1. Chapter 36
Population Ecology
Lecture by Brian R. Shmaefsky 1

2. Introduction: A Tale of Two Fishes
Population ecology is concerned with
Changes in population size
Factors that regulate populations over time
It helps explain the biodiversity of an environment

3. Introduction: A Tale of Two Fishes
Ecologists learn the structure and dynamics of natural populations
With this information they are better equipped to
Develop sustainable food sources
Assess the impact of human activities
Balance human needs with the conservation of biodiversity and resources

7. POPULATION STRUCTURE AND DYNAMICS

8. 36.1 Population ecology is the study of how and why populations change
A group of individuals of a single species that occupy the same general area
Individuals in a population
Rely on the same resources
Are influenced by the same environmental factors
Are likely to interact and breed with one another

9. Population dynamics is the interactions between
A population can be described by the number and distribution of individuals
Population dynamics is the interactions between
Biotic and abiotic factors
It is the cause of variation in population sizes
A population increases through birth and immigration
Death and emigration out of an area decrease the population

10. Examples of population density
36.2 Density and dispersion patterns are important population variables
Population density is the number of individuals of a species per unit area or volume
Examples of population density
The number of oak trees per square kilometer in a forest
The number of earthworms per cubic meter in forest soil
Ecologists use a variety of sampling techniques to estimate population densities

11. Within a population’s geographic range, local densities may vary greatly
The dispersion pattern of a population refers to the way individuals are spaced within their area

13. Dispersion patterns can be
The dispersion pattern of a population refers to the way individuals are spaced within their area
Dispersion patterns can be
Clumped
Uniform
Random

14. In a clumped pattern individuals are grouped in patches

15. In a uniform pattern individuals are equally spaced in the environment

16. In a random pattern of dispersion, the individuals in a population are spaced in an unpredictable way

17. 36. 3 Life tables track survivorship in populations
Life tables track survivorship over the life span of individuals in a population
Survivorship curves plot the proportion of individuals alive at each age
Type I
Type II
Type III

19. 100 I 10 II
Percentage of survivors (log scale) 1
III
0.1 50
100
Percentage of maximum life span

20. 36.4 Idealized models predict patterns of population growth
Exponential growth model
The rate of population increases under ideal conditions
Calculated using the equation G = rN
G is the growth rate of the population
N is the population size
r is the per capita rate of increase

21. 500
450
400
350
300
Population size (N)
250
200
150
100 50 1 2 3 4 5 6 7 8 9 10 11 12
Time (months)

23. Logistic growth model
This growth model takes into account limiting factors
Limiting factors are environmental factors that restrict population growth
Formula

24. Breeding male fur seals10 8
Breeding male fur seals
(thousands) 6 4 2
1915
1925
1935
1945
Year

25. Breeding male fur seals10 8
Breeding male fur seals
(thousands) 6 4 2
1915
1925
1935
1945
Year

26. Idealized models describe two kinds of population growth
Exponential growth
Logistic growth

27. G = rN
(K – N) K
G = rN
Number of individuals (N) K
Time

29. 36.5 Multiple factors may limit population growth
The logistic growth model
Population growth slows and ceases as population density increases
Increasing population density results in a decrease in birth rate, an increase in death rate, or both

30. 4.0
3.8
3.6
Clutch size
3.4
3.2
3.0
2.8 10 20 30 40 50 60 70 80
Density of females

31. Abiotic factors may reduce population size before other limiting factors become important
Most populations fluctuate in numbers

32. Exponential
growth
Sudden
decline
Number of aphids
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec

33. 80 60 40
Number of females 20
1975
1980
1985
1990
1995
2000
Time (years)

34. 36.6 Some populations have “boom-and-bust” cycles
Some populations fluctuate in density with regularity
Boom-and-bust cycles
Food shortages
Predator-prey interactions

35. Snowshoe hare
160
120 9
Lynx
Hare population size
(thousands)
Lynx population size
(thousands) 80 6 40 3
1850
1875
1900
1925
Year

36. 36.7 EVOLUTION CONNECTION: Evolution shapes life histories
Life history
Series of events from birth to death
r/K selection
r-selective traits
K-selective traits

