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Point-of-View
• The root cause of nearly every environmental
issue is human overpopulation.
• Do you agree with this opinion?
– Why or why not?
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Overview: Counting Sheep
• A small population of Soay sheep were introduced
to Hirta Island in 1932
• They provide an ideal opportunity to study
changes in population size on an isolated island
with abundant food and no predators
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Figure 53.1
Big Ideas
• 2D.1: All biological systems from cells to
ecosystems are affected by complex biotic and
abiotic interactions involving exchange of matter
and energy.
–
–
–
–
–
Symbiosis
Predator/Prey interactions (AKA food chain)
Availability of nutrients and energy
Population density and dispersion (Figure 53.4)
Species diversity
Big Ideas
• 4A.5: Communities are composed of populations of
organisms that interact in complex ways.
–
–
–
–
Predator/prey interactions (Lynx/Snowshoe hare; Fig. 53.18)
Carrying capacity (Figure 53.9)
Exponential v. Logistic growth (Sections 53.2-3)
Density-dependent and density independent factors (Section 53.5)
Key Points
• #1: Population density, dispersion, and
demographics are dynamic (ever changing).
• #2: In ideal, unlimited environments, populations
grow exponentially
• #3: As populations near the carrying capacity,
population growth is described as logistic.
• #4: Many factors that regulate population growth are
density dependent
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Population ecology
•study of populations in relation to their environment
•Influenced by
i.
ii.
iii.
iv.
Density
Distribution
Age structure
Population size
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Key Point #1: Population density, dispersion, and
demographics are dynamic (ever changing).
• 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
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Population ecology
•study of populations in relation to their environment
•Influenced by
i.
ii.
iii.
iv.
Density
Distribution
Age structure
Population size
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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
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Population Density
Tokyo: 9,499 persons/mi2
5.1 Population
Growth Notes
Seattle: 6,715 persons/mi2
Unit #5: Interconnected
22
US Population Density
5.1 Population
Growth Notes
Unit #5: Interconnected
23
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
APPLICATION
Hector’s dolphins
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• 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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Figure 53.3
Births
Births and immigration
add individuals to
a population.
Immigration
Deaths
Deaths and emigration
remove individuals
from a population.
Emigration
Figure 53.3
Births
Births and immigration
add individuals to
a population.
Immigration
Deaths
Deaths and emigration
remove individuals
from a population.
Emigration
Population ecology
•study of populations in relation to their environment
•Influenced by
i.
ii.
iii.
iv.
Density
Distribution
Age structure
Population size
© 2011 Pearson Education, Inc.
Figure 53.UN01
Patterns of dispersion
Clumped
Uniform
Random
Patterns of Dispersion
(a) Clumped
• Individuals gather in patches
• May be influenced by resource availability and
behavior
(b) Uniform
•
•
Even distribution of individuals
May be influenced by social interactions such
as territoriality
(c) Random
•
•
Even distribution of individuals
May be influenced by social interactions such
as territoriality
Demographics
• Demography
– study of the vital statistics of a population
– Study of the change in populations over time
vs.
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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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Figure 53.5
Number of survivors (log scale)
1,000
100
Females
10
Males
1
0
2
4
6
Age (years)
8
10
• Survivorship curves can be classified into three
general types
– Type I
– Type II
– Type III
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Survivorship Curves
Type I
• low death
rates during
early and
middle life
• an increase
in death
rates among
older age
groups
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Type II
Type III
Number of survivors (log scale)
Figure 53.6
1,000
I
100
10
1
0
50
Percentage of maximum life span
100
Survivorship Curves
Type I
• low death
rates during
early and
middle life
• an increase
in death
rates among
older age
groups
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Type II
• a constant
death rate
over the
organism’s
life span
Type III
Number of survivors (log scale)
Figure 53.6
1,000
100
II
10
1
0
50
Percentage of maximum life span
100
Survivorship Curves
Type I
• low death
rates during
early and
middle life
• an increase
in death
rates among
older age
groups
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Type II
Type III
• a constant
death rate
over the
organism’s
life span
• high death
rates for the
young
• lower death
rate for
survivors
Number of survivors (log scale)
Figure 53.6
1,000
100
10
III
1
0
50
Percentage of maximum life span
100
Reproductive Rates
• For species with sexual reproduction,
demographers often concentrate on females in a
population
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Key Points
• #1: Population density, dispersion, and demographics
are dynamic (ever changing).
