Download Population Ecology

Chapter 53
Population Ecology
PowerPoint Lectures for
Biology, Eighth Edition
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
and Janette Lewis
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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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Slide 2 of 67
Figure 53.1 What causes a sheep population to
fluctuate in size?
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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
• 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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Density and Dispersion
• Density is the number of individuals per unit
area or volume
– 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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Slide 5 of 67
Density and Dispersion
• Dispersion is the pattern of spacing among
individuals within the boundaries of the
population
– Clumped: individuals aggregate in patches
– Uniform: individuals are evenly distributed
(territoriality)
– Random: position of each individual is
independent of other individuals
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Slide 6 of 67
Figure 52.3 Patterns of dispersion within a
population’s geographic range
(a) Clumped.
(b) Uniform.
(c) Random.
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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, or the mark-recapture method
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Slide 8 of 67
Figure 53.2 Determining population size using the
mark-recapture method
APPLICATION
Hector’s dolphins
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Figure 53.3 Population dynamics
Births
Births and immigration
add individuals to
a population.
Immigration
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Deaths
Deaths and emigration
remove individuals
from a population.
Emigration
Slide 10 of 67
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
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Slide 11 of 67
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
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Slide 12 of 67
Table 53-1
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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Slide 14 of 67
Number of survivors (log scale)
Figure 53.5 Survivorship curves for male and
female Belding’s ground squirrels
1,000
100
Females
10
Males
1
0
2
4
6
Age (years)
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8
10
Survivorship Curves
• 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.
(Humans, large mammals)
– Type II: the death rate is constant over the
organism’s life span. (Squirrels)
– Type III: high death rates for the young, then a
slower death rate for survivors. (Fish, clams,
insects)
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Slide 16 of 67
Number of survivors (log scale)
Figure 53.6 Idealized survivorship curves: Types I,
II, and III
1,000
I
100
II
10
III
1
0
50
Percentage of maximum life span
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100
Concept 53.2: 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
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Life History Diversity
• Life histories are very diverse, two contrasting
types:
1. Species that exhibit semelparity, or big-bang
reproduction, reproduce once and die
• Examples: salmon, agave plant
2. 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
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Slide 19 of 67
Concept 53.3: The exponential model describes
population growth in an idealized, unlimited
environment
• Exponential population growth model
– 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
• Population growth rate = Per capita rate of
increase
– If immigration and emigration are ignored, a
population’s growth rate equals birth rate minus
death rate during a time interval
– N = population size
r=b–d
N
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Slide 21 of 67
Per Capita Growth Rate
•
For a population sample size of 1000 (N =
1000) there were 60 births and 10 deaths
over a one year period. What is the growth
rate?
A. .5 per individual per year
B. .05 per individual per year
C. .005 per individual per year
60-10/1000 = .05
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Slide 22 of 67
Population Growth Rate
• Per capita birth and death rates
– The average # of birth (b) or deaths (m for
mortality) per individual per unit time
– If there are 34 births per year in a population of
1000,
• b = 0.034
– If the death rate in a population is 16 deaths per
year,
• m= 0.016
– Zero population growth occurs when the birth
rate equals the death rate
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Slide 23 of 67
• Most ecologists use differential calculus to
express population growth as growth rate at a
particular instant in time:
– per capita birth rate - per capita death rate =
rN: per capita rate of change in a population
size
ΔN =
Δt rN
where N = population size, t = time, and r = per capita
rate of increase = birth – death
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Slide 24 of 67
Exponential Growth
• Exponential population growth is population
increase under idealized conditions
– Under these conditions, the rate of reproduction
is at its maximum, called the intrinsic rate of
increase
– Equation of exponential population growth:
dN =
dt rmaxN
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Slide 25 of 67
Figure 53.10 Exponential Population Growth
• Exponential population growth results in a Jshaped curve
Population size (N)
2,000
dN
1.0N
dt =
1,500
dN
0.5N
dt =
1,000
500
0
0
5
10
Number of generations
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15
Slide 26 of 67
Exponential Population Growth Model
• Populations with a high rN (per capita rate of
