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Overview: Earth’s Fluctuating Populations. To understand human population growth, we must consider general principles of population ecology

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overview earth s fluctuating populations
Overview: Earth’s Fluctuating Populations
  • To understand human population growth, we must consider general principles of population ecology
  • Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population size
  • The fur seal population of St. Paul Island, off the coast of Alaska, has experienced dramatic fluctuations in size
dynamic biological processes influence population density dispersion and demography
Dynamic biological processes influence population density, dispersion, and demography
  • A population is a group of individuals of a single species living in the same general area
  • 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
Density
  • Determining the density of natural populations is difficult
  • In most cases, it is impractical or impossible to count all individuals in a population
  • Density is the result of an interplay between processes that add individuals to a population and those that remove individuals
slide5
Immigration

Births

Population

size

Emigration

Deaths

patterns of dispersion
Patterns of Dispersion
  • Environmental and social factors influence spacing of individuals in a population
  • In a clumped dispersion, individuals aggregate in patches
  • A clumped dispersion may be influenced by resource availability and behavior
slide7
Clumped. For 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.
slide8
Uniform. Birds 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.
demography
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
  • A life table is an age-specific summary of the survival pattern of a population
  • It is best made by following the fate of a cohort
  • The life table of Belding’s ground squirrels reveals many things about this population
survivorship curves
Survivorship Curves
  • A survivorship curve is a graphic way of representing the data in a life table
  • The survivorship curve for Belding’s ground squirrels shows a relatively constant death rate
  • Survivorship curves can be classified into three general types: Type I, Type II, and Type III
slide13
1,000

100

Number of survivors (log scale)

Females

10

Males

1

2

0

10

6

8

4

Age (years)

slide14
1,000

I

100

II

Number of survivors (log scale)

10

III

1

100

50

0

Percentage of maximum life span

slide15
A reproductive table, or fertility schedule, is an age-specific summary of the reproductive rates in a population. It describes reproductive patterns of a population
slide16
Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds, ensuring that at least some will grow into plants and eventually produce seeds themselves.
slide17
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.
the exponential model describes population growth in an idealized unlimited environment
The exponential model describes population growth in an idealized, unlimited environment
  • It is useful to study population growth in an idealized situation
  • Idealized situations help us understand the capacity of species to increase and the conditions that may facilitate this growth
  • In animals, parental care of smaller broods may facilitate survival of offspring
slide19
dN

rN

dt

  • If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate
  • Zero population growth occurs when the birth rate equals the death rate
  • Most ecologists use differential calculus to express population growth as growth rate at a particular instant in time:
exponential growth
dN

rmaxN

dt

Exponential Growth
  • 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
  • Exponential population growth results in a J-shaped curve
slide21
2,000

dN

= 1.0N

dt

1,500

dN

= 0.5N

dt

Population size (N)

1,000

500

0

15

10

5

0

Number of generations

the logistic growth model includes the concept of carrying capacity
The logistic growth model includes the concept of carrying capacity
  • Exponential growth cannot be sustained for long in any population
  • A more realistic population model limits growth by incorporating carrying capacity
  • Carrying capacity (K) is the maximum population size the environment can support
  • In the logistic population growth model, the per capita rate of increase declines as carrying capacity is reached
  • We construct the logistic model by starting with the exponential model and adding an expression that reduces per capita rate of increase as N increases
slide23
Maximum

Per capita rate of increase (r)

Positive

N = K

0

Negative

Population size (N)

slide24
(K  N)

dN

rmax

N

dt

K

  • The logistic growth equation includes K, the carrying capacity
  • The logistic model of population growth produces a sigmoid (S-shaped) curve
slide26
2,000

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

15

10

5

Number of generations

the logistic model and real populations
The Logistic Model and Real Populations
  • The growth of laboratory populations of paramecia fits an S-shaped curve
  • Some populations overshoot K before settling down to a relatively stable density
  • Some populations fluctuate greatly around K
slide28
1,000

800

600

Number of Paramecium/mL

400

200

0

5

10

15

0

Time (days)

A Paramecium population in the lab

slide29
180

150

120

Number of Daphnia/50 mL

90

60

30

0

20

40

80

60

100

120

140

0

160

Time (days)

A Daphnia population in the lab

slide30
80

60

Number of females

40

20

0

1980

1985

1995

1990

2000

1975

Time (years)

A song sparrow population in its natural habitat

the logistic model and life histories
The Logistic Model and Life Histories
  • Life history traits favored by natural selection may vary with population density and environmental conditions
  • K-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 K-selection and r-selection are somewhat controversial and have been criticized by ecologists as oversimplifications
populations are regulated by a complex interaction of biotic and abiotic influences
Populations are regulated by a complex interaction of biotic and abiotic influences
  • There are two general questions about regulation of population growth:
    • What environmental factors stop a population from growing?
    • Why do some populations show radical fluctuations in size over time, while others remain stable?
population change and population density
Population Change and Population Density
  • 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
slide34
Density-dependent

birth rate

Density-dependent

birth rate

Density-

independent

death rate

Density-

dependent

death rate

Birth or death rate

per capita

Equilibrium

density

Equilibrium

density

Population density

Population density

Density-dependent

death rate

Density-

independent

birth rate

Equilibrium

density

Population density

density dependent population regulation
Density-Dependent Population Regulation
  • Density-dependent birth and death rates are an example of negative feedback that regulates population growth
  • They are affected by many factors, such as competition for resources, territoriality, health, predation, toxic wastes, and intrinsic factors
  • In crowded populations, increasing population density intensifies intraspecific competition for resources
slide36
4.0

