Copyright © 2009 Pearson Education, Inc.. Lectures by Gregory Ahearn University of North Florida Chapter 27 Population Growth

Copyright © 2009 Pearson Education, Inc..

Lectures by

Gregory AhearnUniversity of North Florida

Chapter 27

Population Growth

Copyright © 2009 Pearson Education Inc.

27.1 How Are Populations Distributed In Space And Time? Organisms arrange themselves in space in

many different ways. Ecologists recognize three major types of

spatial distribution:• Clumped• Uniform• Random


Clumped distribution

clumped

(a)

27.1 How Are Populations Distributed In Space And Time? Individuals in many populations clump

together in groups, and include social groups such as elephant herds, wolf packs, lion prides, flocks of birds, and schools of fish.

Fig. 27-1a


27.1 How Are Populations Distributed In Space And Time? What are the advantages of clumping?

• Many eyes can search for localized food.• Part of a large group may reduce the odds

that one individual will be killed by a predator.• A group may increase the individual’s chance

of finding a mate.


27.1 How Are Populations Distributed In Space And Time? Some individuals disperse themselves

evenly.• Populations with uniform distributions maintain

relatively even spacing between individuals.• This type of distribution occurs among animals

that defend territories.• Male Galapagos iguanas establish regularly

spaced breeding territories.


27.1 How Are Populations Distributed In Space And Time? Creosote bushes release chemicals into the

soil around them that inhibit germination of seeds from other plants.

Fig. 27-1bUniform distribution

uniform

(b)


27.1 How Are Populations Distributed In Space And Time? In a few populations, individuals are

distributed at random.• In a population with random distribution, the

distance between individuals varies unpredictably.

• Individuals in such populations do not form social groups.

• The resources they need are not in short supply and are available throughout the year.


27.1 How Are Populations Distributed In Space And Time? Trees and other plants in rain forests may

be randomly distributed.

Fig. 27-1cRandom distribution

random

(c)


27.2 How Do Populations Grow?

Births, deaths, and migration determine population growth.• A population’s size remains stable if, on

average, as many individuals join as leave.• Individuals enter a population by birth or

immigration, and leave it by death or emigration.

• A population grows when the number of births plus immigrants exceeds the number of deaths plus emigrants.

• Populations shrink when the opposite occurs.



A simple equation for the change in population size within a given period is as follows:• (births – deaths) + (immigrants – emigrants) =

change in population size



Population growth can be expressed as a rate.• The per capita growth rate (r) of a population

is measure of how fast a population grows, expressed as a change in population size per individual per unit of time (that is, as a percentage).

• This value is determined by subtracting the per capita death rate (d) from the per capita birth rate (b).• b (births) – d (deaths) = r (growth rate)



Population growth can be expressed as a rate (continued).• For example, to calculate the annual growth

rate of a human population of 10,000 in which there are 1,500 births and 500 deaths each year, we first calculate the annual per capita birth rate. • b = 1,500 births/10,000 people • = 0.15 births per person per year

• Next, the annual per capita death rate. • d = 500 deaths/10,000 people • = 0.05 deaths per person per year



Population growth can be expressed as a rate (continued).• Finally, we calculate the per capita growth rate

by subtracting the death rate from the birth rate.• r = 0.15 births per person per year – 0.05

deaths per person per year • = 0.10 (that is, 10% increase per year)

• If the death rate exceeds the birth rate, the growth rate will be negative and the population will shrink.



A constant growth rate increases population size rapidly.• To calculate the number of individuals added

to a population in a year, multiply the annual per capita growth rate (r) by the original population size (N).• Population growth = rN

• In our example, population growth in the first year (rN) is 0.10 x 10,000 = 1,000 people; at the end of year 1, the population has 11,000 people.



A constant growth rate increases population size rapidly (continued).• If the per capita growth rate r remains

constant, the number of people added to the population increases each year.

• This pattern of continuously accelerating increase in population size is exponential growth.



A population’s growth rate depends on patterns of reproduction.• A population grows during periods when births

exceed deaths.• Growth will persist if, on average, each

individual produces more than one surviving offspring during its lifetime.

