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Chapter 52. Population Ecology. Overview: Earth’s Fluctuating Populations To understand human population growth We must consider the general principles of population ecology. Population ecology is the study of populations in relation to environment

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Chapter 52

Chapter 52

Population Ecology




Figure 52.1



Density and dispersion
Density and Dispersion population density, dispersion, and demography

  • Density

    • Is the number of individuals per unit area or volume

  • Dispersion

    • Is the pattern of spacing among individuals within the boundaries of the population


Density a dynamic perspective
Density: A Dynamic Perspective population density, dispersion, and demography

  • Determining the density of natural populations

    • Is possible, but difficult to accomplish

  • In most cases

    • It is impractical or impossible to count all individuals in a population


Births and immigration add individuals to a population. population density, dispersion, and demography

Births

Immigration

PopuIationsize

Emigration

Deaths

Deaths and emigration remove individuals from a population.

  • Density is the result of a dynamic interplay

    • Between processes that add individuals to a population and those that remove individuals from it

Figure 52.2


Patterns of dispersion
Patterns of Dispersion population density, dispersion, and demography

  • Environmental and social factors

    • Influence the spacing of individuals in a population


(a) Clumped. population density, dispersion, and demographyFor many animals, such as these wolves, living in groups increases the effectiveness of hunting, spreads the work of protecting and caring for young, and helps exclude other individuals from their territory.

Figure 52.3a

  • A clumped dispersion

    • Is one in which individuals aggregate in patches

    • May be influenced by resource availability and behavior


(b) Uniform. population density, dispersion, and demographyBirds nesting on small islands, such as these king penguins on South Georgia Island in the South Atlantic Ocean, often exhibit uniform spacing, maintained by aggressive interactions between neighbors.

Figure 52.3b

  • A uniform dispersion

    • Is one in which individuals are evenly distributed

    • May be influenced by social interactions such as territoriality


  • A random dispersion population density, dispersion, and demography

    • Is one in which the position of each individual is independent of other individuals

(c) Random. Dandelions grow from windblown seeds that land at random and later germinate.

Figure 52.3c


Demography
Demography population density, dispersion, and demography

  • Demography is the study of the vital statistics of a population

    • And how they change over time

  • Death rates and birth rates

    • Are of particular interest to demographers


Life tables
Life Tables population density, dispersion, and demography

  • A life table

    • Is an age-specific summary of the survival pattern of a population

    • Is best constructed by following the fate of a cohort


Table 52.1


Survivorship curves
Survivorship Curves population density, dispersion, and demography

  • A survivorship curve

    • Is a graphic way of representing the data in a life table


1000 population density, dispersion, and demography

100

Number of survivors (log scale)

Females

10

Males

1

2

8

10

4

6

0

Age (years)

Figure 52.4

  • The survivorship curve for Belding’s ground squirrels

    • Shows that the death rate is relatively constant


1,000 population density, dispersion, and demography

I

100

II

Number of survivors (log scale)

10

III

1

100

50

0

Percentage of maximum life span

  • Survivorship curves can be classified into three general types

    • Type I, Type II, and Type III

Figure 52.5


Reproductive rates
Reproductive Rates population density, dispersion, and demography

  • A reproductive table, or fertility schedule

    • Is an age-specific summary of the reproductive rates in a population


Table 52.2 population density, dispersion, and demography

  • A reproductive table

    • Describes the reproductive patterns of a population



Life history diversity
Life History Diversity selection

  • Life histories are very diverse


Figure 52.6 selection

  • Species that exhibit semelparity, or “big-bang” reproduction

    • Reproduce a single time and die



Trade offs and life histories

Researchers in the Netherlands studied the effects of parental caregiving in European kestrels over 5 years. The researchers transferred chicks among nests to produce reduced broods (three or four chicks), normal broods (five or six), and enlarged broods (seven or eight). They then measured the percentage of male and female parent birds that survived the following winter. (Both males and females provide care for chicks.)

EXPERIMENT

100

Male

Female

80

60

Parents surviving the following winter (%)

40

20

The lower survival rates of kestrels with larger broods indicate that caring for more offspring negatively affects survival of the parents.

CONCLUSION

0

Reduced brood size

Normal brood size

Enlarged brood size

“Trade-offs” and Life Histories

  • Organisms have finite resources

  • Which may lead to trade-offs between survival and reproduction

RESULTS

Figure 52.7


(a) parental caregiving in European kestrels over 5 years. The researchers transferred chicks among nests to produce reduced broods (three or four chicks), normal broods (five or six), and enlarged broods (seven or eight). They then measured the percentage of male and female parent birds that survived the following winter. (Both males and females provide care for chicks.)Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds, ensuring that at least somewill grow into plants and eventually produce seeds themselves.