increase) will have a steeper J-shaped curve than
ones with a lower value
• The J-shaped curve of exponential growth
characterizes some rebounding populations
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Slide 27 of 67
Figure 53.11 Exponential growth in the African
elephant population of Kruger National Park
Elephant population
8,000
6,000
4,000
2,000
0
1900
1920
1940
Year
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1960
1980
Concept 53.4: The logistic model describes how a
population grows more slowly as it nears its
carrying capacity (K)
• The logistical growth model
– 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
• Influenced by many factors such as: available
water and food, predators, soil nutrients, suitable
nesting sites
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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
(K − N)
dN
= rmax N
dt
K
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Slide 30 of 67
Population growth predicted by the logistic model
Population size (N)
• The logistic model of population growth
produces a sigmoid (S-shaped) curve
Exponential
growth
dN 1.0N
=
2,000
dt
1,500
K = 1,500
Logistic growth
1,000
dN 1.0N
dt =
1,500 – N
1,500
500
0
0
5
10
Number of generations
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15
Slide 31 of 67
The Logistic Model and Real Populations
Number of Paramecium/mL
• The growth of laboratory populations of
paramecia fits an S-shaped curve if the
experimenter maintains a constant environment.
1,000
800
600
400
200
0
0
5
10
Time (days)
15
A Paramecium population in the lab
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Slide 32 of 67
The Logistic Model and Real Populations
Number of Daphnia/50 mL
• Some populations overshoot K before settling
down to a relatively stable density
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
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Slide 33 of 67
The Logistic Model and Real Populations
• Some populations fluctuate greatly and make it
difficult to define K
– Some populations have regular cycles of boom
and bust
– Some populations show an Allee effect, in which
individuals have a more difficult time surviving
or reproducing if the population size is too small
– The logistic model fits few real populations but
is useful for estimating possible growth
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Slide 34 of 67
The Logistic Model and Real Populations
80
60
Number of females
• Some populations
fluctuate greatly
around K: not
well described by
the logistical
model
40
20
0
1975
1980
1985
1990
1995
2000
Time (years)
(c) A song sparrow population in its natural habitat. The
population of female song sparrows nesting on Mandarte Island,
British Columbia, is periodically reduced by severe winter
weather, and population growth is not well described by the
logistic model.
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Slide 35 of 67
The Logistic Model and Real Populations
160
120
Lynx
9
80
6
40
3
0
1850
Figure 52.21
Snowshoe hare
Lynx population size
(thousands)
Hare population size
(thousands)
• Many populations undergo regular boom-andbust cycles
0
1875
1900
Year
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1925
Slide 36 of 67
Figure 53.14 White rhinoceros mother and calf
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Slide 37 of 67
The Logistic Model and Life Histories
• Life history traits favored by natural selection
may vary with population density and
environmental conditions
– K-selection, or density-dependent selection,
selects for life history traits that are sensitive to
population density
– r-selection, or density-independent selection,
selects for life history traits that maximize
reproduction
– The concepts of K-selection and r-selection are
somewhat controversial and have been criticized by
ecologists as oversimplifications
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Slide 38 of 67
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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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
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Slide 40 of 67
Fig. 53-15
Birth or death rate
per capita
Density-dependent
birth rate
Density-dependent
birth rate
Densitydependent
death rate
Equilibrium
density
Equilibrium
density
Population density
(a) Both birth rate and death rate vary.
Birth or death rate
per capita
Densityindependent
death rate
Densityindependent
birth rate
Density-dependent
death rate
Equilibrium
density
Population density
(c) Death rate varies; birth rate is constant.
Population density
(b) Birth rate varies; death rate is constant.
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, accumulation of toxic wastes, and
intrinsic factors
– Example: Cheetahs are highly territorial, using
chemical communication to warn other cheetahs
of their boundaries
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Slide 42 of 67
Gannets defend their territory by pecking others
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Slide 43 of 67
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
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Slide 44 of 67
Immigration, Emigration, and Metapopulations
• Metapopulations are groups of populations
linked by immigration and emigration
– High levels of immigration combined with
higher survival can result in greater stability in
populations
Aland
˚ Islands
EUROPE
5 km
Occupied patch
Unoccupied patch
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Slide 45 of 67
Concept 53.6: The human population is no longer
growing exponentially but is still increasing rapidly
• Human population growth
– No population can grow indefinitely, and
humans are no exception
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The Global Human Population
7
6
5
4
3
2
The Plague
8000
B.C.E.