• #2: In ideal, unlimited environments, populations
grow exponentially
• #3: As populations near the carrying capacity,
population growth is described as logistic.
• #4: Many factors that regulate population growth are
density dependent
© 2011 Pearson Education, Inc.
Key Concept #2: In ideal, unlimited environments,
populations grow exponentially
• 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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Per Capita Rate of Increase
Change in
population
size
Immigrants
Emigrants
 Births  entering  Deaths  leaving
population
population
• If immigration and emigration are ignored, a
population’s growth rate (per capita increase)
equals birth rate minus death rate
© 2011 Pearson Education, Inc.
Figure 53.3
Births
Births and immigration
add individuals to
a population.
Immigration
Deaths
Deaths and emigration
remove individuals
from a population.
Emigration
Figure 53.3
If the initial population were 110 sparrows, how will
The factors below change the population size?
Births
Deaths
58
70
Deaths and emigration
remove individuals
from a population.
Births and immigration
add individuals to
a population.
Immigration
32
12
Emigration
A Little Population Math
• 1) The population growth rate can be expressed
mathematically as:
r=(births-deaths)
N
-N is the initial population size
- r is the reproductive growth rate
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Growth Pattern #1: Exponential Growth
• Exponential population growth is population
increase under idealized conditions
• Under these conditions, the rate of increase is at
its maximum, denoted as rmax
• The equation of exponential population growth is
dN 
dt rmaxN
• Exponential population growth results in a Jshaped curve
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Population size (N)
Figure 53.UN02
dN
= rmax N
dt
Number of generations
Figure 53.7
2,000
Population size (N)
dN
= 1.0N
dt
1,500
dN
= 0.5N
dt
1,000
500
0
5
10
Number of generations
15
• The J-shaped curve of exponential growth
characterizes some rebounding populations
– For example, the elephant population in Kruger
National Park, South Africa, grew exponentially
after hunting was banned
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Figure 53.8
Elephant population
8,000
6,000
4,000
2,000
0
1900 1910
1920
1930 1940
Year
1950
1960
1970
Key Points
• #1: Population density, dispersion, and demographics
are dynamic (ever changing).
• #2: In ideal, unlimited environments, populations
grow exponentially
• #3: As populations near the carrying capacity,
population growth is described as logistic.
• #4: Many factors that regulate population growth are
density dependent
© 2011 Pearson Education, Inc.
Key Concept #3: As populations near the carrying
capacity, population growth is described as logistic.
• 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
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Growth Pattern #2: Logistic Growth
• In the logistic population growth model, the per
capita rate of increase declines as carrying
capacity is reached
• The logistic model starts with the exponential
model and adds an expression that reduces the
per capita rate of increase as N approaches K
(K  N)
dN
 rmax N
dt
K
• The logistic model of population growth produces
a sigmoid (S-shaped) curve
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Population size (N)
Figure 53.UN03
K = carrying capacity
K–N
dN
= rmax N
K
dt
(
Number of generations
)
Figure 53.9
Exponential
growth
dN
= 1.0N
dt
Population size (N)
2,000
1,500
K = 1,500
Logistic growth
1,500 – N
dN
= 1.0N
1,500
dt
(
1,000
Population growth
begins slowing here.
500
0
0
5
10
Number of generations
15
)
The Logistic Model and Real Populations
• The growth of laboratory populations of paramecia
fits an S-shaped curve
• These organisms are grown in a constant
environment lacking predators and competitors
• Some populations overshoot K before settling
down to a relatively stable density
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Number of Daphnia/50 mL
Number of Paramecium/mL
Figure 53.10
1,000
800
600
400
200
0
0
5
10
Time (days)
15
(a) A Paramecium population in
the lab
180
150
120
90
60
30
0
0
20
40
60 80 100 120 140 160
Time (days)
(b) A Daphnia population in the lab
Figure 53.11
Key Points
• #1: Population density, dispersion, and demographics
are dynamic (ever changing).
