audesirk audesirk byers 26 - linn-benton community...

Lecture Presentations by

Carol R. Anderson

Westwood College, River Oaks Campus

© 2014 Pearson Education, Inc.

BIOLOGY Life on Earth WITH PHYSIOLOGY Tenth Edition

Audesirk Audesirk Byers

26 Population Growth

and Regulation


Chapter 26 At a Glance

26.1 How Does Population Size Change?

26.2 How Is Population Growth Regulated?

26.3 How Are Populations Distributed in Space

and Age?

26.4 How Is the Human Population Changing?



A population consists of all the members of a

particular species that live within an ecosystem, a

defined geographical area

Each population forms an integral part of a larger

community, defined as a group of interacting

populations



The biosphere is the enormous ecosystem that

encompasses all of Earth’s habitable surface

Ecology is the study of the interrelationships of

organisms with each other and with the nonliving

environment



Changes in population size result from natural

increases and net migration

– Population size changes through

– Births

– Deaths

– Net migration




increases and net migration (continued)

– The natural increase of a population is the difference

between births and deaths

– Natural “increase” can be negative (decrease) if deaths

exceed births

– The net migration of a population is the difference

between immigration (migration into the population)

and emigration (migration out)




increases and net migration (continued)

– A population thus grows when the sum of natural

increase and net migration is positive and declines

when this sum is negative

– A simple equation for the change is

– Change in population size natural increase net

migration (births deaths) (immigration

emigration)



Populations grow based on the birth rate, the death

rate, and the population size

– The size of most natural populations of organisms

fluctuates over the course of a year because

reproduction tends to be seasonal

– Growing populations add individuals in proportion to

the population’s size, much like a bank account

accumulates compound interest

– If conditions are the same, a population will grow at a

constant percentage of its size over a given interval




rate, and the population size (continued)

– The growth rate (r) of a population is the percentage

change in the population size per unit time

– The population growth rate is the birth rate (b) minus

its death rate (d)

– r (growth rate) b (birth rate) d (death rate)





– If the birth rate exceeds the death rate, the population

growth rate will be positive and population size will

increase

– If the death rate exceeds the birth rate, the growth

rate will be negative and the population size will

decrease





– Population growth (G), which is the number of

individuals added to a population in a given time

period, can be calculated by multiplying growth rate (r)

by the original population size (N)

– Population growth (G) r (growth rate) × N (population

size)





– If births exceed deaths, exponential growth occurs

– A constant growth rate (r) produces exponential growth

– During exponential growth, an ever-larger number is

added to the population during each succeeding time

period






– This pattern of growth will occur in any population in

which each individual, on average over the course of its

life span, produces more than one offspring that

survives to reproduce

– If the size of an exponentially growing population is

graphed against time, a characteristic shape called a J-

curve will be produced






(continued)

– The age at which an organism first reproduces affects

the size of the future population






(continued)

– For example, consider two populations of golden

eagles that are followed for 30 years

– Individuals in one population begin reproducing at the

age of 4 years

– Individuals in the other population begin reproducing at

age 6 years






(continued)

– Both populations will follow a J-shaped population

growth curve, but more individuals will be added to the

earlier reproducing population, resulting in a steeper

increase in population numbers

– At 30 years, the earlier reproducing population would

be 10 times the size of the other population


Figure 26-1 Exponential growth curves are J-shaped

2,600

2,400

2,200

2,000

1,800

1,600

1,400

1,200

1,000

800

600

400

200

0 5 10 15 25 30

nu

mb

er

of

eag

les

time (years)

20

reproduce at 4 years (pop. 1) reproduce at 6 years (pop. 2)

At 24 years,

this population

has 2,504 eagles 0 2 2

6 8 4

12 52 18

18 362 86

24 2,504 392

30 17,314 1,764

Time

(years)

Number of

eagles (pop. 1)

Number of

eagles (pop. 2)

