, if either, provided the most credible INFORMATION
2. Which provided the most compelling overall argument.
3. Be sure students argue their points forcefully, whether or not they
believe them personally.
See above
Student
Instructions:
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Chapter 8
Specific
Suggestions:
The instructor is likely to have to serve as a facilitator or moderator from
time to time
1. Do not allow personal assaults
2. Feel free to challenge pieces of information that you find dubious
if the JURY does not.
It will probably take a couple times through this debate process before
you and your class are comfortable with it.
Objectives:
Identify Thomas Malthus, relate his ideas on human population growth,
and explain why he may or may not be wrong.
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Instructor Notes for In-Class Activity 2
Title:
Household Pets, Household Pests
Time:
Materials:
Handouts:
0 minutes prep; 10 – 20 minutes in class
None
None
Procedures:
Have students make lists of the most common household pets and pests.
For each, have them list the characteristics that make them attractive or
distasteful to their human hosts.
Then, as a class, discuss:
Are pets or pests more likely to be K strategists? Is this universally true?
Is a relationship between reproductive strategy and the pet-human / pesthuman relationship.
Make a list of the most common household pets and household pests.
Student
Instructions: For each, list the characteristics that make them attractive or distasteful to
their human hosts.
Specific
Suggestions:
None
Objectives:


Differentiate between K- and r-strategists.
Describe density dependent and density independent influence on
population growth.
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Chapter 8
Instructor Notes for In-Class Activity 3
Title:
Demographic Transition Model and Determinism
Time:
5 – 10 Minutes prep; 40 – 60 minutes in class (or can assign research
between class periods)
None
None
Materials:
Handouts:
Procedures:
For – Against – Jury standard procedure. Randomly divide class into
three groups.
Statement: The demographic transition model is deterministic, so
government sponsored population control measures are not
necessary to limit human population growth.
(See figure 8.14)
Assign one group each to argue FOR or AGAINST the statement, and the
third group to serve as a JURY.
Each group should select a leader and a recorder.
The FOR group should research (not just think up!) information that
supports the statement. They should be explicit about their sources,
whether those are data, ethics, theories, or political positions. They
should then synthesize this into a five-minute verbal argument, to be
made before the full class.
The AGAINST group should do the same for the opposite position. Their
original argument SHOULD NOT respond to items brought up by the
FOR group.
After each has made a five-minute argument, each side will have two
minutes to respond to claims or statements made by the other side.
The JURY group will then deliberate openly; the FOR and AGAINST
groups will listen to the deliberations, but may not respond. The JURY
may challenge either group to provide evidence for up to three pieces of
information, and may ask up to three questions of each group (they may
ask the same question to both groups).
The JURY should then make two judgments:
4. Which, if either, provided the most credible INFORMATION
5. Which provided the most compelling overall argument.
6. Be sure students argue their points forcefully, whether or not they
believe them personally.
Student
See above
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Chapter 8
Instructions:
Specific
Suggestions:
The instructor is likely to have to serve as a facilitator or moderator from
time to time
3. Do not allow personal assaults
4. Feel free to challenge pieces of information that you find dubious
if the JURY does not.
It will probably take a couple times through this debate process before
you and your class are comfortable with it.
Objectives:


Explain how highly developed and developing countries differ in
population characteristics such as infant mortality rate, total fertility
rate and age structure.
Describe the Demographic Transition Model.
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Chapter 8
Instructor Notes for In-Class Activity 4
Title:
Changes in Population
Time:
Materials:
Handouts:
5 minutes prep; 60 minutes out of class time
None
None
Procedures:
In chapter 7 the students were assigned continents to see what is the
population of that continent or islands. Now have the students take the
same continent or island and bread down the population in to adult, infant
or child, senior citizen and also into culture or race. It will be interesting
for each group to define what they think is an adult, infant, or senior
citizen. Have them make a collage of their continent with the faces of
pictures cut out of magazines portraying their continent. Questions they
could research is: how many births in the last 100 years? What is the
mean age of the continent? What is the average income? What is the
ratio between men and women, senior citizens and children etc. Have the
students put all their facts on a graph and present to the class.
Using the continent you were assigned in chapter 7, break down the
Student
Instructions: population in to ages, race, male/female etc. Present your data to the
class in the form of a graph.
Specific
Suggestions:
None
Objectives:


Define population and diversity.