37. Predator: Killifish; preys mainly on small guppies
Experimental
transplant of
guppies
Predator: Killifish; preys
mainly on small guppies
Pools with killifish,
but no guppies prior
to transplant
Guppies:
Larger at
sexual maturity
than those in
pike-cichlid pools
Predator: Pike-cichlid preys mainly on large guppies
Guppies: Smaller at sexual maturity than
those in killifish pools
11 years
later
200
185.6
100
Control:
Guppies from pools
with pike-cichlids
as predators
85.7
92.3
161.5
160 80
120
Age of guppies
at maturity (days) 60
58.2
48.5
Mass of guppies
at maturity (mg) 80
67.5
76.1 40
Experimental:
Guppies transplanted
to pools with killifish
as predators 40 20
Males
Females
Males
Females

38. Experimental
transplant of
guppies
Predator: Killifish; preys
mainly on small guppies
Pools with killifish,
but no guppies prior
to transplant
Guppies:
Larger at
sexual maturity
than those in
pike-cichlid pools
Predator: Pike-cichlid preys mainly on large guppies
Guppies: Smaller at sexual maturity than
those in killifish pools

39. 11 years later Control: Guppies from pools with pike-cichlids
as predators
200
185.6
100
92.3
85.7
161.5
160 80
Mass of guppies
at maturity (mg)
Age of guppies
at maturity (days)
120
58.2 60
48.5 80
76.1
67.5 40
Experimental:
Guppies transplanted
to pools with killifish
as predators 40 20
Males
Females
Males
Females

40. Sustainable resource management
36.8 CONNECTION: Principles of population ecology have practical applications
Sustainable resource management
Maximum sustained yield

41. 900
800
700
600
Yield (thousands of metric tons)
500
400
300
200
100
1960
1970
1980
1990
2000

42. THE HUMAN POPULATION

43. 36.9 The human population continues to increase, but the growth rate is slowing
Human population is expected to continue increasing for several decades
95% of the increase is in developing nations

44. 100 10
Population increase 80 8 60 6
Annual increase (in millions)
Total population (in billions) 40 4
Total population size 20 2
1500
1550
1600
1650
1700
1750
1800
1850
1900
1950
2000
2050
Year

46. per 1,000 population Birth or death rate50 40 30
per 1,000 population
Birth or death rate
Rate of increase (r) 20
Birth rate
Death rate 10
1900
1925
1950
1975
2000
2025
2050
Year

47. 36.9 The human population continues to increase, but the growth rate is slowing
Population momentum of Mexico

48. Age
80+
1980
75-79
70-74
Male
Female
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 5 4 3 2 1 1 2 3 4 5
Population in millions
Total population size = 68,347,479

49. Age
80+
2005
75-79
70-74
Male
Female
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 5 4 3 2 1 1 2 3 4 5
Population in millions
Total population size = 106,202,903

50. Age
80+
2030
75-79
70-74
Male
Female
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 5 4 3 2 1 1 2 3 4 5
Population in millions
Total population size = 135,172,155

51. 36.10 CONNECTION: Age structures reveal social and economic trends
Age structure diagram
Reveals a population’s growth trends

52. 1980
Age
Birth years
Male
Female
80+
before 1900
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 12 10 8 6 4 2 2 4 6 8 10 12
Population in millions
Total population size = 227,726,463

53. 2005
Age
Birth years
Male
Female
80+
before 1926
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 12 10 8 6 4 2 2 4 6 8 10 12
Population in millions
Total population size = 295,734,134

54. 2030
Age
Birth years
Male
Female
80+
before 1951
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 12 10 8 6 4 2 2 4 6 8 10 12
Population in millions
Total population size = 363,811,435

55. 36.11 CONNECTION: An ecological footprint is a measure of resource consumption
U.S. Census Bureau projection
8 billion people within the next 20 years
9.5 billion by mid-21st century

56. 36.11 CONNECTION: An ecological footprint is a measure of resource consumption
Ecological footprint helps understand resource availability and usage
The United States has a
Big ecological footprint
Large ecological deficit

60. North
America
Europe
Asia
Africa
> 5.4 global ha per person
3.6–5.4 global ha per person
1.8–3.6 global ha per person
South
America
0.9–1.8 global ha per person
Australia
< 0.9 global ha per person
Insufficient data

61. Few large offspring,
low mortality
until old age I
Percentage of survivors
Many small
offspring,
high mortality II
III
Percentage of maximum life span

62. (K − N)
G = rN K

63. I II
III IV
Birth or death rate
Time