10,000

3.8

3.6

Average number of seeds

per reproducing individual

(log scale)

1,000

Average clutch size

3.4

3.2

3.0

100

2.8

0

10

60

20

30

70

40

100

10

80

1

50

Plants per m2 (log scale)

Females per unit area

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

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

slide37
Cheetahs are highly territorial, using chemical communication to warn other cheetahs of their boundaries
health predation and toxic wastes
Health, Predation and Toxic Wastes
  • Population density can influence the health and survival of organisms
  • In dense populations, pathogens can spread more rapidly
  • As a prey population builds up, predators may feed preferentially on that species
  • Accumulation of toxic wastes can contribute to density-dependent regulation of population size
population dynamics
Population Dynamics
  • The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size
stability and fluctuation
Stability and Fluctuation
  • Long-term population studies have challenged the hypothesis that populations of large mammals are relatively stable over time
  • Extreme fluctuations in population size are typically more common in invertebrates than in large mammals
slide42
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

1960

1980

1990

1970

Year

slide43
730,000

100,000

Commercial catch (kg) of

male crabs (log scale)

10,000

1960

1980

1990

1950

1970

Year

metapopulations and immigration
Metapopulations and Immigration
  • Metapopulations are groups of populations linked by immigration and emigration
  • High levels of immigration combined with higher survival can result in greater stability in populations
  • Many populations undergo boom-and-bust cycles
  • Boom-and-bust cycles are influenced by complex interactions between biotic and abiotic factors
slide45
60

50

40

Mandarte

Island

Number of breeding females

30

20

Small

islands

10

0

1988

1990

1991

1989

Year

slide46
Snowshoe hare

160

120

9

Lynx

Lynx population size

(thousands)

Hare population size

(thousands)

80

6

40

3

0

0

1925

1850

1875

1900

Year

slide47
6

5

4

Human population (billions)

3

2

The Plague

1

0

2000

B.C.

1000

B.C.

4000

B.C.

3000

B.C.

0

1000

A.D.

8000

B.C.

2000

A.D.

slide48
2.2

2

1.8

1.6

2003

1.4

Annual percent increase

1.2

1

0.8

0.6

0.4

0.2

0

2000

2050

1975

1950

2025

Year

regional patterns of population change
Regional Patterns of Population Change
  • No population can grow indefinitely, and humans are no exception
  • To maintain population stability, a regional human population can exist in one of two configurations:
    • Zero population growth = High birth rate – High death rate
    • Zero population growth =Low birth rate – Low death rate
  • The demographic transition is the move from the first state toward the second state
  • The demographic transition is associated with various factors in developed and developing countries
slide50
50

40

30

Birth or death rate per 1,000 people

20

10

Sweden

Mexico

Birth rate

Birth rate

Death rate

Death rate

0

2050

1950

1850

1750

1900

2000

1800

Year

age structure
Age Structure
  • One important demographic factor in present and future growth trends is a country’s age structure
  • Age structure is the relative number of individuals at each age
  • It is commonly represented in pyramids
  • Age structure diagrams can predict a population’s growth trends
  • They can illuminate social conditions and help us plan for the future
slide52
Rapid growth

Afghanistan

Decrease

Italy

Slow growth

United States

Male

Female

Male

Female

Age

Male

Female

Age

85+

80–84

75–79

70–74

65–69

60–64

55–59

50–54

85+

80–84

75–79

70–74

65–69

60–64

55–59

50–54

45–49

40–44

35–39

30–34

25–29

20–24

15–19

10–14

5–9

0–4

45–49

40–44

35–39

30–34

25–29

20–24

15–19

10–14

5–9

0–4

8

0

2

4

8

6

6

8

0

2

4

8

4

2

6

6

8

0

2

4

4

2

6

6

8

4

2

Percent of population

Percent of population

Percent of population

infant mortality and life expectancy
Infant Mortality and Life Expectancy
  • Infant mortality and life expectancy at birth vary greatly among developed and developing countries but do not capture the wide range of the human condition
slide54
60

80

50

60

40

Life expectancy (years)

Infant mortality (deaths per 1,000 births)

30

40

20

20

10

0

0

Developed

countries

Developing

countries

Developed

countries

Developing

countries

ecological footprint
Ecological Footprint
  • How many humans can the biosphere support?
  • The carrying capacity of Earth for humans is uncertain
  • The ecological footprint concept summarizes the aggregate land and water area needed to sustain the people of a nation
  • It is one measure of how close we are to the carrying capacity of Earth
  • Countries vary greatly in footprint size and available ecological capacity
animations and videos
Animations and Videos
  • Chapter Quiz Questions – 1
  • Chapter Quiz Questions – 2
  • Population Cycles
  • Restrained Population Growth
  • Unrestrained Population Growth
  • Human Population Growth
  • Population Cycles
  • Predator Prey Interactions
animations and videos1
Animations and Videos
  • Predator-prey Relationship
  • Bozeman - Logistic Growth
  • Bozeman - Exponential Growth
  • Bozeman - Population Variation
  • r and K Strategies
  • Bozeman - r and K selection
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