• #2: In ideal, unlimited environments, populations
grow exponentially
• #3: As populations near the carrying capacity,
population growth is described as logistic.
• #4: Many factors that regulate population growth
are density dependent
© 2011 Pearson Education, Inc.
Key Concept #4: 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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Population Change and Population Density
Abiotic Factors
•Weather
•Fire
•Floods
•Wind and storms
•Oxygen supply
Biotic Factors
•Predation
•Infection
•Competition
•Toxic build-up
•Available mates
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1) Competition for Resources
• In crowded populations, increasing population
density intensifies competition for resources and
results in a lower birth rate
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Figure 53.17a
2) Toxic Wastes
• Accumulation of toxic wastes can contribute to
density-dependent regulation of population size
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Figure 53.17c
5 m
3) Predation
• As a prey population builds up, predators may
feed preferentially on that species
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Figure 53.17b
4) Intrinsic Factors
• For some populations, intrinsic (physiological)
factors appear to regulate population size
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Figure 53.17d
5) Territoriality
• In many vertebrates and some invertebrates,
competition for territory may limit density
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Figure 53.17e
6) Disease
• Population density can influence the health and
survival of organisms
• In dense populations, pathogens can spread more
rapidly
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Figure 53.17f
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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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, the moose population on Isle Royale
collapsed during a harsh winter, and when wolf
numbers peaked
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Figure 53.18
2,500
Wolves
Moose
40
2,000
30
1,500
20
1,000
10
500
0
1955
0
1965
1975
1985
Year
1995
2005
Number of moose
Number of wolves
50
Paramecium Competition
120
100
80
P. aurelia grown alone,
cells/mL
P. caudatum grown alone,
cells/mL
60
P. aurelia grown in mixed
culture, cells/ mL
P caudatum grown in mixed
culture, cells/mL
40
20
0
Day O Day 2 Day 4 Day 6 Day 8 Day
10
Day
12
Day
14
Day
16
Figure 53.19a
Figure 53.19
Snowshoe hare
120
9
Lynx
80
6
40
3
0
0
1850
1875
1900
Year
1925
Number of lynx
(thousands)
Number of hares
(thousands)
160
Lynx v. Rabbits: Predation
120
Population Size
100
80
60
Rabbits Left
Total Lynx
40
20
0
1
3
5
7
9
11 13 15 17 19 21 23 25
Generations
Key Points
• #1: Population density, dispersion, and demographics
are dynamic (ever changing)
• #2: In ideal, unlimited environments, populations
grow exponentially
• #3: As populations near the carrying capacity,
population growth is described as logistic.
• #4: Many factors that regulate population growth are
density dependent
© 2011 Pearson Education, Inc.
Review Questions
1. Explain why a population that fits the logistic growth
model increases more rapidly at intermediate population
size than at relatively small and large population sizes.
2. Where is exponential growth by a plant population more
likely- in an area where a forest was destroyed by fire or
in a mature, undisturbed forest? Why?
3. Describe three attributes of a habitat that could affect (+
or-) the population density (or dispersion) and rates of
emigration and immigration.
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Review Questions: Answered!
1. Explain why a population that fits the logistic growth model
increases more rapidly at intermediate population size
than at relatively small and large population sizes.
When N (population size) is small, there are relatively few
individuals producing offspring. When N is large, near the
carrying capacity, the per capita growth rate (r) is
relatively small because it is limited by available
resources.
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Review Questions: Answered!
2. Where is exponential growth by a plant population more
likely- in an area where a forest was destroyed by fire or in a
mature, undisturbed forest? Why?
Exponential growth is more likely in the area where a forest
was destroyed by fire. The first plants that found suitable
habitat there would encounter an abundance of space,
nutrients, and light. In the undisturbed forest, competition
among plants for these resources would be intense.
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Review Questions: Answered!
3. Describe three attributes of a habitat that could affect
(+ or-) the population density (or dispersion) and rates
of emigration and immigration.
Three attributes are the size, quality, and isolation of the
patches. A patch that is larger or of higher quality is
more likely to attract individuals and to be a source of
individuals for other patches.
A patch that is relatively isolated will undergo less
exchange of individuals with other patches.
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