5-2 What Limits the Growth of
Populations?
• Concept 5-2 No population can continue to grow
indefinitely because of limitations on resources and
because of competition among species for those
resources.
Most Populations Live Together in
Clumps or Patches (1)
• Population: group of interbreeding individuals of the
same species
• Population distribution
1. Clumping
2. Uniform dispersion
3. Random dispersion
Most Populations Live Together in
Clumps or Patches (2)
• Why clumping?
1. Species tend to cluster where resources are
available
2. Groups have a better chance of finding clumped
resources
3. Protects some animals from predators
4. Packs allow some to get prey
Population of Snow Geese
Fig. 5-11, p. 112
Generalized Dispersion Patterns
Fig. 5-12, p. 112
Populations Can Grow, Shrink, or
Remain Stable (1)
• Population size governed by
•
•
•
•
Births
Deaths
Immigration
Emigration
• Population change =
(births + immigration) – (deaths + emigration)
Populations Can Grow, Shrink, or
Remain Stable (2)
• Age structure
• Pre-reproductive age
• Reproductive age
• Post-reproductive age
Some Factors Can Limit Population
Size
• Range of tolerance
• Variations in physical and chemical environment
• Limiting factor principle
• Too much or too little of any physical or chemical
factor can limit or prevent growth of a population,
even if all other factors are at or near the optimal
range of tolerance
• Precipitation
• Nutrients
• Sunlight, etc
Trout Tolerance of Temperature
Fig. 5-13, p. 113
No Population Can Grow Indefinitely:
J-Curves and S-Curves (1)
• Size of populations controlled by limiting factors:
•
•
•
•
•
Light
Water
Space
Nutrients
Exposure to too many competitors, predators or
infectious diseases
No Population Can Grow Indefinitely:
J-Curves and S-Curves (2)
• Environmental resistance
• All factors that act to limit the growth of a population
• Carrying capacity (K)
• Maximum population a given habitat can sustain
No Population Can Grow Indefinitely:
J-Curves and S-Curves (3)
• Exponential growth
• Starts slowly, then accelerates to carrying capacity
when meets environmental resistance
• Logistic growth
• Decreased population growth rate as population size
reaches carrying capacity
Logistic Growth of Sheep in Tasmania
Fig. 5-15, p. 115
Number of sheep (millions)
2.0
Population
overshoots
carrying
capacity
Carrying capacity
1.5
Population recovers
and stabilizes
1.0
Exponential
growth
Population
runs out of
resources
and crashes
.5
1800
1825
1850
1875
Year
1900
1925
Fig. 5-15, p. 115
Number of sheep (millions)
2.0
Population
overshoots
carrying
capacity
Carrying capacity
1.5
Population recovers
and stabilizes
1.0
Exponential
growth
Population
runs out of
resources
and crashes
.5
1800
1825
1850
1875
Year
1900
1925
Fig. 5-15, p. 115
Science Focus: Why Do California’s Sea
Otters Face an Uncertain Future?
• Low biotic potential
• Prey for orcas
• Cat parasites
• Thorny-headed worms
• Toxic algae blooms
• PCBs and other toxins
• Oil spills
Population Size of Southern Sea Otters Off the Coast of
So. California (U.S.)
Fig. 5-B, p. 114
Case Study: Exploding White-Tailed
Deer Population in the U.S.
• 1900: deer habitat destruction and uncontrolled hunting
• 1920s–1930s: laws to protect the deer
• Current population explosion for deer
• Spread Lyme disease
• Deer-vehicle accidents
• Eating garden plants and shrubs
• Ways to control the deer population
Mature Male White-Tailed Deer
Fig. 5-16, p. 115
When a Population Exceeds Its Habitat’s
Carrying Capacity, Its Population Can Crash
• A population exceeds the area’s carrying capacity
• Reproductive time lag may lead to overshoot
• Population crash
• Damage may reduce area’s carrying capacity
Exponential Growth, Overshoot, and
Population Crash of a Reindeer
Fig. 5-17, p. 116
Population
overshoots
carrying
capacity
Number of reindeer
2,000
1,500
Population
crashes
1,000
500
Carrying
capacity
0
1910
1920
1930
Year
1940
1950
Fig. 5-17, p. 116
Species Have Different Reproductive
Patterns (1)
• Some species
•
•
•
•
Many, usually small, offspring
Little or no parental care
Massive deaths of offspring
Insects, bacteria, algae
Species Have Different Reproductive
Patterns (2)
• Other species
•
•
•
•
•
•
•
Reproduce later in life
Small number of offspring with long life spans
Young offspring grow inside mother
Long time to maturity
Protected by parents, and potentially groups
Humans
Elephants
Under Some Circumstances Population
Density Affects Population Size
• Density-dependent population controls
•
•
•
•
Predation
Parasitism
Infectious disease
Competition for resources
Several Different Types of Population
Change Occur in Nature
• Stable
• Irruptive
• Population surge, followed by crash
• Cyclic fluctuations, boom-and-bust cycles
• Top-down population regulation
• Bottom-up population regulation
• Irregular
Population Cycles for the Snowshoe Hare and
Canada Lynx
Fig. 5-18, p. 118
Population size (thousands)
160
Hare
Lynx
140
120
100
80
60
40
20
0
1845
1855
1865
1875
1885
1895
1905
1915
1925
1935
Year
Fig. 5-18, p. 118
Humans Are Not Exempt from
Nature’s Population Controls
• Ireland
• Potato crop in 1845
• Bubonic plague
• Fourteenth century
• AIDS
• Global epidemic
MILLER/SPOOLMAN
LIVING IN THE ENVIRONMENT
Chapter 6
The Human Population
and Its Impact
17TH
6-1 How Many People Can the Earth
Support?
