Chapter 44
Population Ecology
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44.1 Scope of Ecology
• Ecology
 The study of the interactions of organisms with other
organisms and the physical environment
 Habitat - Place where an organism lives
 Population - All the individuals of a species within a
particular space
 Community – Various populations of multiple species
interacting with each other
 Ecosystem - Community interacting with the
environment
 Biosphere - All the communities on Earth whose
members exist in air and water and on land
2
Ecological Levels
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Organism
Population
Community
Ecosystem
© David Hall/Photo Researchers, Inc.
3
44.2 Demographics of Populations
• Demography is the statistical study of a
population
• Demography includes
 Population density
 Population distribution
 Growth rate of a population
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Demographics of Populations
• Density and Distribution
 Population Density - Number of individuals
per unit area
 Population Distribution - Pattern of
dispersal of individuals across an area of
interest
 Limiting factors are environmental aspects
that particularly determine where an organism
lives
5
Distribution Patterns of the Creosote Bush
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Young, small
shrubs
a. Mature desert shrubs
Medium
shrubs
b. Clumped
Large
shrubs
c. Random
d. Uniform
(a): © The McGraw Hill Companies, Inc. Evelyn Jo Johnson, photographer
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Demographics of Populations
• Population Growth
 The rate of natural increase depends on
• Birth rate and death rate
 Biotic Potential
• The maximum rate of natural increase for a population
that can occur when resources are unlimited
 Biotic potential depends on factors that influence
the population’s reproduction, including
• competition
• number of reproductive opportunities
• Presence of disease and predators
7
Biotic Potential
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a.
b.
(mice): © E. R. Degginger/Photo Researchers, Inc.; (rhinos): © Corbis RF
Which organism has a higher biotic
potential?
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Demographics of Populations
• Mortality Patterns
 A cohort
• Composed of all the members of a population born
at the same time
– Life tables demonstrate how many members of a cohort
are still alive after certain intervals of time
 Survivorship
• The probability that newborn individuals of a cohort
will survive to a particular age
• Survivorship Curves
– A plot of the number of organisms surviving at each age
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A Life Table for a
Bluegrass Cohort
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Demographics of Populations
• Survivorship Curves
 Type I
• Characteristic of a population in which most individuals
survive past the midpoint of the life span and death does not
come until the end of the life span
 Type II
• Death is linear over time (unrelated to age)
 Type III
• Typical of a population in which most individuals die very
young
11
Survivorship Curves
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1,000
1,000
Number of Survivors
Number of Survivors
I Death occurs after midpoint.
100
I
II
III
II Death unrelated to age.
10
100
10
III Death comes early on.
0
0
0
50
Percent of Life Span
0
100
50
100
Percent of Life Span
a.
1,000
1 million
100
10,000
10
0
0
50
100
Number of Survivors
Number of Survivors
b. Bluegrasses
100
0
0
Percent of Life Span
c. Lizards
50
Percent of Life Span
100
d. Mosquitoes
b: © Holt Studios/Photo Researchers, Inc.; c: © Bruce M. Johnson; d: © Digital Vison/Getty RF Images
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Demographics of Populations
• Age Distribution
 The proportion of the population that falls into various
age categories
 There are three major age groups
• Prereproductive
• Reproductive
• Postreprodutive
 At least three age structure diagrams are possible
• Increasing population
• Stable population
• Decreasing population
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Age Structure Diagrams
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Age Structure
Postreproductive Ages
Reproductive Ages
Prereproductive Ages
Increasing Population
Stable Population
Decreasing Population
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44.3 Population Growth Models
• Two working models for population growth:
 Semelparity
• Members of a population have only a single
reproductive event in their lifetime
– Ex: insects
 Iteroparity
• Members of the population experience many
reproductive events throughout their lifetime
– Ex: most vertebrates, shrubs, and trees
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Population Growth Models
• Exponential Growth
 Rate of population growth increases as the total
number of females increases
 Biotic potential is having full effect and birthrate is
a maximum during exponential growth
 Phases of an exponential growth curve
• Lag phase
• Exponential phase
16
Model for Exponential Growth
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Generation Population Number of
Size
Females
0
10.0
5
1
2
3
24.0
57.6
138.2
12
28.8
69.1
4
5
331.7
796.1
165.9
398.1
6
7
1,910.6
4,585.4
955.3
2292.7
8
9
11,005.0
26,412.0
5502.5
13206.1
10
63,388.8
31694.5
a.
