1. Population Ecology
AP Chap 53

2. Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population size

3. Fig. 53-1
A population is a group of individuals of a single species living in the same general area
Figure 53.1 What causes a sheep population to fluctuate in size?

4. Every population has geographic boundaries.
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

5. Population density is often determined by sampling techniques
Population size can be estimated by
Direct counting
Random sampling based on sample plots (quadrats)
Indexes such as tracks, nests, burrows, fecal droppings, etc.
Mark and recapture method

6. Quadrat sampling

7. Mark-recapture Method

8. Mark-Recapture Formula for estimating population size
Estimate of Total Population  = 
(total number recaptured) x (number marked)
(total number recaptured with mark)

9. In a mark-recapture study, an ecologist traps, marks and releases 25 voles in a small wooded area. A week later she resets her traps and captures 30 voles, 10 of which are marked. What is her estimate of the vole population in that area?

10. How does population density change?
Addition – birth, immigration
Removal – death, emigration

11. Births and immigration add individuals to a population.
Fig. 53-3
Births
Deaths
Births and immigration
add individuals to
a population.
Deaths and emigration
remove individuals
from a population.
Figure 53.3 Population dynamics
Immigration
Emigration

12. 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

13. Fig. 53-4a
Figure 53.4a Patterns of dispersion within a population’s geographic range
(a) Clumped

14. UNIFORM
A uniform dispersion is one in which individuals are evenly distributed
It may be influenced by social interactions such as territoriality

15. Fig. 53-4b
Figure 53.4b Patterns of dispersion within a population’s geographic range
(b) Uniform

16. RANDOM
In a random dispersion, the position of each individual is independent of other individuals
It occurs in the absence of strong attractions or repulsions

17. Fig. 53-4c
Figure 53.4c Patterns of dispersion within a population’s geographic range
(c) Random

19. Demographics
Demography is the study of the vital statistics (death and birth rates) 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, a group of individuals of the same age

20. Table 53-1
The life table of Belding’s ground squirrels reveals many things about this population

21. 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

22. Number of survivors (log scale)
Fig. 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)

23. Survivorship curves can be classified into three general types:
Type I: low death rates during early and middle life, then an increase 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

24. Number of survivors (log scale)
Fig. 53-6
More parental care,
better health care
1,000 I
Predation, accidents,
disease at all levels
100 II
Number of survivors (log scale) 10
High mortality
of many offspring
Figure 53.6 Idealized survivorship curves: Types I, II, and III
III 1 50
100
Percentage of maximum life span

25. What type of survivorship curve?

26. Reproductive tables focus on female reproductivity.

27. 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

28. 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.

29. “Trade-offs” and Life Histories
Organisms have finite resources, which may lead to trade-offs between survival and reproduction
Examples:
brood size vs parental life span
number of seeds and chance of gemination and growth

30. Fig. 53-8
RESULTS
100
Male
Female 80 60
Parents surviving the following winter (%) 40
Figure 53.8 How does caring for offspring affect parental survival in kestrels? 20
Reduced
brood size
Normal
brood size
Enlarged
brood size

31. Fig. 53-9
Some plants produce a large number of small seeds, ensuring that at least some of them will grow and eventually reproduce
(a) Dandelion
Other types of plants produce a moderate number of large seeds that provide a large store of energy that will help seedlings become established
Figure 53.9 Variation in the size of seed crops in plants
(b) Coconut palm

32. In what way might high competition for limited resources in a predictable environment influence the evolution of life history traits? Semelparity or iteroparity
Selection would most likely favor iteroparity, with fewer, larger, better-provisioned or cared-for offspring.

33. How do we model population growth?
By construction graphs and using mathematical formulas
If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate
r = b - d

34. 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: N t  rN
where N = population size, t = time, and r = per capita rate of increase

35. Exponential Growth
Exponential population growth is population increase under idealized. unlimited conditions
Under these conditions, the rate of reproduction is at its maximum, called the intrinsic rate of increase

36. Equation of exponential population growth:dN dt 
rmaxN
Exponential population growth results in a J-shaped curve.

37. 2,000 = 1.0N 1,500 = 0.5N Population size (N) 1,000 500 5 10 15
Fig
2,000 dN =
1.0N dt
1,500 dN =
0.5N dt
Population size (N)
1,000
500
Figure Population growth predicted by the exponential model 5 10 15
Number of generations

