Download CHAPTER 35 Population Dynamics

BIOLOGY
CONCEPTS & CONNECTIONS
Fourth Edition
Neil A. Campbell • Jane B. Reece • Lawrence G. Mitchell • Martha R. Taylor
CHAPTER 35
Population Dynamics
Modules 35.1 – 35.5
From PowerPoint® Lectures for Biology: Concepts & Connections
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The Spread of Shakespeare's Starlings
• In the 1800s and early 1900s, introducing
foreign species of animals and plants to North
America was a popular, unregulated activity
• In 1890, a group of Shakespeare enthusiasts
released about 120 starlings in New York's
Central Park
– It was part of a project to
bring to America every
bird species mentioned
in Shakespeare’s works
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• Today, the starling range extends from Mexico
to Alaska
• Their
population
is estimated
at well over
100 million
Current
1955
Current
1955
1945
1935
1925
1945
1905
1915
1935
1925
1925
1935
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• Over 5 million starlings have been counted in a
single roost
• Starlings are omnivorous, aggressive, and
tenacious
• They cause
destruction and
often replace
native bird species
• Attempts to
eradicate starlings
have been
unsuccessful
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• The starling population in North America has
some features in common with the global
human population
– Both are expanding and are virtually
uncontrolled
– Both are harming other species
• Population ecology is concerned with
changes in population size and the factors
that regulate populations over time
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35.1 Populations are defined in several ways
• Ecologists define a population as a singlespecies group of individuals that use common
resources and are regulated by the same
environmental factors
– Individuals in a population have a high
likelihood of interacting and breeding with one
another
• Researchers must define a population by
geographic boundaries appropriate to the
questions being asked
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POPULATION STRUCTURE AND DYNAMICS
35.2 Density and dispersion patterns are important
population variables
• Population density = # of individuals in a
given area or volume
• It is sometimes possible to count all the
individuals in a population
– More often, density is estimated by sampling
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• One useful sampling technique for estimating
population density is the mark-recapture
method
Figure 35.2A
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• The dispersion pattern of a population refers
to the way individuals are spaced within
their area
– Clumped, Uniform (even), Random
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• Clumped dispersion = a pattern in which
individuals are aggregated in patches
– This is the
most common
dispersion
pattern in nature
– It often results
from an unequal
distribution of
resources in the
environment
Figure 35.2B
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Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
• A uniform pattern of dispersion often results
from interactions among individuals of a
population
– Territorial behavior and competition for water
are examples of such interactions
Figure 35.2C
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• Random dispersion is characterized by
individuals in a population spaced in a
patternless, unpredictable way
– Example: clams living in a mudflat
– Environmental conditions and social
interactions make random dispersion rare
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How Does Population Size Change?
• Several processes can change the size of
populations
– Birth and immigration add individuals to a
population
– Death and emigration remove individuals
from the population
How Does Population Size Change?
• Change in population size
= (births – deaths) + (immigrants – emigrants)
How Does Population Size Change?
• Ignoring migration, population size is
determined by two opposing forces
1. Biotic potential: the maximum rate at
which a population could increase when
birth rate is maximal and death rate
minimal
2. Environmental resistance: limits set
by the living and nonliving environment
that decrease birth rates and/or increase
death rates (examples: food, space, and
predation
Population Growth
• The growth rate (r) of a population is the
change in the population size per individual
over some time interval
• Determined by
Growth rate (r) = birth rate (b) – death rate (d)
Ex.