At 24 years,

this population

has 392 eagles






(continued)

– As long as birth rate exceeds death rate, population

size will follow a J-shaped rate of increase

– However, the time for each population to reach a

specific number of individuals will depend on the

magnitude of the death rate






(continued)

– The death rate has a major impact on population size

– As long as births exceed deaths, the population

eventually becomes enormous

– As the death rate increases, it takes longer to reach any

given population size


Figure 26-2 The effect of death rates on population growth

2,500

2,000

1,500

1,000

500

0 1 2 3 4 5 6

time (hours)

nu

mb

er

of

ba

cte

ria

no deaths 10% death rate 25% death rate

It takes about 4 hours to produce 1,500 bacteria

It takes about 3.5 hours to produce 1,500 bacteria

It takes about 5.5 hours to produce 1,500 bacteria



Biotic potential determines the maximum rate at

which a population can grow

– The ability to produce many offspring is an inherited

attribute

– Natural selection favors organisms whose attributes

adapt them to their environments and who pass these

adaptations on to as many healthy offspring as

possible

– Biotic potential refers to the maximum rate at which

a particular population could increase



Biotic potential determines the maximum rate at

which a population can grow (continued)

– Several factors influence biotic potential

– The age at which the organism first reproduces

– The frequency of reproduction

– The average number of offspring produced each time

– The length of the organism’s reproductive life span

– The death rate of individuals



In 1859, Charles Darwin wrote: “There is no

exception to the rule that every organic being

naturally increases at so high a rate, that if not

destroyed, the Earth would soon be covered by the

progeny of a pair.”



Population size results from the interaction between

biotic potential and environmental resistance, or

all the curbs on population growth imposed by the

living and nonliving environment

– Examples include interactions among organisms such

as predation and competition for limited resources

– Environmental resistance also includes natural events

such as freezing weather, storms, fires, floods, and

droughts



Exponential growth only occurs under unusual

conditions

– Under unusual and temporary circumstances, natural

populations exhibit exponential growth, producing J-

shaped growth curves




conditions (continued)

– Exponential growth occurs in populations with boom-

and-bust cycles

– Exponential growth can be observed in populations that

undergo boom-and-bust cycles, in which periods of

rapid population growth are followed by a sudden,

massive die-off






and-bust cycles (continued)

– Seasonal populations are linked to changes in rainfall,

temperature, or nutrient availability

– Ideal conditions encourage rapid growth; deteriorating

conditions encourage massive die-off







– For example, each year, photosynthetic bacteria in a

lake may exhibit exponential growth when conditions

are ideal, but crash when they have depleted their

nutrient supply


Figure 26-3a A boom-and-bust cycle in photosynthetic bacteria

Nutrients are depleted, and

water temperature falls

Favorable growth

conditions occur

“boom” “bust”

Jan Mar May

month

A boom-and-bust cycle in photosynthetic bacteria

Jul Sep Nov

po

pu

lati

on

den

sit

y







– Complex factors produce four-year cycles for small

rodents, such as lemmings

– Lemming populations may grow until lack of food, large

migrations, and predators and starvation cause sudden

high mortality


Figure 26-3b Boom-and-bust cycles in a lemming population in the Canadian Arctic

14

12

10

8

6

4

2

1985

nu

mb

er

pe

r 100 t

rap

nig

hts

1990 1995 2000

year

Boom-and-bust cycles in a lemming population in

the Canadian Arctic.