4000 3000 2000 1000
B.C.E. B.C.E. B.C.E. B.C.E.
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0
1000
C.E.
1
2000
C.E.
0
Human population (billions)
• The human population increased relatively
slowly until about 1650 and then began to grow
exponentially
Slide 47 of 67
Human Population Growth Rate
• Though the global population is still growing, the
rate of growth began to slow during the 1960s
Annual percent increase
2.2
2.0
1.8
1.6
1.4
2005
1.2
Projected
data
1.0
0.8
0.6
0.4
0.2
0
1950
1975
2000
Year
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2025
2050
Slide 48 of 67
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
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Slide 49 of 67
Birth or death rate per 1,000 people
Figure 53.24 Demographic transition in Sweden
and Mexico, 1750–2025 (data as of 2005)
50
40
30
20
10
Sweden
Birth rate
Death rate
0
1750
1800
Mexico
Birth rate
Death rate
1850
1900
Year
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1950
2000
2050
• 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
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Slide 51 of 67
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
– Commonly represented in pyramids
– Male/female represented by color
– Percent of population at each age is represented
by a horizontal bar. Young at bottom, elderly at
top.
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Slide 52 of 67
Figure 53.25 Age-structure pyramids for the human
population of three countries (data as of 2005)
Rapid growth
Afghanistan
Male
10 8
Female
6 4 2 0 2 4 6
Percent of population
Slow growth
United States
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 10
8
Male
Female
6 4 2 0 2 4 6
Percent of population
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No growth
Italy
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
8
Male
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
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Slide 54 of 67
50
Life expectancy (years)
Infant mortality (deaths per 1,000 births)
80
60
40
30
20
60
40
20
10
0
0
Indus- Less industrialized
trialized
countries countries
Indus- Less industrialized
trialized
countries countries
Estimates of Global Carrying Capacity
• The carrying capacity of Earth for humans is
uncertain
• The average estimate is 10–15 billion
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Slide 56 of 67
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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Slide 57 of 67
Figure 53.27 The amount of photosynthetic
products that humans use around the world
Log (g carbon/year)
13.4
9.8
5.8
Not analyzed
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• A population of 500 experiences 55 births
and 5 deaths during a one year period. What
is the reproductive (growth) rate for the
population during the one-year period?
A. .01/ year
B. .05/year
C. .1/year
D. 50/year
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Slide 59 of 67
• For a population sample size of 1000 (N =
1000) there were 60 births and 10 deaths
over a one year period. If the population
maintains the current growth pattern, a plot
of its growth would resemble
A. exponential growth
B. K-selected growth
C. ZPG (zero population growth)
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Slide 60 of 67
You should now be able to:
1. Define and distinguish between the following
sets of terms: density and dispersion; clumped
dispersion, uniform dispersion, and random
dispersion; life table and reproductive table;
Type I, Type II, and Type III survivorship
curves; semelparity and iteroparity; r-selected
populations and K-selected populations
2. Explain how ecologists may estimate the
density of a species
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Slide 61 of 67
3. Explain how limited resources and trade-offs
may affect life histories
4. Compare the exponential and logistic models of
population growth
5. Explain how density-dependent and densityindependent factors may affect population
growth
6. Explain how biotic and abiotic factors may
work together to control a population’s growth
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Slide 62 of 67
Fig. 53-UN1
Patterns of dispersion
Clumped
Uniform
Random
Population size (N)
Fig. 53-UN2
dN
= rmax N
dt
Number of generations
Population size (N)
Fig. 53-UN3
K = carrying capacity
K–N
dN
= rmax N
dt
K
Number of generations
Fig. 53-UN4
Fig. 53-UN5