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Chapter 19
Population Ecology
PowerPoint® Lectures for
Campbell Essential Biology, Fourth Edition
– Eric Simon, Jane Reece, and Jean Dickey
Campbell Essential Biology with Physiology, Third Edition
– Eric Simon, Jane Reece, and Jean Dickey
Lectures by Chris C. Romero, updated by Edward J. Zalisko
© 2010 Pearson Education, Inc.
Biology and Society:
Multiplying Like Rabbits
• In 1859, 12 pairs of European rabbits were released on a ranch in
southern Australia.
• By 1865, 20,000 rabbits were killed on just that one ranch.
• By 1900, several hundred million rabbits were distributed over
most of the continent.
• The European rabbits
– Destroyed farming and grazing land
– Promoted soil erosion
– Made grazing treacherous for cattle and sheep
– Competed directly with native marsupials
© 2010 Pearson Education, Inc.
Figure 19.00
• The European red fox
– Was introduced to control the rabbits
– Spread across Australia
– Ate several species of native birds and small mammals to extinction
– Had little impact on the rabbit population
• Today, rabbits, foxes, and a long list of other non-native animal
and plant species are
– Still entrenched in Australia
– Damaging the environment
– Costing hundreds of millions of dollars in economic losses
© 2010 Pearson Education, Inc.
© 2010 Pearson Education, Inc.
AN OVERVIEW OF POPULATION ECOLOGY
• A population is a group of individuals of a single species that
occupy the same general area.
• Population ecology is the study of factors that affect population:
– Density
– Structure
– Size
– Growth rate
• Population ecology is used to study
– How to develop sustainable fisheries
– How to control pests and pathogens
– Human population growth
© 2010 Pearson Education, Inc.
Figure 19.1
Population Density
• Population density is the number of individuals of a species per
unit of area or volume. Examples include
– The number of largemouth bass per cubic kilometer (km3) of a lake
– The number of oak trees per square kilometer (km2) in a forest
– The number of nematodes per cubic meter (m3) in a forest’s soil
• How do we measure population density?
– In most cases, it is impractical or impossible to count all individuals in a
population.
– In some cases, population densities are estimated by indirect indicators,
such as
– Number of bird nests
– Rodent burrows
© 2010 Pearson Education, Inc.
Figure 19.2
Population Age Structure
• The age structure of a population is the distribution of
individuals among age groups.
• The age structure of a population provides insight into
– The history of a population’s survival
– Reproductive success
– How the population relates to environmental factors
© 2010 Pearson Education, Inc.
11
10
9
Age (years)
8
7
6
5
4
3
2
1
0
10
20
30
40
50
Percent of population
Figure 19.3
Life Tables and Survivorship Curves
• Life tables
– Track survivorship
– Help to determine the most vulnerable stages of the life cycle
• Survivorship curves
– Graphically represent some of the data in a life table
– Are classified based upon the rate of mortality over the life span of an
organism
© 2010 Pearson Education, Inc.
Table 19.1
Percentage of survivors (log scale)
100
I
10
II
1
III
0.1
0
50
100
Percentage of maximum life span
Figure 19.4
Life History Traits as Evolutionary Adaptations
• An organism’s life history is the set of traits that affect the
organism’s schedule of reproduction and survival
• Key life history traits are the
– Age at first reproduction
– Frequency of reproduction
– Number of offspring
– Amount of parental care provided
• Organisms with an opportunistic life history
– Take immediate advantage of favorable conditions
– Typically exhibit a Type III survivorship curve
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• Organisms with an equilibrial life history
– Reach sexual maturity slowly
– Produce few, well cared for offspring
– Are typically large-bodied and longer lived
– Typically exhibit a Type I survivorship curve
© 2010 Pearson Education, Inc.
Table 19.2
Dandelions have an opportunistic life history
Table 19.2a
Elephants have an equilibrial life history
Table 19.2b
POPULATION GROWTH MODELS
• Population size fluctuates as individuals
– Are born
– Immigrate into an area
– Emigrate away
– Die
• Exponential population growth describes the expansion of a
population in an ideal and unlimited environment.
• Exponential growth explains how
– A few dozen rabbits can multiple into millions
– In certain circumstances following disasters, organisms that have
opportunistic life history patterns can rapidly recolonize a habitat
© 2010 Pearson Education, Inc.