Population (thousands)
70
R = 2.4
60
50
40
exponential growth
30
20
10
lag
0
1
2
3
4
5
6
7
8
9
10
Generations
b.
To calculate population size from year to year, use this
formula:
Nt+1 = RNt
Nt = number of females already present
R = net reproductive rate
Nt+1 = population size the following year
c.
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Population Growth Models
• Logistic growth
 Occurs when limiting environmental factors
oppose growth
 Phases of a logistic growth curve
•
•
•
•
Lag phase
Exponential growth phase
Deceleration phase
Stable equilibrium phase (i.e. carrying capacity)
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Model for Logistic Growth
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Growth of Yeast Cells in Laboratory Culture
Time (t )
(hours)
Number of individuals D N
added per 2-hour period D t
Number of
individuals (N)
0
2
4
6
8
10
12
14
16
18
9.6
29.0
71.1
174.6
350.7
513.3
594.4
640.8
655.9
661.8
0
19.4
42.1
103.5
176.1
162.6
81.1
46.4
15.1
5.9
Number of Yeast Cells
a.
700
deceleration
600
stable
equilibrium
phase
500
400
exponential
growth
300
200
lag
100
2
4
6
8
10
12
14
16
18
Time (hours)
b.
To calculate population growth as time passes, use this
formula:
K–N
N
= rN
K
t
N = population size
N/t = change in population size
r = rate of natural increase
K = carrying capacity
K – N = effect of carrying capacity on population growth
K
c.
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Population Growth Models
• Carrying Capacity
 The maximum number of individuals of a
species the environment can continuously
support
• The closer the population to the carrying capacity,
the more likely its resources will become scarce
and that biotic effects such as competition and
predation will become apparent
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44.4 Regulation of Population
Size
• Density-independent Factors
 The population density does not influence
the intensity of the factor’s effect
• Natural disasters
• Density-dependent Factors
 The percentage of the population affected
increases as the population density
increases
• Competition
• Predation
• Parasitism
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Density-independent Effects
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a. Low density of mice
b. High density of mice
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Density-dependent Effect
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Number of Reindeer
2,000
1,500
decline as
a result
of sudden
resource
depletion
exponential
growth
1,000
500
0
1910
1920
1930
© Paul Janosi/Valan Photos
1940
1950
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Density-dependent Effects -Competition
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a. Low density of birds
b. High density of birds
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Density-dependent Effects -Predation
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a. Low density of mice
b. High density of mice
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44.5 Life History Patterns
• Life histories contain characteristics of a
population such as




The number of births per reproduction
The age of reproduction
The life span
The probability of an individual living the entire life
span
• Each population distributes energy among its life
span, reproduction events, and care of offspring.
• Related species may have different life history
patterns.
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Life History Patterns
• r is the rate of natural increase of a
population.
• K is the carrying capacity of the
environment.
• Some populations are subject to rselection, and other populations are
subject to K-selection.
27
Life History Strategies
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Opportunistic Species
(r-strategist)
• Small individuals
• Short life span
• Fast to mature
• Many offspring
• Little or no care of
offspring
• Many offspring die
before reproducing
• Early reproductive age
Equilibrium Species
(K-strategist)
• Large individuals
• Long life span
• Slow to mature
• Few and large offspring
• Much care of offspring
• Most young survive to
reproductive age
• Adapted to stable
environment
(dandelions): © Ted Levin/Animals Animals; (bears): ©Michio Hoshino/Minden Pictures
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