• Each individual, of course, has the potential to replace itself many times during its lifetime.



A population’s growth rate depends on patterns of reproduction.• Every species has a built-in capacity for population

growth, but the speed of this potential growth varies among species, dependent upon the following factors.

• The age of first reproduction• The frequency of reproduction• The average number of offspring produced each

time• The length of an organism’s reproductive life span • The organism death rate



Some species produce large numbers of offspring quickly.• The harmless bacterium Staphylococcus is

found on the human body.• Each bacterial cell can divide every 20

minutes, doubling the population three times each hour.

• The larger the population growth, the more cells there are to divide.



Some species produce large numbers of offspring quickly (continued).• The growth rate is so great that, unchecked,

the offspring of one bacterium could produce a layer around the Earth 7 feet deep in 48 hours.

• Because this does not happen, many bacteria must die.



Other species produce fewer but longer-lived offspring.• The golden eagle is a long-lived, rather slowly

reproducing species.• Figure 27-2 compares the potential population

growth of eagles with that of bacteria, assuming no deaths.

• All three curves on this figure are J-shaped, indicating exponential growth.



J-shaped exponential growth curves in bacteria

Fig. 27-2a

Bacteria

020406080

100120140160180200220

time(minutes)

number ofbacteria

1248

163264

128256512

1,0242,048

bacteria

Exponentialgrowth curvesare J-shaped

(a)

1,2001,1001,000

900800700600500400300200100

0nu

mb

er

of

ind

ivid

ua

ls

0 60 120 180 240time (minutes)



Delayed onset of reproduction slows population growth.• Figure 27-2b shows what happens if there is a

difference in the age at which individuals in a population begin reproduction.

• Growth in both populations is exponential, but the population whose members begin reproduction later takes longer to reach a particular size.

• For humans, delayed childbearing slows population growth.



J-shaped exponential growth curve in eagles

Fig. 27-2b

time(years)

numberof

eagles (i)

numberof

eagles (ii)02468

1012141618202224262830

2248

142852

100190362630

1,3142,5044,7709,088

17,314

22248

1218325486

142238392644

1,0661,764

Reproductionbegins at4 years

Reproductionbegins at6 years

eagles

(i)

(ii)

Eagles(b)

2,0001,8001,6001,4001,2001,000

800600400200

0

nu

mb

er

of

ind

ivid

ua

ls

time (years)0 5 10 15 20 25 30



Death rate also influences growth rate.• Figure 27-3 compares three bacterial

populations with different death rates.• The J shapes of the curves are the same.• However, the time required to reach any given

population size is longer at higher death rates.


bacteria

25% diebetweendoublings

10% diebetweendoublings

Nodeaths

2,500

2,000

1,500

1,000

500

0

0 1 2 3 4 5 6

time (hours)

nu

mb

er o

f in

div

idu

als


The effects of death rates on population growth

Fig. 27-3


27.3 How Is Population Growth Regulated?

A population’s growth is influenced by its biotic potential—the maximum rate at which a population can increase, assuming ideal conditions that allow the highest possible birth rate and the lowest possible death rate.• The ultimate size of a population is also

affected by limits that oppose this potential for growth.

• These limits, which are set by the living and nonliving environments, are collectively known as environmental resistance.



Environmental resistance is imposed by several things.• Availability of food and space• Interactions with competitors• Predators• Disease-causing organisms• Natural catastrophes (e.g., storms, fires,

freezing weather, floods, and droughts)



Environmental resistance limits population size by the following factors:• Increasing death rates• Decreasing birth rates• Both factors together

For example, a drought might restrict growth of an animal population both by increasing the number of deaths from starvation, and by causing malnutrition that reduces the number of births.



Rapid growth cannot continue indefinitely.• Populations grow rapidly only under certain

circumstances and only for a limited time before environmental resistance brings their expansion to a halt.



Some populations fluctuate cyclically.• Short-term explosive growth occurs in

populations that undergo regular cycles of rapid population growth followed by a sudden, massive die-off—a boom-and-bust cycle.

• Many such animals have seasonal population cycles that are linked to predictable changes in rainfall, temperature, or nutrient availability.