500
450
Population size (N)
400
350
300
250
200
150
100
50
0
0
1
2
3
4
5
6
7
8
9 10 11 12
Time (months)
Figure 19.5
Figure 19.5a
The Logistic Growth Model: The Reality of a
Limited Environment
• Limiting factors
– Are environmental factors that hold population growth in check
– Restrict the number of individuals that can occupy a habitat
• The carrying capacity is the maximum population size that a
particular environment can sustain.
• Logistic population growth occurs when the growth rate
decreases as the population size approaches carrying capacity.
• The carrying capacity for a population varies, depending on
– The species
– The resources available in the habitat
© 2010 Pearson Education, Inc.
Breeding male fur seals
(thousands)
10
8
6
4
2
0
1915
1925
1935
1945
Year
Figure 19.6
Figure 19.6a
• Organisms exhibiting equilibrial life history patterns occur in
environments where the population size is at or near carrying
capacity.
• The logistic model and the exponential model are theoretical
ideals of population growth.
• No natural population fits either one perfectly.
© 2010 Pearson Education, Inc.
Number of individuals
Exponential growth
Carrying
capacity
Logistic growth
0
Time
Figure 19.7
Regulation of Population Growth
Density-Dependent Factors
• The logistic model is a description of intraspecific competition,
competition between individuals of the same species for the same
limited resources.
• As population size increases
– Competition becomes more intense
– The growth rate declines in proportion to the intensity of competition
• A density-dependent factor is a population-limiting factor
whose effects intensify as the population increases in density.
© 2010 Pearson Education, Inc.
Average clutch size
12
11
10
9
8
0
10
20
30
40
50
60
70
80
90
Number of breeding pairs
(a) Decreasing birth rate with increasing density in a
population of great tits
Figure 19.8a
100
Survivors (%)
80
60
40
20
0
20
40
60
80
100
120
Density (beetles/0.5 g flour)
(b) Decreasing survival rates with increasing density in a
population of flour beetles
Figure 19.8b
• Density-dependent factors may include
– Accumulation of toxic wastes
– Limited food supply
– Limited territory
© 2010 Pearson Education, Inc.
Figure 19.9
Density-Independent Factors
• Density-independent factors
– Are population-limiting factors whose intensity is unrelated to population
density
– Include abiotic factors such as: fires, floods, storms, etc…
• In many natural populations, density-independent factors limit
population size before density-dependent factors become
important.
• Over the long term, most populations are probably regulated by a
mixture of both
© 2010 Pearson Education, Inc.
Sudden
decline
Number of aphids
Exponential
growth
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Figure 19.10
Population Cycles
• Some populations have regular boom-and-bust cycles
characterized by periods of rapid growth followed by steep
population declines.
• A well studied example of boom and bust cycles are the cycles of
snowshoe hares and lynxs
• The cause of these hare and lynx cycles may be
– Winter food shortages for the hares
– Overexploitation of hares by lynx
– Or a combination of both
© 2010 Pearson Education, Inc.
160
120
9
80
6
40
3
0
0
1850
Lynx population (thousands)
Hare population (thousands)
Lynx
Snowshoe hare
1900
Year
Figure 19.11
APPLICATIONS OF POPULATION ECOLOGY
• Population ecology is used to
– Increase populations of organisms we wish to harvest
– Decrease populations of pests
– Save populations of organisms threatened with extinction
• The U.S. Endangered Species Act defines
– An endangered species as one that is in danger of extinction throughout
all or a significant portion of its range
– A threatened species as one that is likely to become endangered in the
foreseeable future
• A major factor in population decline is habitat destruction or
modification.
© 2010 Pearson Education, Inc.
• The red-cockaded woodpecker
– Requires longleaf pine forests with clear flight paths between trees
– Suffered from fire suppression, increasing the height of the vegetation on
the forest floor
– Recovered from near-extinction to sustainable populations due to
controlled burning and other management methods
© 2010 Pearson Education, Inc.
A red-cockaded
woodpecker perches
at the entrance to its
nest in a long-leaf
pine tree.
High, dense undergrowth impedes
the woodpeckers’ access to feeding
grounds.
Low undergrowth offers birds a
clear flight path between nest sites
and feeding grounds.
Figure 19.12
Sustainable Resource Management
• According to the logistic growth model, the fastest growth rate
occurs when a population size is at roughly half the carrying
capacity.
• Theoretically, populations should be harvested down to this level,
assuming that growth rate and carrying capacity are stable over
time.
© 2010 Pearson Education, Inc.
• In the northern Atlantic cod fishery
– Estimates of cod stocks were too high
– The practice of discarding young cod (not of legal size) at sea caused a
higher mortality rate than was predicted
– The fishery collapsed in 1992 and has not recovered
© 2010 Pearson Education, Inc.