38. The J-shaped curve of exponential growth characterizes some rebounding populations.
Fig
8,000
6,000
Elephant population
4,000
Elephants in Kruger National Park in
S. Africa after they were protected
from hunting.
2,000
Figure Exponential growth in the African elephant population of Kruger National Park, South Africa
1900
1920
1940
1960
1980
Year

39. But, is this the normal state of population growth?
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

40. 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 approaches K. dN dt 
(K  N) K
rmax N

41. Table 53-3
As N approaches
K, rate nears
“0”.
Table 53.3

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

43. K Exponential growth 2,000 = 1.0N 1,500 K = 1,500 Population size (N)
Fig
Exponential
growth
2,000 dN K =
1.0N dt
1,500
K = 1,500
Population size (N)
Logistic growth
1,000 dN
1,500 – N =
1.0N dt
1,500
Figure Population growth predicted by the logistic model
500 5 10 15
Number of generations

44. Number of Paramecium/mL
Fig a
Some populations overshoot K before settling down to a relatively stable density
1,000
800
These organisms are grown in a constant environment lacking predators and competitors
Number of Paramecium/mL
600
400
200
Figure 53.13a How well do these populations fit the logistic growth model? 5 10 15
Time (days)
(a) A Paramecium population in the lab

45. (b) A Daphnia population in the lab
Fig b
Some populations fluctuate greatly and make it difficult to define K.
180
150
120
Number of Daphnia/50 mL 90 60 30
Figure 53.13b How well do these populations fit the logistic growth model? 20 40 60 80
100
120
140
160
Time (days)
(b) A Daphnia population in the lab

46. Some populations show an Allee effect, in which individuals have a more difficult time surviving or reproducing if the population size is too small

47. The Logistic Model and Life Histories
Natural selection shapes the final life history of individual species.
Some members of populations are subject to r-selection and some to k-selection.
When population size is low relative to K,
r-selection favors r-strategies:
high fecundity (ability to reproduce),
small body size,
early maturity onset,
short generation time, and
the ability to disperse offspring widely.

48. Characteristics of r - Selected Opportunists
Very high intrinsic rate of increase.
Opportunistic
Populations can expand rapidly to take advantage of temporarily favorable conditions
Ex – Bacteria, some fungi, many insects, rodents, weeds, and annual plants.

49. In environments that are relatively stable and populations tend to be near K, with minimal fluctuations in population size, K-selection favors K strategies: large body size, long life expectancy, and the production of fewer offspring that require extensive parental care until they mature.
These populations are strong competitors.
They are specialists rather than colonists and may become extinct if their normal way of life is destroyed.

50. Characteristics of K - Selected Species
Population responds slowly, usually with negative feedback control so that constancy is the rule.
Their numbers are controlled by the availability of resources. In other words, they are a density dependent species
Most birds
Most predators
Elephants
Whales
Oaks
Chestnuts
Apple
Coconut

51. r or k-selected?
Nature is more complex though and most populations lie somewhere in between these two extremes.
Ex- Gymnosperms and angiosperms are typically classified as K-strategists but they release many seeds.
Cod fish are large fish but
release large numbers of
gametes into the sea with
no parental investment.
So, cod are considered
r-strategists.

52. K or r-selected ?
When a farmer abandons a field, it is quickly colonized by fast-growing weeds. Are these species more likely to be K-selected or r-selected species? r

53. What about the bluegill fish?
Bluegill exhibit one of the most social and complex mating systems in nature. Parental males delay maturation and compete to construct nests in colonies, court females, and provide sole parental care for the young within their nest.

54. 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?

55. 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

56. Birth or death rate per capita Birth or death rate per capita
Fig
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
(a) Both birth rate and death rate vary.
(b) Birth rate varies; death rate is constant.
Density-dependent
death rate
Density-
independent
birth rate
Figure Determining equilibrium for population density
Birth or death rate
per capita
Equilibrium
density
Population density
(c) Death rate varies; birth rate is constant.