A boom-and-bust population cycle

Fig. 27-4

Favorable growthconditions occur

Nutrients are depleted

“bust”“boom”

po

pu

lati

on

den

sity

0Jan Mar May Jul Sep Nov

month



Some populations fluctuate cyclically (continued).• A lemming population may grow until the

animals overgraze their fragile arctic tundra ecosystem.

• Then, lack of food, increasing populations of predators, and social stress caused by overpopulation all contribute to a suddenly high death rate.

• Eventually, the lemming population shrinks so much that number of predators declines.



Lemming population cycles

Fig. 27-5



Ecosystem changes may allow temporary rapid growth.• A population may grow rapidly if population-

controlling factors, such as predators, are eliminated or if the food supply is increased.

• Growth can also be explosive when individuals invade a new habitat that has favorable conditions and few competitors.

• For example, a species’ population may grow explosively when people introduce the species into an ecosystem.



Environmental resistance limits population growth.• Populations that grow rapidly must eventually

stabilize or crash.• They tend to stabilize at or below their

ecosystem’s carrying capacity, the maximum population of a particular species that an ecosystem can support indefinitely.



Environmental resistance limits population growth (continued).• As a growing population approaches the

carrying capacity of the environment, its growth rate gradually declines and finally stops when the population reaches a state of equilibrium.

• In this equilibrium, the birth rate is balanced by the death rate, and population size is stable.

• This type of population growth is represented graphically by an S-shaped growth curve.


An S-shaped growth curve stabilizes at carrying capacity

Growth stops and thepopulation stabilizes closeto the carrying capacity

Populationgrows rapidly

Growthrate slows

carryingcapacity

(a)

nu

mb

er

of

ind

ivid

ua

ls

time

0


Fig. 27-6a

An S-shaped logistic growth curve stabilizes at carrying capacity.



Environmental resistance limits population growth (continued).• A population may grow to a size larger than its

ecosystem’s carrying capacity.• This overshooting of carrying capacity is

necessarily temporary.• A small overshoot of carrying capacity is likely

to be followed by a decrease in population size until the resources recover and the original carrying capacity is restored.



Consequences of exceeding carrying capacity

Fig. 27-6b

Consequences of exceeding carrying capacity

carryingcapacity

(reduced)

carryingcapacity(original)

High damage; thecarrying capacity ispermanently lowered

Low damage; resourcesrecover, and thepopulation fluctuates

Extremedamage; thepopulationdies out

Populationovershootsthe carryingcapacity; theenvironmentis damaged

(b)

time

0



Environmental resistance limits population growth (continued).• The factors that usually maintain populations

at or below the carrying capacity of their environment can be classified into two broad categories.• Density-independent factors limit population

size regardless of the population density (number of individuals per given area).

• Density-dependent factors increase in effectiveness as the population density increases.



Density-independent factors limit population size.• Natural events—including hurricanes,

droughts, floods, and fire—can reduce the size of populations.

• The effectiveness of such events in limiting the size of a population does not generally depend on the population’s density.



Density-independent factors limit population size (continued).• The most important natural density-

independent factor is weather.• Human activities, such as the use of

pesticides and pollutants, can also limit population growth in ways independent of population density.



Density-dependent factors have a greater effect as population density increases.• Density-dependent factors become

increasingly effective as population density increases, thus exerting negative feedback that limits the size of populations.

• Conversely, density-dependent factors become less effective as population density decreases, allowing population size to stabilize or grow.



Density-dependent factors have greater effect as population density increases (continued).• The most important density-dependent factors

are predation and competition. • In predation, one organism feeds on another,

harming it in the process.• Often, one organism (the predator) kills

another (its prey) in order to eat it, but the prey is not always killed.



Predators help control prey populations.

Fig. 27-7



Density-dependent factors have greater effect as population density increases (continued).• Prey also often survive the special form of

predation known as parasitism.• In parasitism, the predator (the parasite) lives

on or inside another organism (its host) and feeds on the host’s body without killing it—at least not immediately.



Predators exert density-dependent controls on populations.• Predation plays an increasingly important role

in population control as prey populations increase.