0





– Exponential growth occurs temporarily when

environmental resistance is reduced

– In populations that do not experience boom-and-bust

cycles, exponential growth may occur temporarily under

special circumstances such as

– An increase of food supply or habitat

– A reduction in predation






environmental resistance is reduced (continued)

– For example, the whooping crane population has grown

exponentially since they were first protected from

hunting and human disturbance in 1940

– The whooping crane remains among the world’s rarest

birds, so continued population growth will be necessary

for its survival


Figure 26-4 Exponential growth of wild whooping cranes

450

425

400

375

350

325

300

275

250

225

200

175

150

125

100

75

50

25

0 1940 1950 1960 1970 1980 1990 2000 2010

year

nu

mb

er

of

cra

nes







– Exponential growth can occur when individuals invade

a new habitat with little competition

– Invasive species are organisms with a high biotic

potential that are introduced into ecosystems where

they did not evolve and where they encounter little

environmental resistance







– When they are introduced into a new ecosystem,

population numbers may explode due to a lack of

natural predators



Environmental resistance limits population growth

– Many populations that exhibit exponential growth

eventually stabilize to match the resources available

to support them

– As resources become depleted, reproduction slows and

the growth rate eventually drops to zero, causing the

population size to remain constant




(continued)

– Logistic growth occurs when new populations stabilize

as a result of environmental resistance

– This growth pattern, where populations increase to the

maximum number sustainable by their environment and

then stabilize, is called logistic population growth

– The maximum population size that can be sustained by

an ecosystem for an extended time without damage to

the ecosystem is called its carrying capacity (K)


Figure 26-5a An S-shaped growth curve stabilizes at carrying capacity

carrying capacity

nu

mb

er

of

ind

ivid

ua

ls

Growth rate slows

Population grows rapidly

Growth stops and the population stabilizes close to the carrying capacity

time

An S-shaped growth curve stabilizes at carrying capacity

0




(continued)


as a result of environmental resistance (continued)

– When logistic growth is plotted, it results in an S-

shaped growth curve, or S-curve

– In nature, an increase in population size (N) above

carrying capacity (K) can be sustained for a short time




(continued)



– If a population far exceeds the carrying capacity of its

environment, excess demands placed on the

ecosystem are likely to destroy crucial resources




(continued)



– This can permanently and severely reduce carrying

capacity, causing the population to decline to a fraction

of its former size or disappear entirely


Figure 26-5b Consequences of exceeding carrying capacity

The population overshoots its carrying capacity; the environment is damaged

Low damage; resources recover, and the population fluctuates

Extreme damage; the population dies out

High damage; the carrying capacity is permanently lowered

time

Consequences of exceeding carrying capacity

0

carrying capacity (original)

carrying capacity (reduced)




(continued)



– For example, when reindeer were introduced onto an

island with no large predators, their population

increased rapidly, seriously overgrazing the vegetation

they relied on for food

– As a result, the reindeer population plummeted


Figure 26-6 The effects of exceeding carrying capacity

2,000

1,600

1,200

800

400

0

1910 1920 1930 1940 1950

population

crash

exponential

growth

nu

mb

er

of

rein

deer

*Data not taken for 1943–1946

year




(continued)



– Logistic population growth can occur in nature when a

species moves into a new habitat

– For example, new barnacle settlers along a rocky coast

may find ideal conditions that allow their population to

grow exponentially




(continued)



– Logistic population growth can occur in nature when a

species moves into a new habitat (continued)

– As population density increases, however, individuals

begin to compete for space, energy, and nutrients


Figure 26-7 A logistic curve in nature

80

60

40

20

0

3 4 5 time (weeks) n

um

ber

of

barn

acle

s (

per

cm

2)

1 2




(continued)



– Two forms of environmental resistance usually maintain

populations at or below the carrying capacity of their

environment

– Density-independent

– Density-dependent




(continued)



– Density-independent factors limit population size

regardless of the population density

– Density-dependent factors increase in effectiveness

as the population density increases




(continued)

– Density-independent factors limit populations

regardless of their density

– The most important natural density-independent factors

are climate and weather, which are responsible for

most boom-and-bust population cycles

– Many insects and annual plant populations are limited in

size by the number of individuals that can be produced

before the first hard freeze




(continued)


regardless of their density (continued)

– The most important natural density-independent factors

are climate and weather, which are responsible for

most boom-and-bust population cycles (continued)