Figure 52.8a

  • Some plants produce a large number of small seeds

    • Ensuring that at least some of them will grow and eventually reproduce


(b) parental caregiving in European kestrels over 5 years. The researchers transferred chicks among nests to produce reduced broods (three or four chicks), normal broods (five or six), and enlarged broods (seven or eight). They then measured the percentage of male and female parent birds that survived the following winter. (Both males and females provide care for chicks.)Some plants, such as this coconut palm, produce a moderate number of very large seeds. The large endosperm provides nutrients for the embryo, an adaptation that helps ensure the success of a relatively large fraction of offspring.

Figure 52.8b

  • Other types of plants produce a moderate number of large seeds

    • That provide a large store of energy that will help seedlings become established


  • Parental care of smaller broods parental caregiving in European kestrels over 5 years. The researchers transferred chicks among nests to produce reduced broods (three or four chicks), normal broods (five or six), and enlarged broods (seven or eight). They then measured the percentage of male and female parent birds that survived the following winter. (Both males and females provide care for chicks.)

    • May also facilitate survival of offspring


  • Concept 52.3: The exponential model describes population growth in an idealized, unlimited environment

  • It is useful to study population growth in an idealized situation

    • In order to understand the capacity of species for increase and the conditions that may facilitate this type of growth


Per capita rate of increase
Per Capita Rate of Increase growth in an idealized, unlimited environment

  • If immigration and emigration are ignored

    • A population’s growth rate (per capita increase) equals birth rate minus death rate


dN growth in an idealized, unlimited environment

rN

dt

  • Zero population growth

    • Occurs when the birth rate equals the death rate

  • The population growth equation can be expressed as


Exponential growth
Exponential Growth growth in an idealized, unlimited environment

  • Exponential population growth

    • Is population increase under idealized conditions

  • Under these conditions

    • The rate of reproduction is at its maximum, called the intrinsic rate of increase


dN growth in an idealized, unlimited environment

rmaxN

dt

  • The equation of exponential population growth is


2,000 growth in an idealized, unlimited environment

dN

1.0N

dt

1,500

dN

0.5N

dt

Population size (N)

1,000

500

0

0

10

15

5

Number of generations

Figure 52.9

  • Exponential population growth

    • Results in a J-shaped curve


8,000 growth in an idealized, unlimited environment

6,000

Elephant population

4,000

2,000

0

1900

1920

1940

1960

1980

Year

Figure 52.10

  • The J-shaped curve of exponential growth

    • Is characteristic of some populations that are rebounding



  • Carrying capacity ( of carrying capacityK)

    • Is the maximum population size the environment can support


The logistic growth model
The Logistic Growth Model of carrying capacity

  • In the logistic population growth model

    • The per capita rate of increase declines as carrying capacity is reached


Maximum of carrying capacity

Per capita rate of increase (r)

Positive

N  K

0

Negative

Population size (N)

Figure 52.11

  • We construct the logistic model by starting with the exponential model

    • And adding an expression that reduces the per capita rate of increase as N increases


( of carrying capacityK  N)

dN

rmax

N

dt

K

  • The logistic growth equation

    • Includes K, the carrying capacity


Table 52.3 of carrying capacity

  • A hypothetical example of logistic growth


2,000 of carrying capacity

dN

1.0N

Exponential growth

dt

1,500

K  1,500

Logistic growth

Population size (N)

1,000

dN

1,500  N

1.0N

dt

1,500

500

0

0

5

10

15

Number of generations

  • The logistic model of population growth

    • Produces a sigmoid (S-shaped) curve

Figure 52.12


The logistic model and real populations

1,000 of carrying capacity

800

600

Number of Paramecium/ml

400

200

0

0

5

15

10

Time (days)

(a) A Paramecium population in the lab. The growth of Paramecium aurelia in small cultures (black dots) closely approximates logistic growth (red curve) if the experimenter maintains a constant environment.

Figure 52.13a

The Logistic Model and Real Populations

  • The growth of laboratory populations of paramecia

    • Fits an S-shaped curve


180 of carrying capacity

150

120

90

Number of Daphnia/50 ml

60

30

0

160

0

40

60

100

120

140

20

80

Time (days)

(b) A Daphnia population in the lab. The growth of a population of Daphnia in a small laboratory culture (black dots) does not correspond well to the logistic model (red curve). This population overshoots the carrying capacity of its artificial environment and then settles down to an approximately stable population size.

