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

• 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

Population size
Births Immigration
Emigration
Deaths
Population

Environmental and social factors influence spacing of individuals in a population
1.
2.
3.

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.

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.

Random . Dandelions grow from windblown seeds that land at random and later germinate.

• A survivorship curve is a graphic way of representing the data in a life table
• Survivorship curves can be classified into 3 general types:
• Type I:
• Type II
• Type III

• characterized by high survival in early and middle life, followed by a rapid decline in survivorship in later life.
– Example: Humans

• The greatest mortality is experienced early in life, with relatively low rates of death for those surviving. This type of curve is characteristic of species that produce a large number of offspring.
– Example: most marine invertebrates like an Oyster

1,000
I
100
II
10
1
0
III
50
Percentage of maximum life span
100

1,000
100
Females
10
Males
1
0 2 4 6
Age (years)
8 10

• It is useful to study population growth in an idealized situation that may facilitate this growth
– In animals, parental care of smaller broods may facilitate survival of offspring
• Exponential population growth results in a Jshaped curve

2,000
1,500
1,000
500
0
0 dN dt
= 1.0N
5 10
Number of generations dN dt
= 0.5N
15

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

Maximum
Positive
0
Negative
Population size (N)
N = K

2,000
1,500
K = 1,500 dN dt
= 1.0N
Exponential growth
1,000
Logistic growth dN dt
= 1.0N
1,500 – N
1,500
500
0
0 5 10
Number of generations
15

/mL 1,000
800
600
400
200
0
0 5 10
Time (days)
15
A Paramecium population in the lab

mL 180
150
120
90
60
30
0
0 20 40 60 80 100 120 140 160
Time (days)
A Daphnia population in the lab

80
60
40
20
0
1975 1980 1985 1990
Time (years)
1995 2000
A song sparrow population in its natural habitat

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

2,500
2,000
1,500
1,000
500
0
1960
Steady decline probably caused largely by wolf predation
Dramatic collapse caused by severe winter weather and food shortage, leading to starvation of more than
75% of the population
1970 1980
Year
1990 2000

730,000
100,000
10,000
1950 1960 1970
Year
1980 1990

Snowshoe hare
160
120
80
40
0
1850 1875
Year
1900
Lynx
9
6
3
1925
0

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

50
40
30
20
10
0
1750
Sweden
Birth rate
Death rate
1800
Mexico
1850
Birth rate
Death rate
1900
Year
1950 2000 2050

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

Rapid growth
Afghanistan
Male Female
8 6 4 2 0 2 4 6 8
Percent of population
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
8
Slow growth
United States
Male Female
6 4 2 0 2 4 6 8
Percent of population
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
8
Decrease
Male
Italy
Female
6 4 2 0 2 4 6 8
Percent of population

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

60
50
40
30
20
10
0
Developed countries
Developing countries
80
60
40
20
0
Developed countries
Developing countries