1. Chapter 53
Population Ecology

2. Concept 53.1: Dynamic biological processes influence population density, dispersion, and demographics
A population is a group of individuals of a single species living in the same general area
Populations are described by their boundaries and size
Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population size
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3. 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 (1-2)
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4. Density: A Dynamic Perspective
In most cases, it is impractical or impossible to count all individuals in a population
Sampling techniques can be used to estimate densities and total population sizes
Population size can be estimated by either extrapolation from small samples, an index of population size (e.g., number of nests), or the mark-recapture method
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5. Mark-recapture method
Scientists capture, tag, and release a random sample of individuals (s) in a population
Marked individuals are given time to mix back into the population
Scientists capture a second sample of individuals (n), and note how many of them are marked (x)
Population size (N) is estimated by (apply to 3) sn x
N 
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6. Immigration is the influx of new individuals from other areas
Density is the result of an interplay between processes that add individuals to a population and those that remove individuals
Immigration is the influx of new individuals from other areas
Emigration is the movement of individuals out of a population
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7. (Apply to 4) Births Deaths
Figure 53.3
(Apply to 4)
Births
Deaths
Deaths and emigration remove individuals from a population.
Births and immigration add individuals to a population.
Figure 53.3 Population dynamics.
Immigration
Emigration 7

8. 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 (5 visualized on next slide)
Video: Flapping Geese (Clumped)
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9. (a) Clumped (b) Uniform (c) Random Figure 53.4
Figure 53.4 Patterns of dispersion within a population’s geographic range.
(c) Random 9

10. A uniform dispersion is one in which individuals are evenly distributed
It may be influenced by social interactions such as territoriality, the defense of a bounded space against other individuals
Video: Albatross Courtship (Uniform)
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11. Figure 53.4b
(b) Uniform
Figure 53.4 Patterns of dispersion within a population’s geographic range. 11

12. It occurs in the absence of strong attractions or repulsions
In a random dispersion, the position of each individual is independent of other individuals
It occurs in the absence of strong attractions or repulsions
Video: Prokaryotic Flagella
(Salmonella typhimurium) (Random)
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13. Figure 53.4c
(c) Random
Figure 53.4 Patterns of dispersion within a population’s geographic range. 13

14. Demographics
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 (6)
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15. 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, a group of individuals of the same age
The life table of Belding’s ground squirrels reveals many things about this population
For example, it provides data on the proportions of males and females alive at each age (7)
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16. Table 53.1a
Table 53.1 Life Table for Belding’s Ground Squirrels (Spermophilus beldingi) at Tioga Pass, in the Sierra Nevada of California 16

17. Table 53.1b
Table 53.1 Life Table for Belding’s Ground Squirrels (Spermophilus beldingi) at Tioga Pass, in the Sierra Nevada of California 17

18. 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
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19. Number of survivors (log scale)
Figure 53.5
1,000
100
Number of survivors (log scale)
Females 10
Males
Figure 53.5 Survivorship curves for male and female Belding’s ground squirrels. 1 2 4 6 8 10
Age (years) 19

20. Survivorship curves can be classified into three general types
Type I: low death rates during early and middle life, then an increase in death rates among older age groups
Type II: the death rate is constant over the organism’s life span
Type III: high death rates for the young, then a slower death rate for survivors (8 Explained here visualized next slide)
Many species are intermediate to these curves
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21. (9 From Understanding) Number of survivors (log scale) 1,000 I 100 II
Figure 53.6
(9 From Understanding)
1,000 I
100 II
Number of survivors (log scale) 10
Figure 53.6 Idealized survivorship curves: Types I, II, and III.
III 1 50
100
Percentage of maximum life span 21

22. Reproductive Rates
For species with sexual reproduction, demographers often concentrate on females in a population
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 (10)
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23. Table 53.2
Table 53.2 Reproductive Table for Belding’s Ground Squirrels at Tioga Pass 23

