BIO320 Chapter 7

1. 7-1
ENVIRONMENTAL
SCIENCE
A Study of Interrelationships
15th Edition
Populations: Characteristics and Issues
Chapter 7
©2019 McGraw-Hill Education. All rights reserved. Authorized only for instructor use in the classroom. No reproduction or
further distribution permitted without the prior written consent of McGraw-Hill Education.

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Outline (1 of 2)
7.1 Population Characteristics
7.2 A Population Growth Curve
7.3 Factors That Limit Population Size
7.4 Categories of Limiting Factors
7.5 Carrying Capacity
7.6 Reproductive Strategies and Population Fluctuations
7.7 Human Population Growth

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Outline (2 of 2)
7.8 Human Population Characteristics and Implications
7.9 Factors That Influence Human Population Growth
7.10 Population Growth Rates and Standard of Living
7.11 Hunger, Food Production, and Environmental
Degradation
7.12 The Demographic Transition Concept
7.13 The U.S. Population Picture
7.14 What Does the Future Hold?

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7.1 Population Characteristics (1 of 2)
A population is a group of individuals of the same species
inhabiting the same area.
• Genetic differences commonly exist within different populations of a
given species
• Natality: The number of individuals added to a population through
reproduction over a particular time period.
• Birth Rate = Humans born per 1,000 individuals per year.
• Mortality: The number of deaths in a population over a particular
time period.
• Death Rate = Humans died per 1,000 individuals per year.

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7.1 Population Characteristics (2 of 2)
A survivorship curve shows the proportion of individuals
likely to survive to each age.
Three types of survivorship curves can be recognized:
• Species with high mortality in young.
• Species with mortality equitable among age classes.
• Species with mortality high only in old age.

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Genetic Differences between Black
Bear Populations

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Population Growth Curve
Population growth rate is the birthrate minus the death rate.
• In human population studies it is often expressed as a percentage of the total
population.
Sex ratio refers to the relative number of males and females in a
population.
• The number of females is important because females determine the number of
offspring produced in sexually reproducing populations.
Age distribution is the number of individuals of each age in the
population.
• Age distribution greatly influences the population growth rate.

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Effect of Birth, Death Rates on
Population Size
Source: Data from World Population Data Sheet 2016, Population Reference Bureau, Inc.,
Washington, D.C.

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Types of Survivorship Curves

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Age Distribution in Human Populations
Source: Data from United States Census Bureau International Data Base.

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Population Density and Spatial
Distribution
Population density is the number of individuals per unit
area.
• High population density injures all individuals within the population
because they compete severely for necessary resources.
• Dispersal: Movement of individuals from densely populated locations to
new areas.
• Emigration: Movement from an area.
• Immigration: Movement into an area.

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7.2 A Population Growth Curve (1 of 3)
Biotic potential is the inherent reproductive capacity of a species
(biological ability to produce offspring).
Generally, biotic potential is much above replacement level.
• There is a natural tendency for increase.

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7.2 A Population Growth Curve (2 of 3)
Population growth follows a
pattern consisting of a lag
phase, an exponential
growth phase, a deceleration
phase, and a stable
equilibrium phase.
• Lag Phase: This is the first
portion of the curve; slow
population growth.

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7.2 A Population Growth Curve (3 of 3)
Exponential Growth Phase (Log Phase): More organisms are reproducing,
causing accelerated growth; this continues as long as birth rate exceeds death
rate.
Deceleration Phase: The population growth rate slows as the death rate and
birth rate come to equal one another.
Stable Equilibrium Phase: The death rate and birth rate become equal and the
population stops growing.

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7.3 Factors That Limit Population Size
(1 of 2)
Factors that prevent unlimited population growth are known as limiting
factors.
All of the different limiting factors that act on a population are
collectively known as environmental resistance.
Extrinsic limiting factors are those that come from outside the
population.
• Predators
• Loss of food source
• Lack of sunlight
• Accidents of nature

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7.3 Factors That Limit Population Size
(2 of 2)
Intrinsic limiting factors are those factors that originate within the
population and exercise control over it.
• Behavioral changes amongst the population cause lower birthrates and
higher death rates.
Density-dependent limiting factors are those that become more
effective as the density of the population increases.
Density-independent limiting factors are population-controlling
influences that are not related to the density of the population.