– Hurricanes, droughts, floods, and fire can have profound

effects on local population, regardless of density




(continued)



– Human activities can also limit the growth of natural

populations

– Pesticides and pollutants can cause drastic declines in

natural populations

– Overhunting has driven some species to extinction




(continued)



– Habitat destruction by humans, a density-independent

factor, is the single greatest threat to wildlife worldwide

– The ivory-billed woodpecker has been driven to

extinction in the United States




(continued)

– Density-dependent factors become more effective as

population density increases (continued)

– Populations of organisms with a life span of more than

a year have evolved adaptations that allow them to

survive density-independent controls imposed by

seasonal changes, such as cold and lack of food during

the winter




(continued)



– Many mammals develop thick coats and store fat for

the winter

– Some mammals hibernate

– Migration is another coping mechanism

– Many birds migrate long distances to find food and a

hospitable climate




(continued)



– Most trees and bushes survive the rigors of winter by

entering a period of dormancy

– For long-lived species in undisturbed habitats, the most

important elements of environmental resistance are

density dependent




(continued)



– Density-dependent factors exert a negative feedback

effect on population size, because they become

increasingly effective as the population density

increases




(continued)



– Predators exert density-dependent controls on

populations

– Predators are organisms that eat other organisms,

called their prey

– Prey are killed directly and eaten, but not always


Figure 26-8a Predators often kill weakened prey

Predators often kill weakened prey




(continued)




populations (continued)

– Predation becomes important as prey populations grow

because predators eat a variety of prey, depending on

what is most abundant and easiest to find




(continued)





– Predator populations often grow as their prey becomes

more abundant, which makes them even more effective

as control agents




(continued)





– For example, snowy owls hatch up to 12 chicks when

lemmings (their prey) are abundant, but may not

reproduce at all in years when the lemming population

has crashed


Figure 26-8b Predator populations often increase when prey are abundant

Predator populations often increase when prey are abundant




(continued)





– Some predator-prey population cycles are out-of-phase

when predators cause a dramatic decline in prey

populations, which in turn results in a decline in the

predator population at a future date




(continued)





– Predation may maintain healthy prey populations near a

density that can be sustained by the resources of the

ecosystem


Figure 26-9 Experimental predator–prey cycles

1,600

1,200

800

400

0

5 10 15 20 25 30

generation

ad

ult

po

pu

lati

on

bean weevils (prey)

braconid wasp (predator)

A high predator

population

reduces the prey

population

The prey population

peaks when the

predator population

is low




(continued)



– Parasites spread more rapidly among dense

populations

– A parasite feeds on a larger organism, its host,

harming it

– Parasites include tapeworms that live in the intestines of

mammals, ticks that cling to the host’s skin, and disease-

causing microorganisms




(continued)





– Parasites influence population size by weakening their

hosts and making them more susceptible to death from

other causes, such as harsh weather or predators

– Organisms weakened by parasites are less likely to

reproduce




(continued)





– Parasites, like predators, more often contribute to the

death of less-fit individuals, producing a balance in which

the host population is regulated but not eliminated




(continued)



– Competition for resources helps control populations

– Competition is the interaction among individuals who

attempt to use the same limited resource, which limits

population size in a density-dependent manner




(continued)




(continued)

– There are two major forms of competition

– Interspecific competition, between individuals of

different species

– Intraspecific competition, between individuals of

the same species




(continued)




(continued)

– Because the needs of members of the same species for

resources are almost identical, intraspecific competition

is an important density-dependent mechanism of

population control




(continued)




(continued)

– Most plants and many insects engage in scramble

competition—a free-for-all scramble as individuals try to

beat others to a limited pool of resources




(continued)




(continued)

– For example, gypsy moth females each lay a mass of up

to 1,000 eggs on tree trunks in eastern North America




(continued)




(continued)