Figure 52.13b

  • Some populations overshoot K

    • Before settling down to a relatively stable density


80 of carrying capacity

60

40

Number offemales

20

0

1995

2000

1980

1985

1990

1975

Time (years)

(c) A song sparrowpopulation in its natural habitat. The population of female song sparrows nesting on Mandarte Island, British Columbia, is periodically reduced by severe winter weather, and population growth is not well described by the logistic model.

Figure 52.13c

  • Some populations

    • Fluctuate greatly around K



The logistic model and life histories
The Logistic Model and Life Histories of carrying capacity

  • Life history traits favored by natural selection

    • May vary with population density and environmental conditions


  • K of carrying capacity-selection, or density-dependent selection

    • Selects for life history traits that are sensitive to population density

  • r-selection, or density-independent selection

    • Selects for life history traits that maximize reproduction


  • The concepts of of carrying capacityK-selection and r-selection

    • Are somewhat controversial and have been criticized by ecologists as oversimplifications




Population change and population density
Population Change and Population Density interaction of biotic and abiotic influences

  • In density-independent populations

    • Birth rate and death rate do not change with population density

  • In density-dependent populations

    • Birth rates fall and death rates rise with population density


Density-dependent death rate interaction of biotic and abiotic influences

Density-dependent birth rate

Density-dependent birth rate

Density-independent death rate

Density-independent birth rate

Density-dependent death rate

Birth or death rate per capita

Equilibrium density

Equilibrium density

Equilibrium density

Population density

Population density

Population density

(a) Both birth rate and death rate change with population density.

(c) Death rate changes with populationdensity while birht rate is constant.

(b) Birth rate changes with populationdensity while death rate is constant.

Figure 52.14a–c

  • Determining equilibrium for population density


Density dependent population regulation
Density-Dependent Population Regulation interaction of biotic and abiotic influences

  • Density-dependent birth and death rates

    • Are an example of negative feedback that regulates population growth

    • Are affected by many different mechanisms


Competition for resources

4.0 interaction of biotic and abiotic influences

10,000

3.8

3.6

Average number of seeds per reproducing individual (log scale)

1,000

3.4

Average clutch size

3.2

3.0

100

2.8

0

0

40

50

60

80

20

30

10

70

0

10

100

Seeds planted per m2

Density of females

(a) Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases.

(b) Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply.

Competition for Resources

  • In crowded populations, increasing population density

    • Intensifies intraspecific competition for resources

Figure 52.15a,b


Territoriality
Territoriality interaction of biotic and abiotic influences

  • In many vertebrates and some invertebrates

    • Territoriality may limit density


Figure 52.16 interaction of biotic and abiotic influences

  • Cheetahs are highly territorial

    • Using chemical communication to warn other cheetahs of their boundaries


Figure 52.17 interaction of biotic and abiotic influences

  • Oceanic birds

    • Exhibit territoriality in nesting behavior


Health
Health interaction of biotic and abiotic influences

  • Population density

    • Can influence the health and survival of organisms

  • In dense populations

    • Pathogens can spread more rapidly


Predation
Predation interaction of biotic and abiotic influences

  • As a prey population builds up

    • Predators may feed preferentially on that species


Toxic wastes
Toxic Wastes interaction of biotic and abiotic influences

  • The accumulation of toxic wastes

    • Can contribute to density-dependent regulation of population size


Intrinsic factors
Intrinsic Factors interaction of biotic and abiotic influences

  • For some populations

    • Intrinsic (physiological) factors appear to regulate population size


Population dynamics
Population Dynamics interaction of biotic and abiotic influences

  • The study of population dynamics

    • Focuses on the complex interactions between biotic and abiotic factors that cause variation in population size


Stability and fluctuation

FIELD STUDY interaction of biotic and abiotic influences

Researchers regularly surveyed the population of moose on Isle Royale, Michigan, from 1960 to 2003. During that time, the lake never froze over, and so the moose population was isolated from the effects of immigration and emigration.

RESULTS

Over 43 years, this population experiencedtwo significant increases and collapses, as well as several less severe fluctuations in size.

2,500

Steady decline probably caused largely by wolf predation

2,000

1,500

Moose population size

1,000

Dramatic collapse caused by severe winter weather and food shortage, leading to starvation of more than 75% of the population

500

0

2000

1970

1980

1990

1960

Year

CONCLUSION

The pattern of population dynamics observedin this isolated population indicates that various biotic and abiotic factors can result in dramatic fluctuations over time in a moose population.