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7.4 Categories of Limiting Factors
(1 of 2)
For most populations, limiting factors recognized as
components of environmental resistance can be placed into
four main categories:
• Raw material availability
• Energy availability
• Accumulation of waste products
• Interactions among organisms

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7.4 Categories of Limiting Factors
(2 of 2)

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7.5 Carrying Capacity
Carrying capacity is the maximum sustainable population
for an area.
It is not an inflexible number; it can be influenced by
environmental differences such as:
• Successional changes
• Climate variations
• Disease epidemics
• Forest fires, floods, or natural disasters
• Nutrient levels in aquatic ecosystems

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Carrying Capacity

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7.6 Reproductive Strategies and
Population Fluctuations (1 of 3)
Species can be divided into two
broad categories based on their
reproductive strategies:
• K-strategists: Organisms that
typically reach a stable population
as the population reaches the
carrying capacity.
• r-strategists: Typically, these are
small organisms that have a short
life, produce many offspring,
exploit unstable environments, and
do not reach a carrying capacity.

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7.6 Reproductive Strategies and
Population Fluctuations (2 of 3)
K-strategist characteristics include:
• Usually occupy relatively stable environments
• Large organisms
• Long-lived
• Produce few offspring
• Provide substantial parental care
• Reproductive strategy is to invest a great deal of energy in producing a few
offspring that have a good chance of living to reproduce.
K-strategists are controlled by density-dependent limiting factors.

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7.6 Reproductive Strategies and
Population Fluctuations (3 of 3)
r-strategist characteristics include:
• Small, short-lived organisms
• Produce many offspring
• Little if any parental care
• Exploit unstable environments
• Usually do not reach carrying capacity (boom-bust cycles)
• Reproductive strategy is to produce large numbers of offspring to overcome
high mortality.
r-strategists are controlled by density-independent limiting factors.

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A Comparison of Life History Characteristics
of Typical K- and r- Strategists

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Population Cycles
Many kinds of animals show
distinct population cycles-periods
of relatively large populations
followed by periods of small
populations.

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7.7 Human Population Growth (1 of 2)
A major reason for the increasing human
population growth rate is an increase in
medical care, and a consequential decrease
in death rates.

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7.7 Human Population Growth (2 of 2)
The doubling time for the human population is calculated
by:
70 / annual rate of increase (%)
( 70 / 2.0% = 35 years )

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The Historical Human Population
Curve
The human population growth curve has a long lag phase followed by a
sharply rising exponential growth phase that is still rapidly increasing.
Source: Data from U.S. Census Bureau and the Population Reference Bureau

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Doubling Time for the Human
Population

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7.8 Human Population Characteristics
and Implications (1 of 7)
The world can be divided into two segments based on
economic development.
• More-developed countries typically have per capita income
exceeding $25,000.
• Europe, Canada, the U.S., Japan, Australia, New Zealand
• Combined population of 1.25 billion
• Relatively stable populations
• Expected to grow 5% between 2016 and 2050.

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7.8 Human Population Characteristics
and Implications (2 of 7)
Less-developed countries typically have a per
capita income less than $15,000.
• All other remaining countries of the world
• Combined population of 6 billion

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7.8 Human Population Characteristics
and Implications (3 of 7)
Less-developed countries typically have a per
capita income less than $15,000.
• Nearly 900 million live on less than $1.90 per day
• High population growth rates
• Expected to grow to 8.5 billion by 2050 and that
will represent about 87% of the world’s people.