Describe the population on a specific continent or island.
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Chapter 8
Answers to Thinking About the Environment Questions
End of Chapter Questions:
1. What are some of the factors that have contributed to the huge increase in the incidence of
AIDS in sub-Saharan Africa?
Ans: People in sub-Saharan Africa have little access to prevention information; knowledge about
how the virus is spread and how to protect yourself must reach at-risk populations. People in this
region also have little access to effective treatment, such as anti-retroviral drugs.
2. What is population density?
Ans: Population density is the number of individuals of a species per unit of area or volume at a
given time.
3. How are these factors related in determining the growth rate: birth rate, death rate,
immigration, and emigration?
Ans: Growth rate on a global scale is due to two factors: the rate at which individuals produce
offspring (the birth rate) and the rate at which organisms die (the death rate). The growth rate (r)
of a population is equal to the birth rate (b) minus the death rate (d). In addition to birth and
death rates, dispersal, or movement from one region or country to another, is considered when
changes in populations on a local scale are examined. There are two types of dispersal:
immigration (i), in which individuals enter a population and increase its size, and emigration (e),
in which individuals leave a population and decrease its size. The growth rate of a local
population must take into account birth rate (b), death rate (d), immigration (i), and emigration
(e).
4. If a population (or bank account) is growing by the same percentage every year, what type of
growth does it exhibit?
Ans: This type of growth is exponential population growth: The accelerating population growth
that occurs when optimal conditions allow a constant reproductive rate over a period of time.
5. Draw a graph to represent the long-term growth of a population of bacteria cultured in a test
tube containing a nutrient medium that is replenished. Now draw a graph to represent the growth
of bacteria in a test tube when the nutrient medium is not replenished. Explain the difference.
Ans: Graph 1 should be a J shaped curve (see Fig. 8.2b). Graph 2 should be an S shaped curve
(see Fig. 8.3). Curve 2 shows an approximate exponential increase initially (note the curve's J
shape at the start, when environmental resistance is low), followed by a leveling out as the
carrying capacity of the environment is approached. Environmental resistance in this example
takes place as nutrients run out.
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6. What is environmental resistance?
Ans: Organisms cannot reproduce indefinitely at their intrinsic rates of increase because the
environment sets limits, collectively called environmental resistance. Environmental resistance
includes such unfavorable environmental conditions as the limited availability of food, water,
shelter, and other essential resources (resulting in increased competition), as well as limits
imposed by disease and predation.
7. In the Caribbean study involving spiders and lizards, was density dependence or density
independence more significant in determining spider populations? Explain your answer.
Ans: Spider population density was best explain by density dependent factors. In particular, the
results are explained by a combination of two density-dependent factors, predation (lizards eat
spiders) and competition (lizards compete with spiders for insect prey—that is, both spiders and
lizards eat insects).
8. Describe the general interactions between wolf and moose populations on Isle Royale.
Ans: Generally, as the population of wolves decrease, the population of moose increase. The
reverse was also true: as the population of wolves increase, the population of moose decrease.
Although the overall effect of wolves is to reduce the population of moose, the wolves do not
eliminate moose from the island. Studies indicate that wolves primarily feed on the old and
young in the moose population. Healthy moose in their peak reproductive years are not eaten.
9. Give an example of a K-selected species. What features are characteristic of K-selected
species?
Ans: The tawny owl and the redwood tree are both K-selected species. They characteristically
have long life spans with slow development, late reproduction, large body size, and low
reproductive rate. Animals that are K strategists typically invest in the parental care of their
young. K strategists are found in relatively constant or stable environments, where they have a
high competitive ability.
10. How do survivorship curves relate to r selection and K selection in animals?
Ans: Many life history trait, including, body size, age of maturity, life span, number of offspring,
and amount of parental care, link reproductive strategies and survivorship curves. Species that
are r-selected often have type III survivorship curves. Species that are K-selected often have type
I survivorship curves.
11. Explain how the spread of human diseases such as the Black Death, tuberculosis, and AIDS
is related to high population densities found in urban environments. Are such diseases densitydependent or density-independent?
Ans: Infectious diseases are density-dependent; as populations concentrate into urban
environments diseases are increasingly likely to spread. When population density is high, the
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members of a population encounter one another more frequently, and the chance of their
transmitting infectious disease organisms increases.
12. What is zero population growth? When do population experts think the human population
will have zero population growth?