• The increased frequency of encounters with predators increases the prey populations’ death rate, because predators tend to eat larger numbers of whichever prey species is most abundant and easiest to find.

• Increasing numbers of prey organisms also increase the numbers of predators, and this tends to control prey numbers.



Parasites spread faster when the host population density is high.• Like other forms of predation, parasitism is

density-dependent.• Most parasites have limited ability to move, so

they spread more readily from host to host at high host-population densities.

• Parasites can affect the death rate of a host population because the damage inflicted by the parasite on its host’s body may kill the host.



Populations can soar or crash when predator-prey relationships are disrupted.• The population balance in ecosystems can be

disrupted when predators are introduced to regions in which they were not previously present and where prey species have had no opportunity to evolve defenses against them.

• Rats, snakes, and mongooses were introduced in Hawaii and many other Pacific islands, and have drastically reduced or exterminated many native bird populations.



Populations can soar or crash when predator-prey relationships are disrupted (continued).• Prey populations can grow out of control when

introduced to areas where they have no predators.• The prickly pear cactus was introduced into Australia

from Latin America, and, lacking predators, it spread uncontrollably.

• In the 1920s, a cactus moth was imported from Argentina to feed on the cacti.

• Within a few years, the cacti were almost eliminated, and today, the moth keeps the population density of its prey very low.



Competition for resources helps curb population size.• The resources that determine the carrying

capacity are limited. • Therefore, organisms must compete for

access to resources, and the competition among individuals limits population size in a density-dependent manner.

• There are two forms of competition: interspecific competition among individuals of different species, and intraspecific competition among individuals of the same species.



Competition may be indirect or direct.• Some organisms, including most plants and

many insects, engage in scramble competition, a kind of free-for-all in which individuals independently seek resources without interacting directly.

• For example, after gypsy moth caterpillars hatch from large masses of eggs laid on tree trucks, armies of caterpillars crawl up to the treetops to feed on leaves; many caterpillars die off before they can metamorphose into moths.



Scramble competition

Fig. 27-8



Competition may be indirect or direct (continued).• Many animals have evolved contest

competition, in which individuals interact to contest access to important resources.

• Only the best competitors are able to defend adequate territories; poor competitors fail to secure territories and are unlikely to reproduce or survive.



Different age groups may experience different death rates.• The effects of population-limiting factors do

not necessarily fall equally on all members of a population.

• For example, the death rate in a population differs among individuals of different ages; survivorship varies with age.

• A survivorship curve shows the likelihood that an individual will survive to a given age.



Survivorship in populations follows three basic patterns.• Convex late-loss survivorship curves reflect

populations in which the death rate is low for juveniles and in which most individuals survive to old age.• These curves are characteristic of humans

and other large, long-lived animals, such as elephants and mountain sheep.

• These species produce relatively few offspring that receive a great deal of parental care.



Survivorship in populations follows three basic patterns (continued).• Early-loss survivorship produces a concave

curve and is characteristic of organisms that produce large numbers of offspring that receive little or no parental care.• The death rate of juveniles is very high, but

individuals than manage to reach adulthood have a good chance of surviving to old age.

• Early-loss survivorship curves are common among invertebrates, plants, and fish.



Survivorship in populations follows three basic patterns (continued).• Populations with straight-line, constant-loss

survivorship curves have a fairly constant death rate that does not vary much with age.• This pattern has been found in some bird

and lizard species, and in populations of some asexually-reproducing invertebrate species.



Survivorship curves

Fig. 27-9

late loss(human)

constant loss(American robin)

early loss(dandelion)

age(in percentage of maximum life span)

0

10

100

1,000

nu

mb

er

of

su

rviv

ors


Technical and cultural advances Agricultural advances Industrial andmedicaladvances

bubonic plague

8000B.C.

7000B.C.

6000B.C.

5000B.C.

4000B.C.

3000B.C.

2000B.C.

1000B.C.

B.C./A.D.1000A.D.

2000A.D.

12,000B.C.

11,000B.C.

10,000B.C.

9000B.C.