24. Concept 53.2: 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
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25. Per Capita Rate of Increase
Change in
population
size
Births
Immigrants
entering
Deaths
Emigrants
leaving   
If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate (11)
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26. The per capita rate of interest (r) is given by
r  b  m
Zero population growth (ZPG) occurs when the birth rate equals the death rate (r  0) (13)
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27. Exponential Growth
Exponential population growth is population increase under idealized conditions (13)
This might occur with rebounding or newly introduced populations. (15)
Under these conditions, the rate of increase is at its maximum, denoted as rmax
The equation of exponential population growth is (Skip 14, but know that exponential growth= J shaped curve) dN dt 
rmaxN
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28. Figure 53.8
8,000
6,000
Elephant population
4,000
2,000
Figure 53.8 Exponential growth in the African elephant population of Kruger National Park, South Africa.
1900
1910
1920
1930
1940
1950
1960
1970
Year 28

29. Concept 53.3: The logistic model describes how a population grows more slowly as it nears its 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
Carrying capacity varies with the abundance of limiting resources (16 and 17 from thinking)
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30. The Logistic Growth Model
In the logistic population growth model, the per capita rate of increase declines as carrying capacity is reached (18)
The logistic model starts with the exponential model and adds an expression that reduces per capita rate of increase as N approaches K dN dt 
(K  N) K
rmax N
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31. Table 53.3
Table 53.3 Logistic Growth of a Hypothetical Population (K = 1,500) 31

32. The logistic model of population growth produces a sigmoid (S-shaped) curve (Skip 19 and 20, but know that logistic growth=S shaped curve)
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33. ( ) Exponential growth 2,000 dN dt = 1.0N 1,500 K = 1,500
Figure 53.9
Exponential growth
2,000 dN dt
= 1.0N
1,500
K = 1,500
Logistic growth dN dt (
1,500 – N
1,500 )
Population size (N)
= 1.0N
1,000
Figure 53.9 Population growth predicted by the logistic model.
Population growth begins slowing here.
500 5 10 15
Number of generations 33

34. Think of examples for the 2 extremes.
K-selection, or density-dependent selection, selects for life history traits that are sensitive to population density
Animals …elephants large body size, small but stable populations, parental care, few offspring
r-selection, or density-independent selection, selects for life history traits that maximize reproduction
Animals…mosquito small body size, unstable populations, little investment in parental care, large numbers of offspring
Think of examples for the 2 extremes.
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35. Concept 53.4: Life history traits are products of natural selection
An organism’s life history comprises the traits that affect its schedule of reproduction and survival
The age at which reproduction begins
How often the organism reproduces
How many offspring are produced during each reproductive cycle
Life history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organism (22-23)
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36. Evolution and Life History Diversity
Species that exhibit semelparity, or big-bang reproduction, reproduce once and die
Species that exhibit iteroparity, or repeated reproduction, produce offspring repeatedly
Highly variable or unpredictable environments likely favor big-bang reproduction, while dependable environments may favor repeated reproduction (24 and 25 from thinking)
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37. “Trade-offs” and Life Histories
Organisms have finite resources, which may lead to trade-offs between survival and reproduction
For example, there is a trade-off between survival and paternal care in European kestrels
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38. Parents surviving the following winter (%)
Figure 53.13
(26)
RESULTS
100
Male
Female 80 60
Parents surviving the following winter (%) 40
Figure Inquiry: How does caring for offspring affect parental survival in kestrels? 20
Reduced brood size
Normal brood size
Enlarged brood size 38

39. Concept 53.5: Many factors that regulate population growth are density dependent
There are two general questions about regulation of population growth
What environmental factors stop a population from growing indefinitely?
Why do some populations show radical fluctuations in size over time, while others remain stable?
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40. 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
Think about factors that would cause density dependent and independent regulation (27)
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41. Birth or death rate per capita
Figure 53.15
When population density is low, b > m. As a result, the population grows until the density reaches Q.
When population density is high, m > b, and the population shrinks until the density reaches Q.
Equilibrium density (Q)
Birth or death rate per capita
Density-independent death rate (m)
Figure Determining equilibrium for population density.
Density-dependent birth rate (b)
Population density 41

42. Mechanisms of Density-Dependent Population Regulation
Density-dependent birth and death rates are an example of negative feedback that regulates population growth (28)
Density-dependent birth and death rates are affected by many factors, such as competition for resources, territoriality, disease, predation, toxic wastes, and intrinsic factors
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43. Competition for Resources
In crowded populations, increasing population density intensifies competition for resources and results in a lower birth rate
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44. Figure 53.17a
Figure Exploring: Mechanisms of Density-Dependent Regulation 44