– As the eggs hatch, armies of caterpillars crawl up the

tree

– Huge outbreaks of this invasive species can

completely strip large trees of their leaves in a few

days




(continued)




(continued)

– Competition for food may be so great that most of the

caterpillars die before they can metamorphose into egg-

laying moths


Figure 26-10 Scramble competition

Gypsy moths laying eggs Gypsy moths caterpillars




(continued)




(continued)

– Many animals have evolved contest competitions,

where social or chemical interactions determine access

to important resources




(continued)




(continued)

– Territorial species—such as wolves, fish, rabbits, and

songbirds—defend areas that contain important

resources

– Only the best adapted individuals are able to defend their

territories that supply adequate food and shelter




(continued)




(continued)

– As population densities increase and competition

becomes more intense, some animals react by

emigrating


Figure 26-11 Emigration




(continued)

– Density-independent and density-dependent factors

interact to regulate population size

– The size of a population at any given time is the result

of complex interactions between density-independent

and density-dependent forms of environmental

resistance




(continued)

– Density-independent and density-dependent factors

interact to regulate population size (continued)

– For example, a caribou weakened by hunger (density-

dependent) and attacked by parasites (density-

dependent) is more likely to be killed by an

exceptionally cold winter (density-independent)


26.3 How Are Populations Distributed in Space and Age?

Populations of different types of organisms show

characteristic spacing of their members, determined

by their behavioral characteristics and their

environments

Each population exhibits patterns of reproduction

and survival that are characteristic of its species



Populations exhibit different spatial distributions

– Spatial distribution describes how individuals within a

population are distributed within a given area

– Spatial distribution may vary with time, changing with

the breeding seasons

– Ecologists recognize three major types of spatial

distribution:

– Clumped

– Uniform

– Random




(continued)

– Populations whose members live in groups exhibit

clumped distribution

– Examples include elephant herds, wolf packs, prides of

lions, flocks of birds, and schools of fish




(continued)

– Advantages of clumped distributions include

– Many eyes that can search for localized food sources

– Movement of the group (e.g., schools of fish or flocks of

birds) can confuse predators by their sheer numbers

– Predators, in turn, may hunt in groups, cooperating to

bring down larger prey


Figure 26-12a Clumped distribution

Clumped distribution

clumped




(continued)

– Organisms with a uniform distribution maintain a

relatively constant distance between individuals

– This is common among territorial animals defending

scarce resources or breeding territories

– An example among plants is desert creosote bushes,

which are spaced evenly resulting from competition

among their root systems for water and nutrients




(continued)

– Territorial behavior is more common among animals

during their breeding seasons

– Seabirds may space their nests evenly along the shore,

just out of reach of one another

– Mature desert creosote bushes are often spaced very

evenly

– This spacing comes from competition among their root

systems, which occupy a circular area around each plant


Figure 26-12b Uniform distribution

uniform

Uniform distribution




(continued)

– Organisms with a random distribution are relatively

rare

– Such individuals do not form social groups

– The resources needed are more or less equally

available throughout the area they inhabit

– Resources are not scarce enough to require territorial

spacing

– Examples include trees and other plants in rain forests




(continued)

– There are probably no vertebrate species that

maintain a random distribution throughout the year

– Most interact socially, at least during the breeding

season


Figure 26-12c Random distribution

random

Random distribution



Populations exhibit different age distributions

– Animals of different species differ considerably in their

chances of dying at any given phase of their life cycle

– Some species produce many offspring that are

provided with very few resources; most die before they

can reproduce

– Others produce few offspring, which are each given far

more resources and often survive to reproduce




(continued)

– Three types of survivorship curves are described

according to the part of the life cycle during which

most deaths occur

– Late-loss populations

– Constant-loss populations

– Early-loss populations




(continued)

– To determine the pattern of survivorship, researchers

construct survivorship tables, which track groups of

organisms (born at the same time) throughout their

lives, recording how many survive in each succeeding

year

– If these numbers are graphed, they reveal the

survivorship curves characteristic of the species in

the environment where the data were collected




(continued)