57. So, to determine if the environmental factor is density dependent or independent….
Density-independent factors may affect all individuals in a population equally –
rainfall, temperature, humidity, acidity, salinity, catastrophic events

58. Density-dependent factors have a greater affect when the population density is higher.
Food supply, disease, parasites, competition, predation

59. 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, disease, predation, toxic wastes, and intrinsic factors

60. (a) Cheetah marking its territory
Fig a
In many vertebrates and some invertebrates, competition for territory may limit density
Cheetahs are highly territorial, using chemical communication to warn other cheetahs of their boundaries
Figure 53.17a Territoriality
(a) Cheetah marking its territory

61. (b) Gannets Oceanic birds exhibit territoriality in nesting behavior
Fig b
Oceanic birds exhibit territoriality in nesting behavior
Figure 53.17b Territoriality
(b) Gannets

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

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

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

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

66. Population Dynamics
The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size
Long-term population studies have challenged the hypothesis that populations of large mammals are relatively stable over time
Weather can affect population size over time

67. Fig
2,100
1,900
1,700
1,500
Number of sheep
1,300
1,100
900
Figure Variation in size of the Soay sheep population on Hirta Island, 1955–2002
700
500
1955
1965
1975
1985
1995
2005
Year

68. 50 2,500 Wolves Moose 40 2,000 30 1,500 Number of wolves
Fig 50
2,500
Wolves
Moose 40
2,000 30
1,500
Number of wolves
Number of moose 20
1,000 10
500
Figure Fluctuations in moose and wolf populations on Isle Royale, 1959–2006
1955
1965
1975
1985
1995
2005
Year
Changes in predation pressure can drive population fluctuations

69. Population Cycles: Scientific Inquiry
Some populations undergo regular boom-and-bust cycles
Lynx populations follow the 10 year boom-and-bust cycle of hare populations
Three hypotheses have been proposed to explain the hare’s 10-year interval
- winter food supply
- predators *
- sunspot activity (quality of food) *
* affected cycles

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

71. The human population is no longer growing exponentially but is still increasing rapidly
No population can grow indefinitely, and humans are no exception

72. 7 6 5 4 3 2 1 Human population (billions) The Plague
Fig 7 6
The human population increased relatively slowly until about 1650 and then began to grow exponentially 5 4
Human population (billions) 3 2
The Plague
Figure Human population growth (data as of 2006) 1
8000
B.C.E.
4000
B.C.E.
3000
B.C.E.
2000
B.C.E.
1000
B.C.E.
1000
C.E.
2000
C.E.

73. Fig
2.2
2.0
1.8
1.6
1.4
2005
Annual percent increase
1.2
Projected
data
1.0
0.8
Though the global population is still growing, the rate of growth began to slow during the 1960s
0.6
Figure Annual percent increase in the global human population (data as of 2005)
0.4
0.2
1950
1975
2000
2025
2050
Year

74. 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

75. Birth or death rate per 1,000 people
Fig
The demographic transition in Sweden took about 150 years, from 1810 to It will take about the same length of time for Mexico. 50 40 30
Birth or death rate per 1,000 people 20
Figure Demographic transition in Sweden and Mexico, 1750–2025 (data as of 2005) 10
Sweden
Mexico
Birth rate
Birth rate
Death rate
Death rate
1750
1800
1850
1900
1950
2000
2050
Year

76. The demographic transition is associated with an increase in the quality of health care and improved access to education, especially for women
Most of the current global population growth is concentrated in developing countries

77. 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
Age structure diagrams can predict a population’s growth trends
They can illuminate social conditions and help us plan for the future

78. Rapid growth Slow growth No growth Afghanistan United States Italy
Fig
Rapid growth
Slow growth
No growth
Afghanistan
United States
Italy
Male
Female
Age
Male
Female
Age
Male
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
Figure Age-structure pyramids for the human population of three countries (data as of 2005)
15–19
15–19
10–14
10–14
5–9
5–9
0–4
0–4
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

79. 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

80. Life expectancy (years) Infant mortality (deaths per 1,000 births)
Fig 60 80 50 60 40
Life expectancy (years)
Infant mortality (deaths per 1,000 births) 30 40 20 20
Figure Infant mortality and life expectancy at birth in industrialized and less industrialized countries (data as of 2005) 10
Indus-
trialized
countries
Less indus-
trialized
countries
Indus-
trialized
countries
Less indus-
trialized
countries

81. Global Carrying Capacity
How many humans can the biosphere support?
The carrying capacity of Earth for humans is uncertain
The average estimate is 10–15 billion

82. 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

83. 13.4 9.8 5.8 Not analyzed Log (g carbon/year) Fig. 53-27
Figure The amount of photosynthetic products that humans use around the world
5.8
Not analyzed

84. Our carrying capacity could potentially be limited by food, space, nonrenewable resources, or buildup of wastes