1

2

3

4

5

1830

1975

1960

1930

1987

1999

2008

6

7

0

bill

ion

s of

peop

le

27.4 How Is The Human Population Changing? The graph of human population growth in

Figure 27-10 and the exponential growth curves in Figure 27-2a are similar—each are J shaped, characteristic of explosive growth.

Fig. 27-10


27.4 How Is The Human Population Changing?

PLAYPLAY Animation—Human Population Growth and Regulation


27.4 How Is The Human Population Changing? Survivorship in populations follows three

basic patterns (continued).• According to the U.S. Census Bureau, world

population currently grows at about 1.2%, or almost 78 million people yearly.

• Why hasn’t environmental resistance put an end to our tremendous population growth?


27.4 How Is The Human Population Changing? Technological advances have increased

Earth’s carrying capacity for humans.• Humans have responded to environmental

resistance by devising ways to overcome it.• Human population growth has increased by a

series of cultural “revolutions” in which the products of human culture conquered various aspects of environmental resistance and increased Earth’s carrying capacity for people.


27.4 How Is The Human Population Changing? Early tools allowed human populations to

spread and prosper.• Prehistoric people produced a tool-use

revolution when they discovered fire, invented tools and weapons, built shelters, and designed protective clothing.

• Tools and weapons allowed more effective hunting, and thereby increased the food supply.

• These developments set the stage for future population growth.


27.4 How Is The Human Population Changing? Agriculture fostered population growth.

• By 8,000 B.C., cultivation of domesticated crops and animals had replaced hunting and gathering as humans’ primary means of acquiring food.

• This agricultural revolution provided people with a larger, more dependable food supply, and further increased Earth’s carrying capacity for humans.

• Increased food resulted in a long life span and more childbearing years, and the human population began to climb steadily.


27.4 How Is The Human Population Changing? Explosive growth began with the Industrial

Revolution.• Human population growth continued slowly for

thousands of years until the industrial-medical revolution began in England in the mid-eighteenth century, spreading through Europe and North America in the nineteenth century.

• This revolution marked the beginning of a period of extremely rapid population growth, as medical advances dramatically decreased the death rate.


27.4 How Is The Human Population Changing? Population growth continues today but is unevenly

distributed.• In Western Europe, the population explosion fostered

by the declining death rates of the industrial-medical revolution was eventually followed by a decline in birth rates.

• Birth rate decline in affluent societies has been attributed to better education, increased availability of contraceptives, a shift to a primary urban lifestyle, and more career options for women.

• In most developed countries, population sizes have stabilized or even declined.


27.4 How Is The Human Population Changing? Population growth continues today but is

unevenly distributed (continued).• In developing countries—such as those in

Central and South America, Asia, and Africa—the death rate has gone down and the life span has increased.

• In agricultural societies, children are an important source of labor and provide support for elderly parents.


developing countries

developed countries

2006: 6.5 billion

year

1950 1970 1990 2010 2030 2050

109876543210

po

pu

lati

on

(b

illi

on

s)

27.4 How Is The Human Population Changing? By 2050, 8 billion people will be living in the

developing nations.

Fig. 27-11


27.4 How Is The Human Population Changing? The age structure of a population predicts

its future growth.• The age structure of a population is

determined by the numbers of males and females in each age group, and can be shown graphically in an age-structure diagram.

• The shape of the diagram shows whether the population is expanding, stable, or shrinking.


27.4 How Is The Human Population Changing? The age structure of a population predicts

its future growth (continued).• The shape of an age-structure diagram is

determined by the relative numbers of reproductive age adults and their children.

• If the adults of reproductive age are producing just enough children to replace themselves, the population is said to have replacement level fertility (RLF).


Population age structure for Sweden

femalemale

Sweden 2007

(a)

percent

80+75–7970–7465–6960–6455–5950–5445–4940–4435–3930–3425–2920–2415–1910–145–90–4

12 10 8 6 4 2 0 2 4 6 8 10 12

27.4 How Is The Human Population Changing? The age-structure diagram of such a

population has relatively straight sides.