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7.8 Human Population Characteristics
and Implications (4 of 7)
Human population growth is tied to economic development and is a
contributing factor to nearly all environmental problems.
Population density relates the size of the population to available
resources.
• Countries with abundant resources can sustain higher population densities
than resource-poor countries.
The degree of technological development and affluence is also
significant; people in highly developed countries consume huge
amounts of resources.

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7.8 Human Population Characteristics
and Implications (5 of 7)
Affluence: People in highly developed countries consume huge
amounts of resources.
Technology: The technology used is an important contributor to
environmental impact.
• Firewood for heat can lead to deforestation.
• Coal-fired power plants contribute to pollution.
• Wind and solar energy lower environmental impact.
The Environmental Foot print Concept: The environmental impact
of highly developed regions is often felt in distant places where goods
are produced.

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7.8 Human Population Characteristics
and Implications (6 of 7)
The ecological footprint is a measure of the land area
required to provide the resources and absorb the wastes of a
population.
Most of the more-developed countries of the world have a
much larger ecological footprint than is represented by their
land area.
• Japan is highly developed but has few resources; its
ecological footprint is 5 times larger than its locally
available resources.
• The ecological footprint of the U.S. is 1.5 times locally
available resources.

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7.8 Human Population Characteristics
and Implications (7 of 7)
Most of the more-developed countries of the world
have a much larger ecological footprint than is
represented by their land area.
• Japan is highly developed but has few
resources; its ecological footprint is 5 times
larger than its locally available resources.
• The ecological footprint of the U.S. is 1.5 times
locally available resources.

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7.9 Factors That Influence Human
Population Growth (1 of 10)
Humans, unlike other kinds of organisms, are influenced
by social, political, economic, and ethical factors.
We can make conscious decisions based on the likely
course of events and adjust our lives accordingly.

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7.9 Factors That Influence Human
Population Growth (2 of 10)
Biological Factors
• Demography is the study of populations and their characteristics,
how those characteristics affect growth, and the consequences of
that growth.
• The most important determinant of the rate at which human
populations grow is related to how many women in the population
are having children and the number of children each woman will
have.
• The total fertility rate of a population is the number of children a
woman has during her lifetime.

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7.9 Factors That Influence Human
Population Growth (3 of 10)
Biological Factors
• A total fertility rate of 2.1 is known as replacement fertility; parents
produce 2 children who will replace the parents when they die.
• Zero population growth is when the birth rate equals death rate.
• The age distribution is the number of people of each age in the
population.

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7.9 Factors That Influence Human
Population Growth (4 of 10)
Social Factors
• Reducing fertility rates would be advantageous, especially in LDCs,
but not everyone agrees.
• Several factors (religious, traditional, social, and economic)
influence the number of children a couple would like to have.
• The major social factors determining family size are the status and
desires of women in the culture.
• In male-dominated cultures, traditional role of women is to marry
and raise children.

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7.9 Factors That Influence Human
Population Growth (5 of 10)
Social Factors
• Early marriages foster high fertility rates.
• In Africa, about 23% of births are to women in the 15-19 year-old range.
• Total Fertility Rate of 5.0%
• Childrearing practices also influence population growth rates.
• In countries where breast feeding is practiced, several benefits accrue.
• Breast milk is an excellent source of nutrients and antibodies.
• During the months a woman is breast feeding, she is less likely to become
pregnant again.

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7.9 Factors That Influence Human
Population Growth (6 of 10)
Social Factors
• Lack of education opportunities for women reduces their options.
• When level of education increases, fertility rates fall.
• Financial independence leads to marriage later in life.
• Better-educated women are more likely to have access to and use birth control.
• In some cultures women desire large families:
• In countries where infant mortality is high, it is traditional to have large families
since several of a woman’s children may not live to adulthood.
• Parents are more secure when there are many children to look after their own
needs in old age.

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7.9 Factors That Influence Human
Population Growth (7 of 10)
Early Marriage and Fertility Rate
Female Literacy, Total Fertility Rate, and
Economic Well-Being
Source: Data from Population
Reference Bureau, 2016
Data Sheet, and World
Development Indicators 2016
from The World Bank.
Source: Data from Population
Reference Bureau, 2016 Data
Sheet, and World
Development Indicators 2016
from The World Bank.

