1. Chapter 52
Population Ecology

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

3. 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

5. Concept 52.1: 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

6. Density and Dispersion
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

7. Density: A Dynamic Perspective
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

8. LE 52-2
Births
Immigration
Population
size
Emigration
Deaths

9. Patterns of Dispersion
Environmental and social factors influence spacing of individuals in a population

10. Video: Flapping Geese (clumped)
In a clumped dispersion, individuals aggregate in patches
A clumped dispersion may be influenced by resource availability and behavior
Video: Flapping Geese (clumped)

11. LE 52-3a
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.

12. Video: Albatross Courtship (uniform)
A uniform dispersion is one in which individuals are evenly distributed
It may be influenced by social interactions such as territoriality
Video: Albatross Courtship (uniform)

13. LE 52-3b
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.

14. Video: Prokaryotic Flagella (Salmonella typhimurium) (random)
In a random dispersion, the position of each individual is independent of other individuals
Video: Prokaryotic Flagella (Salmonella typhimurium) (random)

15. LE 52-3c
Random. Dandelions grow from windblown seeds that land at random and later germinate.

16. 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

17. Life Tables
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

19. 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

20. Number of survivors (log scale)
1,000
100
Number of survivors (log scale)
Females 10
Males 1 2 4 6 8 10
Age (years)

21. Survivorship curves can be classified into three general types: Type I, Type II, and Type III

22. Number of survivors (log scale) Percentage of maximum life span
1,000 I
100
Number of survivors (log scale) II 10
III 1 50
100
Percentage of maximum life span

23. Reproductive Rates
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

25. Concept 52.2: Life history traits are products of natural selection
Life history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organism

26. Life History Diversity
Life histories are very diverse
Species that exhibit semelparity, or “big-bang” reproduction, reproduce once and die
Species that exhibit iteroparity, or repeated reproduction, produce offspring repeatedly

28. “Trade-offs” and Life Histories
Organisms have finite resources, which may lead to trade-offs between survival and reproduction

29. Parents surviving the following winter (%)
LE 52-7
100
Male
Female 80 60
Parents surviving the following winter (%) 40 20
Reduced
brood size
Normal
brood size
Enlarged
brood size

30. Some plants produce a large number of small seeds, ensuring that at least some of them will grow and eventually reproduce

31. LE 52-8a
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.

32. Other types of plants produce a moderate number of large seeds that provide a large store of energy that will help seedlings become established

33. LE 52-8b
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.

34. In animals, parental care of smaller broods may facilitate survival of offspring

35. It is useful to study population growth in an idealized situation
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
Idealized situations help us understand the capacity of species to increase and the conditions that may facilitate this growth

36. Per Capita Rate of Increase
If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate

37. 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: dN dt  rN

38. 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

39. Equation of exponential population growth:dN dt 
rmaxN

40. Exponential population growth results in a J-shaped curve

41. Population size (N) 5 10 15 Number of generations
LE 52-9
2,000 dN
= 1.0N dt
1,500 dN
= 0.5N dt
Population size (N)
1,000
500 5 10 15
Number of generations

42. The J-shaped curve of exponential growth characterizes some rebounding populations

43. Elephant population 1900 1920 1940 1960 1980 Year
8,000
6,000
Elephant population
4,000
2,000
1900
1920
1940
1960
1980
Year

44. Concept 52.4: 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

45. Carrying capacity (K) is the maximum population size the environment can support

46. The Logistic Growth Model
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

47. Per capita rate of increase (r)
LE 52-11
Maximum
Per capita rate of increase (r)
Positive
N = K
Negative
Population size (N)

48. The logistic growth equation includes K, the carrying capacitydN dt 
(K  N) K
rmax N

50. The logistic model of population growth produces a sigmoid (S-shaped) curve

51. Population size (N) 5 10 15 Number of generations
LE 52-12
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 5 10 15
Number of generations

52. The Logistic Model and Real Populations
The growth of laboratory populations of paramecia fits an S-shaped curve

53. Number of Paramecium/mL
LE 52-13a
1,000
800
600
Number of Paramecium/mL
400
200 5 10 15
Time (days)
A Paramecium population in the lab

54. Some populations overshoot K before settling down to a relatively stable density

55. Number of Daphnia/50 mL 20 40 60 80 100 120 140 160 Time (days)
LE 52-13b
180
150
120
Number of Daphnia/50 mL 90 60 30 20 40 60 80
100
120
140
160
Time (days)
A Daphnia population in the lab

56. Some populations fluctuate greatly around K

57. Number of females 1975 1980 1985 1990 1995 2000 Time (years)
LE 52-13c 80 60
Number of females 40 20
1975
1980
1985
1990
1995
2000
Time (years)
A song sparrow population in its natural habitat

58. The logistic model fits few real populations but is useful for estimating possible growth

59. 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

60. The concepts of K-selection and r-selection are somewhat controversial and have been criticized by ecologists as oversimplifications

61. Concept 52.5: 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?

62. 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

63. LE 52-14
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

64. 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

65. Competition for Resources
In crowded populations, increasing population density intensifies intraspecific competition for resources

66. Average number of seeds per reproducing individual
LE 52-15
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 1 10
100 10 20 30 40 50 60 70 80
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.