Ans: Zero population growth is when the birth rate equals the death rate. Population experts
project that this will occur toward the end of the 21st century, however, how large the population
will be is still unknown.
13. What is infant mortality rate? Which group of countries has the highest infant mortality
rates? Which group has the lowest?
Ans: Infant mortality rate is the number of infant deaths (under age 1) per 1000 live births.
Highly developed countries have low infant mortality rates. Moderately developed countries
have infant mortality rates that are higher than those of highly developed countries, but they are
declining. Less developed countries have the highest infant mortality rates.
14. What is total fertility rate? Which group of countries has the highest total fertility rates?
Which group has the lowest?
Ans: Total fertility rate is the average number of children born to each woman. Less developed
countries have high total fertility rates compared low rates in highly developed counties.
15. Examine Figure 8.11, which shows the birth and death rates of Mexico during the 20th
century. Now compare it to Figure 8.14, which shows the demographic transition. What stage is
Mexico in? Is Mexico at the beginning, middle, or end of this demographic stage?
Ans: Mexico is in a late transitional stage.
16. Russia and Japan are entering a period of negative population growth momentum. Explain
what this means.
Ans: Negative population momentum means the population is aging-there will be more elderly
people than working people in the future. An aging population has a higher percentage of people
who are chronically ill or disabled, and these people require more healthcare and other social
services. Because the elderly produce less wealth (most are retired), an aging population reduces
a country's productive workforce, increases its tax burden, and strains its social security, health,
and pension systems.
17. Which population is more likely to have a positive population growth momentum, one with a
young age structure or one with an old age structure? Which is more likely to have a negative
population growth momentum? Explain your answers.
Ans: A population with a young age structure is most likely to have a positive growth
momentum. The age structure diagram of a country with a high growth rate (young age
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structure), is shaped like a pyramid The probability of future population growth is great because
the largest percentage of the population is in the prereproductive age group (0 to 14 years of
age). A population with and old age structure is most likely to have negative population
momentum. Most individuals in these populations are past reproductive age.
18. Do you think the United States should change its immigration policy? Why or why not?
Ans: Answers will vary
19. Should the average citizen in the United States be concerned about the rapid increase in
world population? Why or why not?
Ans: Answers will vary
20. Explain how the loss of a predator from a small island will affect not only the population of
the prey species, but also of the other species in that system.
Ans: When a predator is removed from an island the population of the prey species is likely to
increase. This increase can lead to over grazing thereby decreasing the number of primary
producers in the system. An example of these relationships can be seen on Isle Royale, during
the 1980s and early 1990s, the wolf population plunged, from 50 animals in 1980 to a low of 12
animals in 1989.As expected, the moose population increased as the wolf population declined; in
1995 there were more than 2400 moose on Isle Royale, many more than the island's vegetation
could support. The moose overgrazed the island, particularly mountain ash and aspen, their
preferred food. Lack of food in combination with a particularly bad winter (1995-1996) caused
hundreds of moose to die; only 500 moose were counted in the 1997 survey.
Answers to Review Questions
Principles of Population Ecology (p. 177)
1. What is the effect of each of the following on population size: birth rate, death rate,
immigration, and emigration?
Ans: On a global scale (when dispersal is not a factor), growth rate (r) is due to birth rate (b) and
the death rate (d): r = b - d. Emigration (e), the number of individuals leaving an area, and
immigration (i), the number of individuals entering an area, affect a local population's size and
growth rate. For a local population (where dispersal is a factor), r = (b - d) + (i - e).
2. How do intrinsic rate of increase and carrying capacity produce the J-shaped and S-shaped
population growth curves?
Ans: Intrinsic rate of increase is the exponential growth of a population that occurs under ideal
conditions. Although populations with a constant reproductive rate exhibit exponential
population growth for limited periods (the J curve), eventually the growth rate decreases to
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around zero or becomes negative. The S curve shows an initial lag phase (when the population is
small), followed by an exponential phase, followed by a leveling phase as the carrying capacity
of the environment is reached. The S curve is an oversimplification of how most populations
change over time.
Factors That Affect Population Size (p. 180)
1. What are three examples of density-dependent factors that affect population growth? What are
three examples of density-independent factors?
Ans: Density-dependent factors that affects population growth include, predation, disease, and
competition. Density-independent factors include, hurricanes, severe blizzard, and fires.