• Concept 6-1 We do not know how long we can
continue increasing the earth’s carrying capacity for
humans without seriously degrading the life-support
system that keeps us and many other species alive.
Core Case Study: Slowing Population
Growth in China: A Success Story
• 1.3 billion people
• Promotes one-child families
• Contraception, abortion, sterilization
• Fast-growing economy
• Serious resource and environmental problems
Crowded Street in China
Fig. 6-1, p. 125
Human Population Growth Continues but It
Is Unevenly Distributed (1)
• Reasons for human population increase
• Movement into new habitats and climate zones
• Early and modern agriculture methods
• Control of infectious diseases through
•
•
•
•
Sanitation systems
Antibiotics
Vaccines
Health care
• Most population growth over last 100 years due to
drop in death rates
Human Population Growth Continues but It
Is Unevenly Distributed (2)
• Population growth in developing countries is
increasing 9 times faster than developed countries
• 2050
• 95% of growth in developing countries
• 7.8-10.8 billion people
• Should the optimum sustainable population be
based on cultural carrying capacity?
Human Population Growth
Fig. 1-18, p. 21
Average annual global growth rate (percent)
2.5
2.0
1.5
1.0
0.5
0.0
1950
1970
1990
2010
Year
2030
2050
Fig. 6-2, p. 127
Population Time Line: 10,000 BC - 2042
Figure 3, Supplement 9
Annual Growth Rate of World Population, 1950-2010
Fig. 6-2, p. 127
Where Population Growth Occurred, 1950-2010
Fig. 6-3, p. 127
World population (in billions)
10
9
8
7
6
5
4
3
2
1
0
1950
Population in less-developed countries
Population in more-developed countries
1960
1970
1980
1990
2000
2010 2020
2030
2040 2050
Year
Fig. 6-3, p. 127
Five Most Populous Countries, 2010 and 2050
Fig. 6-4, p. 127
2010
China
1.3 billion
India
United States
Indonesia
Brazil
1.2 billion
310 million
235 million
193 million
2050
India
1.7 billion
China
United States
1.4 billion
439 million
Pakistan
335 million
Indonesia
309 million
Fig. 6-4, p. 127
World population (in billions)
11
UN high-fertility variant (2008 revision)
U.S. Census Bureau (2008 update)
UN medium-fertility variant (2008 revision)
IIASA (2007 update)
UN low-fertility variant (2008 revision)
10
9
8
7
6
2010
2020
2030
Year
2040
2050
Fig. 6-A, p. 128
Science Focus: Projecting Population
Change
• Why range of 7.8-10.8 billion for 2050?
• Demographers must:
1. Determine reliability of current estimates
2. Make assumptions about fertility trends
3. Deal with different databases and sets of
assumptions
World Population Projections to 2050
Fig. 6-A, p. 128
Science Focus: How Long Can The
Human Population Keep Growing?
• Thomas Malthus and population growth: 1798
• Overpopulation and overconsumption
• Will technology increase human carrying capacity?
• Can the human population grow indefinitely?
Natural Capital Degradation:
Altering Nature to Meet Our Needs
Fig. 6-B, p. 129
Natural Capital Degradation
Altering Nature to Meet Our Needs
Reducing biodiversity
Increasing use of net primary
productivity
Increasing genetic resistance in
pest species and disease-causing
bacteria
Eliminating many natural
predators
Introducing harmful species into
natural communities
Using some renewable resources
faster than they can be replenished
Disrupting natural chemical cycling
and energy flow
Relying mostly on polluting and
climate-changing fossil fuels
Fig. 6-B, p. 129