Fig. 27-12a


27.4 How Is The Human Population Changing? If children in the various age classes exceed

the number of reproductive-age adults, the population is exceeding RLF and is expanding.


femalemale

Mexico 2007

Population age structure for Mexico(b)

percent

80+75–7970–7465–6960–6455–5950–5445–4940–4435–3930–3425–2920–2415–1910–145–90–4

12 10 8 6 4 2 0 2 4 6 8 10 12

27.4 How Is The Human Population Changing? The age-structure diagrams of such

expanding populations have a pyramidal shape.

Fig. 27-12b


27.4 How Is The Human Population Changing? In shrinking populations, there are fewer

children than reproducing adults, and the age-structure diagrams narrows at the base.

Fig. 27-12c

femalemale

Italy 2007

Population age structure for Italy(c)percent

80+75–7970–7465–6960–6455–5950–5445–4940–4435–3930–3425–2920–2415–1910–145–90–4

12 10 8 6 4 2 0 2 4 6 8 10 12


27.4 How Is The Human Population Changing? Almost all future population growth will be in

developing countries.• Figure 27-13 shows the average age

structures of the populations of developed and developing countries.

• The outermost boundaries represent the projected population structure for the year 2050, and the innermost green areas give actual values for 2006.


27.4 How Is The Human Population Changing? In 2006, developing countries had an

average annual natural increase of 1.45% and a doubling time of 50 years.

Fig. 27-13b

Developing countries

20502006

75 and older

60 - 74

45 - 59

30 - 44

15 - 29

0 - 14

male female

(b)

60

45

75

30

15

0

millions of people0 200 400 600 800 1,000100 300 500 700 9001,000 800 600 400 200900 700 500 300 100

age


27.4 How Is The Human Population Changing? In developed countries, the numbers were

0.07% and 985 years.

Fig. 27-13a

75 and older

60 - 74

45 - 59

30 - 44

15 - 29

0 - 14

postreproductive (45–79 yr)

prereproductive (0–14 yr)

femalemale

reproductive (15–44 yr)

20502006

Developed countries(a)

60

45

75

30

15

0

millions of people0 200 400 600 800 1,000100 300 500 700 9001,000 800 600 400 200900 700 500 300 100

age


27.4 How Is The Human Population Changing? Almost all future population growth will be in

developing countries.• More than 97% of the growth of the human

population occurs in developing countries. • The growth would come as the children of the

large families of the recent past entered their reproductive years and began having children, even if each couple only had one child.


27.4 How Is The Human Population Changing? Population growth by world regions

Fig. 27-14

Europe –0.2%

Latin America/Caribbean 1.7%

Asia (excluding China and Japan) 1.6%

Developing countries 1.6%

Africa 2.4%

North America 0.5%

World average 1.3%

Developedcountries 0.1%

China 0.6%

annual natural increase2.52.01.51.00.50–0.5

wo

rld

reg

ion

s


27.4 How Is The Human Population Changing? Fertility in Europe is below replacement level.

• In Europe, the fertility rate is well below RLF, and several European countries have declines in population.

• A low fertility rate also occurs in Japan, and the government there provides a variety of subsidies and incentives to encourage people to have larger families.

• The situations in Europe and Japan reveal one of the key barriers to controlling the world’s population: Current economic structures are based on growing populations.


27.4 How Is The Human Population Changing? The U.S. population is growing rapidly.

• The United States is the fastest growing industrialized country in the world.

• The U.S. has a current population of 300 million, a natural increase of 0.6% annually, and the population grows at more than six times the average rate of developed countries.

• Even with RLF, immigration will guarantee continued U.S. population growth.


27.4 How Is The Human Population Changing? U.S. population

growth

Fig. 27-15


27.4 How Is The Human Population Changing? The U.S. population is growing rapidly (continued).• The average person in the U.S. uses five

times as much energy as the average non-U.S. person; therefore, with less than 5% of the world’s population, the United States accounts for 25% of world energy use.

• The ecological impact of the 2.7 million people added annually to the U.S. population is far greater than the total ecological impact of the 18 million people added annually to India’s population.

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Copyright © 2009 Pearson Education, Inc.. Lectures by Gregory Ahearn University of North Florida Chapter 27 Population Growth