– Late-loss populations produce convex survivorship

curves

– These populations have relatively low juvenile death

rates; many or most individuals survive to old age

– Late-loss curves are characteristic of humans and other

large and long-lived animals such as elephants and

mountain sheep

– Relatively few offspring are produced by these species




(continued)

– Constant-loss populations produce straight-line

survivorship curves

– In these populations, individuals have an equal chance

of dying at any time during their life span

– This pattern is seen in some birds such as gulls and the

American robin, in some species of turtles, and in

laboratory populations of organisms that reproduce

asexually, such as hydra and bacteria




(continued)

– Early-loss populations produce concave

survivorship curves

– These curves are characteristic of organisms that

produce large numbers of offspring that receive little or

no parental care

– Many of these species engage in scramble competition

early in life




(continued)

– Early-loss populations produce concave

survivorship curves (continued)

– The death rate is high among the young, but those that

reach adulthood have a reasonable chance to survive

to old age

– Most invertebrates, many fish and amphibians, and most

plants exhibit early loss survivorship curves


Figure 26-13 Survivorship tables and survivorship curves

1,000

100

10

0

nu

mb

er

of

su

rviv

ors

Age Number

of

survivors

0 (birth) 100,000

99,124

98,713

97,754

96,489

93,698

87,967

76,241

54,117

22,312

10

20

30

40

50

60

70

80

90

early loss

(dandelion)

constant loss

(American robin)

late loss

(human)

percent of maximum life span

Survivorship curves

100 2,523

A survivorship

table



No force on Earth rivals that exerted by humans

– Humans possess enormous brainpower

– We possess dexterous hands that can shape the

environment by our demands

Natural selection favored those with the ability and

the drive to bear and nurture offspring, which helped

make sure that few would survive

– This characteristic now threatens us and the

biosphere on which we depend



The human population continues to grow rapidly

– In the last few centuries, the human population has

grown at nearly an exponential rate following a J-

shaped growth curve

– Over the last decade, however, the human population

has been growing at a relatively constant rate,

suggesting that it may no longer be growing

exponentially



The human population continues to grow rapidly

(continued)

– Are humans starting to enter the final bend of the S-

shaped logistic growth curve that will eventually lead

to a stable population?

– Despite the fact that our annual growth rate has

declined from 1.8% in 1960 to 1.2% in 2011, Earth’s

human population is adding people faster than ever

– Having reached 7 billion in 2011, our numbers now

grow by about 83 million each year


Figure 26-14 Human population growth

Date Billions Time to add each billion

(years)

1804

1927

1960

1975

1987

1999

2011

2025

*projected

1

2

3

4

5

6

7

8*

All of human history

123

33

13

12

12

14

12,000 11,000 10,000 9,000 8000 7000 6000 5000 4000 3000 2000 1000 1000 2000

0

1

2

3

4

5 1987

1975

1960

1927

1804

bubonic plague

bil

lio

ns

of

pe

op

le

ye

ar

Technical advances Agricultural advances

B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C. B.C.

B.C./A.D.

A.D. A.D.

Industrial and medical

advances

1999

2011 7

6

14



A series of advances has increased Earth’s carrying

capacity to support people

– Human population growth has been spurred by a

series of advances, each of which circumvented some

type of environmental resistance, increasing Earth’s

carrying capacity for people




capacity to support people (continued)