Yield (thousands of metric tons)
900
800
700
600
500
400
300
200
100
0
1960
1970
1980
1990
2000
Figure 19.13
Invasive Species
• An invasive species
– Is a non-native species that has spread far beyond the original point of
introduction
– Causes environmental or economic damage by colonizing and dominating
suitable habitats
• In the United States, invasive species cost about $137 billion a
year.
• Invasive species typically exhibit an opportunistic life history
pattern.
© 2010 Pearson Education, Inc.
• Cheatgrass
– Is an invasive species in the western United States
– Currently covers more than 60 million acres of rangeland formerly
dominated by native grasses and sagebrush
– Produces seeds earlier and in greater abundance than native species
– Forms highly flammable brush creating fires that native plants cannot
tolerate
© 2010 Pearson Education, Inc.
Figure 19.14
• European starlings
– Are another invasive species
– Were first released into New York in 1890
– Now number more than 200 million in the United States
© 2010 Pearson Education, Inc.
Figure 19.15
Biological Control of Pests
• Invasive species may benefit from the absence of
– Pathogens
– Predators
– Herbivores
• Biological control
– Is the intentional release of a natural enemy to attack a pest population
– Is used to manage an invasive species
© 2010 Pearson Education, Inc.
• In 1950, the Australian government introduced a virus lethal to
European rabbits into the rabbits’ Australian environment.
• Although initially successful, natural selection favored
– Resistant rabbits
– Variations of the virus that
– Were not fatal or
– Killed their hosts more slowly
• Evolutionary changes such as these, in which an adaptation of
one species leads to a counteradaptation in a second species, are
known as coevolution.
© 2010 Pearson Education, Inc.
Figure 19.16
The Process of Science:
Can Biological Control Defeat Kudzu?
• Kudzu
– Is an invasive Asian vine
– Covers about 12,000 square miles of the southeastern United States
– Has a range limited by cold winters
• Many strategies to control kudzu have been considered with little
success.
• A fungal pathogen called Myrothecium verrucaria appears to be a
promising candidate for biological control.
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• Observation: The fungus Myrothecium verrucaria causes severe
disease in other weeds belonging to the same family as kudzu.
• Question: Will the application of fungal spores of M. verrucaria
control an established stand of kudzu in a natural setting?
• Hypothesis: M. verrucaria treatment that was effective in small
outdoor plantings would also be most effective in a natural
setting.
• Prediction: The greatest kudzu mortality would result from the
treatment that sprayed the highest concentration of spores in
combination with a wetting agent.
• Results: The hypothesis was supported by the data, as indicated
in the following table.
© 2010 Pearson Education, Inc.
Treatment
Spores (2  106/ml)  water
Spores (2  107/ml)  water
Spores (2  106/ml)  wetting agent
Spores (2  107/ml)  wetting agent
Wetting agent only
Untreated
0
0
20
40
60
80
100
Mortality (%)
Figure 19.17
Figure 19.17a
Integrated Pest Management
• Agricultural operations create their own highly managed
ecosystems that
– Have genetically similar individuals (a monoculture)
– Are planted in close proximity to each other
– Function as a “banquet” for
– Plant-eating animals
– Pathogenic bacteria
– Viruses
• Like invasive species, most crop pests
– Have an opportunistic life history pattern
– Can cause extensive crop damage
© 2010 Pearson Education, Inc.
Figure 19.18
• Pesticides may
– Result in pesticide-resistant pests
– Kill the pest and their natural predators
– Kill pollinators
• Integrated pest management (IPM)
– Tolerates a low level of pests instead of total eradication
– Produces a sustainable control of agricultural pests
– Uses a combination of
– Biological methods
– Chemical methods
– Cultural methods
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• IPM methods include
– Using pest-resistant varieties of crops
– Using mixed-species plantings
– Rotating crops to deprive the pest of a dependable food source
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HUMAN POPULATION GROWTH
The History of Human Population Growth
• From 2000 to 500 years ago (in 1500)
– Mortality was high
– Births and deaths were about equal
– The world population held steady at about 300 million
• The world’s population began to grow exponentially in the 1900s
due to advances in
– Nutrition
– Sanitation
– Health care
© 2010 Pearson Education, Inc.
10
Population increase
80
8
60
6
40
4
Total population size
20
2
Total population (billions)
Annual increase (millions)
100
0
1500
1550
1600
1650
1700
1750
1800
1850
1900
1950
2000
2050
Year
Figure 19.19
• Worldwide population growth rates reflect a mosaic of the
changes occurring in different countries.