Figure 52.18

Stability and Fluctuation

  • Long-term population studies

    • Have challenged the hypothesis that populations of large mammals are relatively stable over time


730,000 interaction of biotic and abiotic influences

100,000

Commercial catch (kg) of male crabs (log scale)

10,000

1990

1950

1980

1960

1970

Year

Figure 52.19

  • Extreme fluctuations in population size

    • Are typically more common in invertebrates than in large mammals


Metapopulations and immigration
Metapopulations and Immigration interaction of biotic and abiotic influences

  • Metapopulations

    • Are groups of populations linked by immigration and emigration


60 interaction of biotic and abiotic influences

50

40

Mandarte island

Number of breeding females

30

20

Small islands

10

0

1991

1988

1989

1990

Year

  • High levels of immigration combined with higher survival

    • Can result in greater stability in populations

Figure 52.20


Population cycles

Snowshoe hare interaction of biotic and abiotic influences

160

120

Lynx

9

Lynx population size (thousands)

Hare population size (thousands)

80

6

40

3

0

0

1850

1875

1900

1925

Year

Figure 52.21

Population Cycles

  • Many populations

    • Undergo regular boom-and-bust cycles


  • Boom-and-bust cycles interaction of biotic and abiotic influences

    • Are influenced by complex interactions between biotic and abiotic factors



The global human population

6 centuries of exponential increase

5

4

Human population (billions)

3

2

The Plague

1

0

8000 B.C.

4000 B.C.

3000 B.C.

2000 B.C.

1000 B.C.

1000 A.D.

2000 A.D.

0

Figure 52.22

The Global Human Population

  • The human population

    • Increased relatively slowly until about 1650 and then began to grow exponentially


2.2 centuries of exponential increase

2

1.8

1.6

2003

1.4

Percent increase

1.2

1

0.8

0.6

0.4

0.2

0

1950

2025

2050

1975

2000

Year

Figure 52.23

  • Though the global population is still growing

    • The rate of growth began to slow approximately 40 years ago


Regional patterns of population change
Regional Patterns of Population Change centuries of exponential increase

  • To maintain population stability

    • A regional human population can exist in one of two configurations



50 rates

40

30

Birth or death rate per 1,000 people

20

10

Sweden

Mexico

Birth rate

Birth rate

Death rate

Death rate

0

1750

2000

1900

1950

2050

1800

1850

Year

Figure 52.24

  • The demographic transition

    • Is the move from the first toward the second state



Age structure
Age Structure rates

  • One important demographic factor in present and future growth trends

    • Is a country’s age structure, the relative number of individuals at each age


Rapid growth ratesAfghanistan

Decrease Italy

Slow growth United States

Male

Female

Female

Male

Male

Age

Age

Female

85

85

80–84

80–84

75–79

75–79

70–74

70–74

65–69

65–69

60–64

60–64

55–59

55–59

50–54

50–54

45–49

45–49

40–44

40–44

35–39

35–39

30–34

30–34

25–29

25–29

20–24

20–24

15–19

15–19

10–14

10–14

5–9

5–9

0–4

0–4

8

8

8

6

6

6

4

4

4

2

2

2

0

0

0

2

2

2

4

4

4

6

6

6

8

8

8

Percent of population

Percent of population

Percent of population

Figure 52.25

  • Age structure

    • Is commonly represented in pyramids


  • Age structure diagrams rates

    • Can predict a population’s growth trends

    • Can illuminate social conditions and help us plan for the future


Infant mortality and life expectancy

60 rates

80

50

60

40

Infant mortality (deaths per 1,000 births)

Life expectancy (years)

40

30

20

20

10

0

0

Developed countries

Developed countries

Developing countries

Developing countries

Figure 52.26

Infant Mortality and Life Expectancy

  • Infant mortality and life expectancy at birth

    • Vary widely among developed and developing countries but do not capture the wide range of the human condition


Global carrying capacity
Global Carrying Capacity rates

  • Just how many humans can the biosphere support?


Estimates of carrying capacity
Estimates of Carrying Capacity rates

  • The carrying capacity of Earth for humans is uncertain


Ecological footprint
Ecological Footprint rates

  • The ecological footprint concept

    • Summarizes the aggregate land and water area needed to sustain the people of a nation

    • Is one measure of how close we are to the carrying capacity of Earth


16 rates

14

12

New Zealand

10

USA

Germany

Ecological footprint (ha per person)

Australia

8

Netherlands

Japan

Canada

Norway

6

Sweden

UK

4

Spain

World

2

China

India

0

16

4

2

6

8

10

12

14

0

Available ecological capacity (ha per person)

Figure 52.27

  • Ecological footprints for 13 countries

    • Show that the countries vary greatly in their footprint size and their available ecological capacity



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