45. Toxic Wastes
Accumulation of toxic wastes can contribute to density-dependent regulation of population size
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46. Predation
As a prey population builds up, predators may feed preferentially on that species
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47. Figure 53.17b
Figure Exploring: Mechanisms of Density-Dependent Regulation 47

48. Intrinsic Factors
For some populations, intrinsic (physiological) factors appear to regulate population size
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49. Figure 53.17d
Figure Exploring: Mechanisms of Density-Dependent Regulation 49

50. Territoriality
In many vertebrates and some invertebrates, competition for territory may limit density
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51. Figure 53.17e
Figure Exploring: Mechanisms of Density-Dependent Regulation 51

52. Disease
Population density can influence the health and survival of organisms
In dense populations, pathogens can spread more rapidly
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53. Figure 53.17f
Figure Exploring: Mechanisms of Density-Dependent Regulation 53

54. Population Dynamics
The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size
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55. Stability and Fluctuation
Long-term population studies have challenged the hypothesis that populations of large mammals are relatively stable over time
Both weather and predator population can affect population size over time
For example, moose on Isle Royale collapsed during a harsh winter, and when wolf numbers peaked
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56. (30….Just understand biotic and abiotic factors can influence population sizes)
Figure 53.18 50 40 30 20 10
2,500
2,000
1,500
1,000
500
Wolves
Moose
Number of wolves
Number of moose
Figure Fluctuations in moose and wolf populations on Isle Royale, 1959–2008.
1955
1965
1975
1985
1995
2005
Year 56

57. Number of lynx (thousands) Number of hares (thousands)
Figure 53.19
Snowshoe hare
160
120 80 40
(Skip 31) 9 6 3
Number of lynx (thousands)
Number of hares (thousands)
Lynx
Figure Population cycles in the snowshoe hare and lynx.
1850
1875
1900
1925
Year 57

58. (32) 7 6 5 4 Human population (billions) 3 2 1 The Plague 8000 BCE
Figure 53.22
(32) 7 6 5 4 3 2 1
Human population (billions)
The Plague
Figure Human population growth (data as of 2009).
8000
BCE
4000
BCE
3000
BCE
2000
BCE
1000
BCE
1000 CE
2000 CE 58

59. The global population is more than 6.8 billion people
Though the global population is still growing, the rate of growth began to slow during the 1960s
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60. Annual percent increase
Figure 53.23
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
2009
Annual percent increase
Projected data
Figure Annual percent increase in the global human population (data as of 2009).
1950
1975
2000
2025
2050
Year 60

61. 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 to the second state (33)
The demographic transition is associated with an increase in the quality of health care and improved access to education, especially for women
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62. 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
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63. (34) Rapid growth Afghanistan Slow growth United States No growth
Figure 53.24
(34)
Rapid growth
Afghanistan
Slow growth
United States
No growth
Italy
Male
Female
Age
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
Male
Female
Age
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
Male
Female
Figure Age-structure pyramids for the human population of three countries (data as of 2009). 10 8 6 4 2 2 4 6 8 10 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 63

64. 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 (35 from thinking)
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65. Infant mortality (deaths per 1,000 births)
Figure 53.25 60 50 40 30 20 10 80 60 40 20
Infant mortality (deaths per 1,000 births)
Life expectancy (years)
Figure Infant mortality and life expectancy at birth in industrialized and less industrialized countries (data as of 2008).
Indus- trialized countries
Less indus- trialized countries
Indus- trialized countries
Less indus- trialized countries 65

66. Global Carrying Capacity
How many humans can the biosphere support?
Population ecologists predict a global population of 7.810.8 billion people in 2050
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67. Estimates of Carrying Capacity
The carrying capacity of Earth for humans is uncertain
The average estimate is 10–15 billion
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68. Limits on Human Population Size
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
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69. (36….Nope) Gigajoules > 300 150–300 50–150 10–50 < 10
Figure 53.26
Gigajoules
Figure Annual per capita energy use around the world.
> 300
150–300
50–150
10–50
< 10
(36….Nope) 69