– In the most developed nations, the overall growth rates are near zero.
– In the developing world
– Death rates have dropped
– High birth rates persist
© 2010 Pearson Education, Inc.
Table 19.3
Age Structures
• Age structures help predict a population’s future growth.
• The following figure shows the estimated and projected age
structures of Mexico’s population in
– 1985
– 2010
– 2035
• Population momentum is the continuation of population growth
as girls in the prereproductive age group reach their reproductive
years.
© 2010 Pearson Education, Inc.
Age
1985
80
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
2010
Male
5
4
3
Female
2
1
0
1
2
3
2035
Male
4
Population in millions
Total population size  76,767,225
5
5
4
3
Male
Female
2
1
0
1
2
3
4
Population in millions
Total population size  112,468,855
5
5
4
Female
3
2
1
0
1
2
3
4
5
Population in millions
Total population size  139,457,070
Figure 19.20
• Age structure diagrams may also indicate social conditions. An
expanding population needs
– Schools
– Employment
– Infrastructure
• Trends in the age structure of the United States from 1983 to 2033
reveal important patterns.
© 2010 Pearson Education, Inc.
2008
1983
Age
Birth years
80
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
Male Female
before 1904
1904–08
1909–13
1914–18
1919–23
1924–28
1929–33
1934–38
1939–43
1944–48
1949–53
1954–58
1959–63
1964–68
1969–73
1974–78
1979–83
Birth years
Male Female
before 1929
1929–33
1934–38
1939–43
1944–48
1949–53
1954–58
1959–63
1964–68
1969–73
1974–78
1979–83
1984–88
1989–93
1994–98
1999–2003
2004–08
12 10 8 6 4 2 0 2 4 6 8 10 12
Population in millions
Total population size  234,307,207
12 10 8 6 4 2 0 2 4 6 8 10 12
Population in millions
Total population size  305,548,183
2033
Birth years
Male
Female
before 1954
1954–58
1959–63
1964–68
1969–73
1974–78
1979–83
1984–88
1989–93
1994–98
1999–2003
2004–08
2009–13
2014–18
2019–23
2024–28
2029–33
12 10 8 6 4 2 0 2 4 6 8 10 12
Population in millions
Total population size  383,116,539
Figure 19.21
Our Ecological Footprint
• An ecological footprint is an estimate of the amount of land
required to provide the raw materials an individual or a
population consumes, including:
– Food
– Fuel
– Water
– Housing
– Waste disposal
© 2010 Pearson Education, Inc.
• The ecological footprint of the United States
– Is 9.4 hectares per person
– Represents almost twice what the U.S. land and resources can support.
• The ecological impact of affluent nations is a problem of
overconsumption, not overpopulation.
• Compared to a family in rural India, Americans have an
abundance of possessions.
© 2010 Pearson Education, Inc.
Figure 19.22
• There is tremendous disparity in consumption throughout the
world.
• The world’s richest countries
– Have 20% of the global population
– Use 86% of the world’s resources
• The rest of the world
– Has 80% of the population
– Uses just 14% of global resources
© 2010 Pearson Education, Inc.
North
America
Europe
Asia
Africa
South
America
Australia
Key
> 5.4 global hectares per person
3.6–5.4 global hectares per person
1.8–3.6 global hectares per person
0.9–1.8 global hectares per person
< 0.9 global hectares per person
Insufficient data
Figure 19.23
Evolution Connection:
Humans as an Invasive Species
• Pronghorn antelope
– Roamed the open plains and shrub deserts of North America millions of
years ago
– Have a top speed of 97 km/h (60 mph)
– Are the fastest mammal on the continent
• Pronghorn speed is likely an adaptation to outrun American
cheetahs, which went extinct about 10,000 years ago.
© 2010 Pearson Education, Inc.
Figure 19.24
• About 10,000 years ago
– Changes in the biotic and abiotic environments happened too rapidly for
an evolutionary response
– Many large North American mammals, in addition to American cheetahs,
went extinct, including
– Lions
– Saber-toothed cats
– Towering short-faced bears
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• Although still hotly disputed, many scientists think that the
extinction at the end of the ice age about 10,000 years ago was in
part the result of the human invasion of North America, with
humans serving as the damaging invasive species.
• Like other invasive species, humans may change the environment
at an accelerating pace and too rapidly for other species to evolve
and survive.
© 2010 Pearson Education, Inc.