– Early humans

– Discovered fire

– Invented tools and weapons

– Built shelters

– Designed protective clothing





– A series of technical advances increased carrying

capacity

– Tools and weapons allowed humans to hunt more

effectively and obtain additional high-quality food, while

shelter and clothing expanded the habitable areas of

the globe





– Domesticated crops and animals had supplanted

hunting and gathering in many parts of the world by

8000 B.C.

– These agricultural advances provided a larger and

more stable food supply for people, further increasing

Earth’s carrying capacity for humans





– Human population growth continued slowly for

thousands of years until major industrial and medical

advances permitted a population explosion

– These advances began in England in the mid-

eighteenth century

– Medical progress dramatically decreased the death rate

by reducing environmental resistance caused by

disease





– The discovery of bacteria and their role in infection

resulted in better control of bacterial diseases through

improved sanitation and antibiotics

– Vaccines for diseases such as smallpox reduced

deaths from viral infections



The demographic transition explains trends in

population size

– In developed countries, people benefit from a

relatively high standard of living, with access to

modern technology and medical care, including

readily available contraception

– Developed countries include Australia, New Zealand,

Japan, and countries in North America and Europe




population size (continued)

– Average income in developed countries is relatively

high

– Education and employment opportunities are

available to both sexes

– Death rates from infectious diseases are low

– Less than 20% of the world’s population lives in

developed countries





– In the developing countries of Central and South

America, Africa, and much of Asia—home to more

than 80% of humanity—the average person lacks

these advantages

– The historical rate of population growth in developed

countries has changed over time in reasonably

predictable stages, producing a pattern called

demographic transition





– Pre-industrial stage: The population was relatively

small and stable, with high birth rates and high death

rates

– Transitional stage: Food production increased and

health care improved, which caused death rates to

fall; because birth rates remained high, there was an

explosive population increase





– Industrial stage: Birth rates fell as contraceptives were

more available, and as people moved from farms to

cities, where children were less important as a source

of labor

– Post-industrial stage: Populations are relatively

stable, with low birth and death rates


Figure 26-15 The demographic transition

Pre-industrial Stage

Transitional Stage Industrial Stage Post-industrial Stage

Population stabilizes

Birth rate declines

Birth and death rates are low

Population growth slows

Birth rate remains high

Birth and death rates are high

Population grows rapidly

Population remains low

natural rate of population increase

birth rate

death rate

population size

inc

rea

se

in

ra

tes

or

siz

e

Death rate declines

time





– A population’s fertility rate reflects the average

number of children that each woman bears

– If immigration and emigration rates are balanced, a

population will eventually stabilize if parents have just

the number of children to replace themselves

– This is called replacement-level fertility (RLF)

– RLF is 2.1 children per woman because not all children

survive to maturity



World population growth is unevenly distributed

– In developing countries, medical advances have

decreased death rates and increased life span, but

birth rates remain relatively high

– Although China is a developing country, its population

has approached one billion




(continued)

– Most other developing countries are within the late-

transitional or the industrial stage of the demographic

transition

– Adult children provide financial security for aging

parents

– Young children may also contribute significantly to the

family income by working on farms or factories




(continued)

– Most other developing countries are within the late-

transitional or the industrial stage of the demographic

transition (continued)

– Social factors drive population growth in countries

where children confer prestige because religious beliefs

promote large families

– Many individuals who would like to limit their family size

lack access to contraceptives




(continued)

– A lack of education and a lack of access to

contraceptives then contributes to continued high birth

rates

– Of the 7 billion people on Earth in 2011, about 5.8

billion resided in developing countries

– The prospect for world population stabilization in the

near future is nonexistent



The age structure of a population predicts its future

growth

– Age structure diagrams show age groups on the

vertical axis and the numbers (or percentages) of

individuals in each age group on the horizontal axis,

with males and females shown on opposite sides

– Age structure diagrams all rise to a peak that reflects

the maximum human life span

– The shape of the rest of the diagram reveals whether

the population is expanding, stable, or shrinking




growth (continued)

– If adults of reproductive age (15 to 44 years) are

having more children (the 0- to 14-year age group)

than are needed to replace themselves, the

population is above RLF and is expanding

– The age-structure diagram will be roughly triangular


Figure 26-16a Africa: A rapidly growing population

Africa 2010

male

100

90

80

70

60

50

40

30

20

10

0

6

ag

e

4 2 0 2 4 6 percent of population

female

Africa: A rapidly growing population




growth (continued)