2. What is a boom-or-bust population cycle? What density-dependent factors may influence such
cyclic population oscillations?
Ans: Cyclic fluctuation in abundance of a population is often described as a boom-or-bust cycle.
Cyclic population oscillations are poorly understood but may involve density-dependent factors
such as predation, competition for food, and parasites.
Reproductive Strategies (p. 182)
1. What are the three main survivorship curves?
Ans: There are three general types of survivorship curves. In type I survivorship, death is
greatest in old age. In type III survivorship, death is greatest among the young. In type II
survivorship, death is spread evenly across all age groups.
The Human Population (p. 185)
1. Describe human population growth for the past 200 years.
Ans: The human population stayed relatively stable until 200 years ago. It took thousands of
years for the human population to reach 1 billion (around 1800). It took 130 years to reach 2
billion (in 1930), 30 years to reach 3 billion (in 1960), 15 years to reach 4 billion (in 1975), 12
years to reach 5 billion (in 1987), and 12 years to reach 6 billion (in 1999). Although our
numbers continue to increase, the world growth rate (r) declined from a peak of 2.2% per year in
the mid-1960s to 1.3% per year in 2004.
2. Who was Thomas Malthus, and what were his views on human population growth?
Ans: Thomas Malthus, was a British economist (1766-1834), that pointed out that human
population growth is not always desirable—a view contrary to the beliefs of his day and to those
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of many people even today—and that the human population can increase faster than its food
supply. The inevitable consequences of population growth, he maintained, are famine, disease,
and war.
3. When determining Earth's carrying capacity for humans, why is it not enough to just consider
human numbers? What else must be considered?
Ans: Estimates of Earth’s carrying capacity must also take into account assumptions about
standard of living, resource consumption, technological innovations, and waste generation. If we
want all people to have a high level of material well-being equivalent to the lifestyles common in
highly developed countries, then Earth will clearly support far fewer humans than if everyone
lives just above the subsistence level. Earth's carrying capacity for humans is not decided simply
by environmental constraints. Human choices and values must be factored into the assessment.
Demographics of Countries (p. 191)
1. What is infant mortality rate? How does it affect life expectancy?
Ans: Infant mortality rate is the number of infant deaths (under age 1) per 1000 live births. The
infant mortality rate of the United States was 6.7 in 2006, compared with a world rate of 52.
Highly developed countries also have longer life expectancies (78 years in the United States
versus 67 years worldwide). Countries with higher infant mortality rates have lower life
expectancy rates.
2. What is the difference between the total fertility rate and replacement-level fertility?
Ans: Total fertility rate is the average number of children born to each woman.
Replacement-level fertility is the number of children a couple must produce to “replace”
themselves. Replacement-level fertility is usually given as 2.1 children. Worldwide, the total
fertility rate is currently 2.7, well above the replacement level.
3. If all the women in the world suddenly started bearing children at replacement-level fertility
rates, would the population stop increasing immediately? Why or why not?
Ans: The population may not stop increasing immediately because a population can have
population growth momentum. This is the potential for future increases or decreases in a
population based on the present age structure. A country can have replacement-level fertility and
still experience population growth if the largest percentage of the population is in the
prereproductive years; when all these children mature, they will become the parents of the next
generation, and this group of parents will be larger than the previous group.
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Demographics of the United States (p. 195)
1. Should the United States increase or decrease the number of legal immigrants? Present
arguments in favor of both sides.
Ans: Some environmentalists think the population of the United States is already too large.
According to this view, the presence of legal immigrants and unauthorized migrants is
undesirable because it contributes greatly to pollution and resource depletion as these people
adopt the affluent, high-consumption lifestyles of “typical” U.S. citizens. People with this view
say the United States must reduce both birth rates and immigration rates to bring about
sustainability.
Other environmentalists point out that the United States is not the only country to take in
immigrants. They say that it is morally unacceptable to deny people a chance for a better life and
that the United States can absorb the environmental consequences of immigrants, who often have
simpler lifestyles than most U.S. citizens (at least for the first generation). Immigrants tend to
have smaller families than if they had stayed in their countries of origin, so immigration reduces
the rate of global population growth. Furthermore, immigrants provide U.S. environmental
groups with a much-needed global consciousness, a “we're in this together” viewpoint that is
easy to overlook when simply considering U.S. environmental issues.
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