5-2 what limits the growth of populations?

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. Clumping2. Uniform dispersion3. Random dispersion

Most Populations Live Together in Clumps or Patches (2)

• Why clumping?1. Species tend to cluster where resources are

available2. Groups have a better chance of finding clumped

resources3. Protects some animals from predators4. 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


• Environmental resistance • All factors that act to limit the growth of a population

• Carrying capacity (K)• Maximum population a given habitat can sustain


• 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

Fig. 5-15, p. 115

2.0 Population overshoots carrying capacity

Carrying capacity

1.5Population recovers and stabilizes

Num

ber o

f she

ep (m

illio

ns)

.5

Exponential growth

Population runs out of resources and crashes

1.0

1800 1825 1850 1875 1900 1925

Year

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

Fig. 5-17, p. 116

2,000 Population overshoots carrying capacity

1,500Population crashes

1,000

500 Carrying capacity

Num

ber o

f rei

ndee

r

1910 1920 1930 1940 19500

Year

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

Fig. 5-18, p. 118

160

140 Hare Lynx

100

120

80

60

Popu

latio

n si

ze (t

hous

ands

)

20

40

1845 1855 1865 1875 1885 1895 1905 1915 1925 19350

Year

Humans Are Not Exempt from Nature’s Population Controls

• Ireland• Potato crop in 1845

• Bubonic plague• Fourteenth century

• AIDS• Global epidemic

LIVING IN THE ENVIRONMENT 17THMILLER/SPOOLMAN

Chapter 6The Human Population and Its Impact

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

Fig. 6-2, p. 127

2.5

2.0

1.5

1.0

0.5

Aver

age

annu

al g

loba

l gro

wth

rate

(per

cent

)

0.01970 1990 2010 2030 2050

Year1950

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

Fig. 6-3, p. 127

109876543 Population in less-developed countries

Wor

ld p

opul

atio

n (in

bill

ions

)

21 Population in more-developed countries0

1960 1970 1980 1990 2000 2010 2020 2030 2040 20501950

Year

Five Most Populous Countries, 2010 and 2050

Fig. 6-4, p. 127

Fig. 6-4, p. 127

2010China 1.3 billion

United States 310 million

India 1.2 billion

Indonesia 235 million

Brazil 193 million

2050India 1.7 billion

China 1.4 billion

United States 439 millionPakistan 335 million

Indonesia 309 million

Fig. 6-A, p. 128

11 UN high-fertility variant (2008 revision)U.S. Census Bureau (2008 update)

10UN low-fertility variant (2008 revision)IIASA (2007 update)

9

8

Wor

ld p

opul

atio

n (in

bill

ions

)

7

62010 2020 2030 2040 2050

Year

UN medium-fertility variant (2008 revision)

Science Focus: Projecting Population Change

• Why range of 7.8-10.8 billion for 2050?

• Demographers must:1. Determine reliability of current estimates2. Make assumptions about fertility trends3. 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

Fig. 6-B, p. 129

Natural Capital DegradationAltering Nature to Meet Our Needs

Reducing biodiversity

Increasing use of net primary productivity

Increasing genetic resistance in pest species and disease-causing bacteriaEliminating 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

5-2 what limits the growth of populations?

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