– If adults of reproductive age have just the number of

children needed to replace themselves, the population

is at RLF

– A population that has been at RLF for many years will

have an age structure diagram with relatively straight

sides


Figure 26-16b North America: A slowly growing population

female

North America 2010

male

100

90

80

70

60

50

40

30

20

10

0

6

ag

e


North America: A slowly growing population

female




growth (continued)

– In a shrinking population, the reproducing adults have

fewer children than are required to replace

themselves

– The age-structure diagram will be narrow at the base

– The median age depends on the age structure

– The lower the median age, the more rapidly the

population will expand


Figure 26-16c Europe: A slowly declining population

Europe 2010

male

100

90

80

70

60

50

40

30

20

10

0

6

ag

e


Europe: A slowly declining population

female




growth (continued)

– Average-age structure diagrams have been plotted for

developed and developing countries for 2012, with

predictions for 2050




growth (continued)

– These diagrams reveal that even if developing

countries were to achieve RLF immediately, their

population increases would continue for decades

– A large population of children today creates a

momentum for future growth as they enter their

reproductive years


Figure 26-17a Developed countries

100

90

80

70

50

40

30

20

10

0

ag

e 60

millions of people

Developed countries

male female

postreproductive (45100 years)

reproductive (1544 years)

prereproductive (014 years)

2010

300 275 250 225 200 175 150 125 100 75 50 25 0 25 50 75 100 125 150 175 200 225 250 275 300

2050


Figure 26-17b Developing countries

100

90

80

70

50

40

30

20

10

0

ag

e 60

millions of people

Developing countries

male female

2010

300 275 250 225 200 175 150 125 100 75 50 25 0 25 50 75 100 125 150 175 200 225 250 275 300

2050




growth (continued)

– The United Nations has developed high, medium, and

low projections for future growth based on

assumptions about fertility rates

– For the year 2050, the medium projection is that

Earth’s population will have increased by about 33%

to over 9.3 billion

– Eight billion people live in developing nations


Figure 26-18 United Nations world population projections

12

11

10

9

8

7

6 2000 2010

wo

rld

po

pu

lati

on

(b

illi

on

s)

2020 2030 2040 2050

10.6

9.3

8.1 low

medium

high



Fertility in some nations is below replacement level

– A comparison of growth rates for various world

regions shows Europe as the only one with an

average rate of change in population that is negative

– The average fertility rate is 1.6, which is substantially

below RLF

– Concerns about the availability of future workers and

taxpayers have prompted several countries to offer

incentives for couples to have children at an earlier age




(continued)

– Japan’s government is concerned about the country’s

low fertility rate (1.4) and provides subsidies to

encourage larger families

– Japan is about the size of the state of Montana in the

United States and home to 128 million people

(equivalent to 41% of the entire U.S. population)




(continued)

– The current economic structures in countries

throughout the world are based on growing

populations

– The difficult adjustments necessary as populations

decline—or even merely stabilize—motivate

governments to adopt policies that encourage more

childbearing and continued growth


Table 26-1



The U.S. population is growing rapidly

– The United States has a population of more than 313

million and a growth rate of about 0.7% (adding one

person every 15 seconds) and is the fastest-growing

developed country in the world

– Continued immigration, which accounts for about 30%

of the population increase, will ensure growth for the

indefinite future


Figure 26-19 United States population growth

325

300

275

250

225

200

175

150

125

100

75

50

25

0 1800 1850 1900 1950 2000

year

U.S

. p

op

ula

tio

n (

in m

illi

on

s)



The U.S. population is growing rapidly (continued)

– The rapid growth of the U.S. population has major

environmental implications for local ecosystems and

for the planet

– The average U.S. resident uses nearly four times as

much energy as the average person worldwide

audesirk audesirk byers 26 - linn-benton community...

Documents