67. Territoriality
In many vertebrates and some invertebrates, territoriality may limit density

68. Cheetahs are highly territorial, using chemical communication to warn other cheetahs of their boundaries

70. Oceanic birds exhibit territoriality in nesting behavior

72. Health
Population density can influence the health and survival of organisms
In dense populations, pathogens can spread more rapidly

73. Predation
As a prey population builds up, predators may feed preferentially on that species

74. Toxic Wastes
Accumulation of toxic wastes can contribute to density-dependent regulation of population size

75. Intrinsic Factors
For some populations, intrinsic (physiological) factors appear to regulate population size

76. Population Dynamics
The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size

77. Stability and Fluctuation
Long-term population studies have challenged the hypothesis that populations of large mammals are relatively stable over time

78. Moose population size 1960 1970 1980 1990 2000 Year
LE 52-18
2,500
2,000
Steady decline probably caused largely by wolf predation
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
1960
1970
1980
1990
2000
Year

79. Extreme fluctuations in population size are typically more common in invertebrates than in large mammals

80. Commercial catch (kg) of
LE 52-19
730,000
100,000
Commercial catch (kg) of
male crabs (log scale)
10,000
1950
1960
1970
1980
1990
Year

81. 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

82. Number of breeding females60 50 40
Mandarte
Island
Number of breeding females 30 20
Small
islands 10
1988
1989
1990
1991
Year

83. Many populations undergo boom-and-bust cycles
Boom-and-bust cycles are influenced by complex interactions between biotic and abiotic factors

84. LE 52-21
Snowshoe hare
160
120
Lynx 9
Hare population size
(thousands)
Lynx population size
(thousands) 80 6 40 3
1850
1875
1900
1925
Year

85. Concept 52.6: Human population growth has slowed after centuries of exponential increase
No population can grow indefinitely, and humans are no exception

86. The Global Human Population
The human population increased relatively slowly until about 1650 and then began to grow exponentially

87. Human population (billions)
LE 52-22 6 5 4
Human population (billions) 3 2
The Plague 1
8000
B.C.
4000
B.C.
3000
B.C.
2000
B.C.
1000
B.C.
1000
A.D.
2000
A.D.

88. Though the global population is still growing, the rate of growth began to slow about 40 years ago

89. Annual percent increase
LE 52-23
2.2 2
1.8
1.6
2003
1.4
Annual percent increase
1.2 1
0.8
0.6
0.4
0.2
1950
1975
2000
2025
2050
Year

90. Regional Patterns of Population Change
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

91. Birth or death rate per 1,000 people50 40 30
Birth or death rate per 1,000 people 20 10
Sweden
Mexico
Birth rate
Birth rate
Death rate
Death rate
1750
1800
1850
1900
1950
2000
2050
Year

92. The demographic transition is associated with various factors in developed and developing countries

93. 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

94. LE 52-25 Rapid growth Afghanistan Slow growth United States Decrease
Italy
Male
Female
Age
Male
Female
Age
Male
Female
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 6 4 2 2 4 6 8 8 6 4 2 2 4 6 8 8 6 4 2 2 4 6 8
Percent of population
Percent of population
Percent of population

95. Age structure diagrams can predict a population’s growth trends
They can illuminate social conditions and help us plan for the future

96. 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

97. Infant mortality (deaths per 1,000 births) Life expectancy (years)
LE 52-26 60 80 50 60 40
Infant mortality (deaths per 1,000 births)
Life expectancy (years) 30 40 20 20 10
Developed
countries
Developing
countries
Developed
countries
Developing
countries

98. Global Carrying Capacity
How many humans can the biosphere support?

99. Estimates of Carrying Capacity
The carrying capacity of Earth for humans is uncertain

100. Ecological Footprint
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

101. Ecological footprint (ha per person) Available ecological capacity16 14 12
New Zealand 10
USA
Germany
Ecological footprint (ha per person) 8
Australia
Netherlands
Japan
Canada
Norway 6
Sweden UK 4
Spain
World 2
China
India 2 4 6 8 10 12 14 16
Available ecological capacity
(ha per person)

102. At more than 6 billion people, the world is already in ecological deficit