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7.9 Factors That Influence Human
Population Growth (8 of 10)
Economic Factors:
• In less developed countries, the economic benefits of children are
extremely important.
• Even young children can be given jobs that contribute to the family
economy, such as protecting livestock, gathering firewood, or carrying
water.
• In the developed world, large numbers of children are an economic
drain.
• They are prevented from working.
• They must be sent to school at great expense.
• They consume large amounts of the family income.

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7.9 Factors That Influence Human
Population Growth (9 of 10)
Political Factors
• Governments can either reward or punish high fertility rates.
• Several European countries are concerned about low birth rates and
have instituted policies to encourage couples to have more children.
• Paid maternity leave and guaranteed job availability upon the mother’s
return to work.
• Childcare facilities make it possible for both parents to work.
• Child tax deductions provide an indirect payment to families.

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7.9 Factors That Influence Human
Population Growth (10 of 10)
Political Factors
• Most developing countries are concerned that population
growth is too rapid and have instituted programs to limit
growth:
• They provide information on family planning and maternal and
child health.
• Sometimes governments provide free or low-cost contraceptives.
• One-child policy in China

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Population Characteristics of the Ten
Most Populous Countries, 2013
Source: Population Reference Bureau, 2016 Population Data Sheet.

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7.9 Factors That Influence Human
Population Growth
Immigration Policies
• Birthrates are so low in some countries that immigration is
encouraged to offset a potential decline in the number of working-age
citizens.
• Developed countries are under intense pressure to accept
immigrants.
• The higher standard of living is a magnet for refugees or people who
seek a better life than is possible where they currently live.

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7.10 Population Growth Rates and
Standard of Living (1 of 2)
There appears to be an inverse relationship between a country’s growth
rate and its average standard of living.
Standard of living is a difficult concept to quantify since various cultures
have different desires.
Factors usually included in an analysis of standard of living:
• Economic well-being
• Health conditions
• Social status and mobility

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7.10 Population Growth Rates and
Standard of Living (2 of 2)
An important economic measure of standard of living is the average
purchasing power per person.
Gross national income (GNI) is an index of purchasing power
measuring total goods and services generated by citizens of a country.
• Gross national income purchasing power parity (GNIPPP) is used to
account for varying prices of goods and services between countries.

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Standard of Living and Population
Growth in Three Countries
Source: Data from Population Reference Bureau, 2016 Population Data Sheet; and
United Nations Development Program Human Development Reports 2015.

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7.11 Hunger, Food Production, and
Environmental Degradation (1 of 4)
As human population grows, so does the demand for food.
• Most people in developed countries must purchase their own food.
• Most people in less-developed countries must grow their own food.
Human populations can only increase in size if other plant
and animal populations decrease in size.

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7.11 Hunger, Food Production, and
Environmental Degradation (2 of 4)
People in less-developed countries generally feed at lower
trophic levels than those in the developed world.
• People in less-developed countries must eat plants themselves
instead of feeding the plants to animals and eating the animals.
• A lack of protein in the diet can lead to malnutrition.
Many people in more-developed countries suffer from over-
nutrition.
• About 66% of North Americans are overweight, and 33% are obese.

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7.11 Hunger, Food Production, and
Environmental Degradation (3 of 4)
In countries where food is in short supply, agricultural land is already
being exploited to its limit and there is still a need for more food.
Many more-developed countries are net food exporters.
Improved plant varieties and agricultural techniques have dramatically
increased food production in some parts of the world.
Political activities such as war, repayment of foreign debt, corruption,
and poor management often determine food availability.

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7.11 Hunger, Food Production, and
Environmental Degradation (4 of 4)
Solving the problem of food distribution is not an
easy question.
• Humanitarian food aid provides an excellent service, but:
• It provides a temporary relief of symptoms.
• It works against self-reliance.
• The emphasis must be on self-sufficiency.

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Population and Trophic Levels

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7.12 The Demographic Transition
Concept (1 of 2)
Demographic transition is a model of population growth
based on historical, social, and economic development of
Europe and North America.
• Initially, countries have a stable population (high birth and death
rates).
• The death rate falls and the population grows.
• Industrialization occurs and the birth rate falls.
• Death rates and birth rates find equilibrium (low birth rates and low
death rates).

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7.12 The Demographic Transition
Concept (2 of 2)
This is a comfortable model, because it suggests
industrialization will naturally lead to population
stabilization.
Can a model developed in Europe and North America be
applied to less-developed countries?
• Resources are no longer abundant.
• Human population is growing much faster than before.

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Demographic Transition

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7.13 The U.S. Population Picture
United States population had a post-war baby boom period,
significantly affecting population trends.
• 1947–1961: birthrates were much higher than today.
• This created a population bulge.
• As members of this group have raised families, they have had a
significant influence on U.S. population growth and trends.
Legal and illegal immigration significantly influences future
population growth trends.

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Changing Age Distribution of U.S.
Population (1980–2010)
Source: Data from U.S. Department of Commerce,
U.S. Census Bureau, International Data Base.
Source: Data from U.S. Department of Commerce,
U.S. Census Bureau, International Data Base.
Source: Data from U.S. Department of Commerce,
U.S. Census Bureau, International Data Base.
Source: Data from U.S. Department of Commerce,
U.S. Census Bureau, International Data Base.

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7.14 What Does the Future Hold?
(1 of 4)
We must remember that interactions with other species and
with other humans will help determine our carrying
capacity.
Four basic factors are involved in this consideration:
Available raw materials
Available energy
Waste disposal
Interactions with other organisms

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7.14 What Does the Future Hold?
(2 of 4)
Available raw materials
• This issue is not limited to the amount of food available.
• We are also dependent on technology.
• Our lifestyles are tied to use of other resources such as irrigation water,
genetic research, and antibiotics.
Available energy
• This issue is similar to that of raw materials.
• Increases in energy efficiency and the development of renewable
fuels can reduce dependence on fossil fuels.

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7.14 What Does the Future Hold?
(3 of 4)
Waste disposal
• Pollution is the waste product of human activity.
• Lack of adequate sewage treatment and safe drinking water causes large
numbers of deaths.
Interactions with other organisms
• We are not the only species of importance.
• Converting land to meet our needs displaces other species from their
habitats.
• What today seems like an unimportant organism may one day be seen as
an important link to our own survival.

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7.14 What Does the Future Hold?
(4 of 4)
Social Factors Influence Human Population
• Current technology and medical knowledge are available to control
human population growth and improve the health of the people of
the world.
• But humans are social animals who have freedom of choice and
frequently do not do what is considered “best” from an unemotional,
uninvolved, biological point of view.
• People make decisions based on history, social situations, ethical
and religious considerations, and personal desires.

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7.14 What Does the Future Hold?
If the world continues to grow at its current rate
(1.2%), population will nearly 10 billion by 2050.
• The human population is subject to same biological
constraints as other species.
• The human population will ultimately reach a carrying
capacity and stabilize.
• Disagreement about exact size and primary limiting factors

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Summary (1 of 2)
The birth rate (natality) is the number of individuals entering
the population by reproduction during a certain period.
The death rate is the number of deaths in a population in a
certain period.
A typical population growth curve shows a lag phase
followed by an exponential growth phase, a deceleration
phase, and a stable equilibrium phase at the carrying
capacity.
The world’s population is growing rapidly. The causes of
population growth are biological, social, political,
philosophical, and theological.

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Summary (2 of 2)
Most of the growth is occurring in less-developed countries of
the world.
Demography is the study of human populations and the
things that affect them.
Population growth rates are determined by biological factors,
but also by social and economic factors.
The demographic transition model suggests that as a country
becomes industrialized, its population becomes stabilized.