In a pop. of 1000, 150 births and 50 deaths
r=0.15 – 0.05
r=0.1 or 10% growth per year
Population Growth
• Population growth per unit of time can be
calculated by multiplying growth rate (r) by
the original population size (N)
Population growth (G) = rN
• In the previous example, population growth
= rN = 0.1(1000) = 100, so the population
has grown by one hundred individuals
35.3 Idealized models help us understand
population growth
• Idealized models describe two kinds of
population growth
– exponential growth
– logistic growth
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• Exponential growth = the accelerating
increase that occurs during a time when growth
is unregulated
• A J-shaped growth curve, described by
the equation G = rN, is typical of exponential
growth
– G = the population growth rate
– r = the intrinsic rate of increase, or an
organism's maximum capacity to reproduce
– N = the population size
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Figure 35.3A
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Exponential Growth
• Exponential growth cannot continue
indefinitely
• All populations that exhibit exponential
growth must eventually stabilize or crash
Exponential Growth
• Exponential growth can be observed in
populations that undergo boom-and-bust
cycles
– Periods of rapid growth followed by a sudden
massive die-off
Exponential Growth
• Boom-and-bust cycles can be seen in
short lived, rapidly reproducing species
– Ideal conditions encourage rapid growth
– Deteriorating conditions encourage massive
die-off
Exponential Growth
• Example
– Each year cyanobacteria in a lake may
exhibit exponential growth when conditions
are ideal, but crash when they have depleted
their nutrient supply
Cyanobacteria
Abiotic factors may limit
many natural populations
Exponential Growth
• Example
– Lemming cycles are more complex and
involve overgrazing of food supply, large
migrations, and massive mortality caused by
predators and starvation
– About every 10 years, both hare and lynx populations
have a rapid increase (a "boom") followed by a sharp
decline (a "bust")
• Population cycles may also
result from a time lag in the
response of predators to
rising prey numbers
Figure 35.5
Exponential Growth
• Temporary exponential growth can occur
when population-controlling factors are
relaxed, such as
– When food supply is increased
– When predators are reduced
Exponential Growth
• When exotic species are introduced into
a new ecosystem, population numbers
may explode due to lack of natural
predators
Exponential Growth
• When species are protected, e.g. the
whooping crane population has grown
exponentially since they were protected
from hunting and human disturbance in
1940
Environmental Resistance
• Many populations that exhibit exponential
growth eventually stabilize
• Environmental resistance limits population
growth
– As resources become depleted, reproduction
slows
Environmental Resistance
• This growth pattern, where populations
increase to the maximum number
sustainable by their environment, is called
logistic growth
• When this growth pattern is plotted, it
results in an S-shaped growth curve (or Scurve)
Environmental Resistance
• Carrying capacity (K) is the maximum
population size that can be sustained by
an ecosystem for an extended time
without damage to the ecosystem
Environmental Resistance
• Logistic population growth can occur in nature
when a species moves into a new habitat, e.g.
barnacles colonizing bare rock along a rocky
ocean shoreline
• Initially, new settlers may find ideal conditions
that allow their population to grow almost
exponentially
• As population density increases, individuals
compete for space, energy, and nutrients
Environmental Resistance
• In nature, conditions are never completely
stable, so both K and the population size
will vary somewhat from year to year
• However, environmental resistance ideally
maintains populations at or below the
carrying capacity of their environment
Environmental Resistance
• These forms of environmental resistance
can reduce the reproductive rate and
average life span and increase the death
rate of young
• As environmental resistance increases,
population growth slows and eventually
stops
Environmental Resistance
• If a population far exceeds the carrying
capacity, excess demands decimate
crucial resources
• This can permanently and severely
reduce K, causing the population to
decline to a fraction of its former size or
disappear entirely
Environmental Resistance
• Example: Pribilof Island reindeer
populations
Environmental Resistance
• Environmental resistance can be
classified into two broad categories
– Density-independent factors
– Density-dependent factors
Density-Independent Factors
• Density-independent factors limit
populations regardless of their density
– Examples: climate, weather, floods, fires,
pesticide use, pollutant release, and
overhunting
Density-Independent Factors
• Some species have evolved means of
limiting their losses
– Examples: seasonally migrating to a better
climate or entering a period of dormancy
when conditions deteriorate
Density-Dependent Factors
• Density-dependent factors become
more effective as population density
increases
• Exert negative feedback effect on
population size
Density-Dependent Factors
• Density-dependent factors can cause
birth rates to drop and/or death rates to
increase
– Population growth slows resulting in an Sshaped growth curve (or S-curve)
Density-Dependent Factors
• At carrying capacity, each individual's
share of resources is just enough to allow
it to replace itself in the next generation
• At carrying capacity birth rate (b) = death
rate (d)
Density-Dependent Factors
• Carrying capacity is determined by the
continuous availability of resources
Density-Dependent Factors
• Include community interactions
– Predation
– Parasitism
– Competition
Predation
• Predation involves a predator killing a
prey organism in order to eat it
– Example: a pack of grey wolves hunting an
elk
Predation
• Predators exert density-dependent
controls on a population
– Increased prey availability can increase birth
rates and/or decrease death rates of
predators
• Prey population losses will increase
Predation
• There is often a lag between prey
availability and changes in predator
numbers
– Overshoots in predator numbers may cause
predator-prey population cycles
– Predator and prey population numbers
alternate cycles of growth and decline
Predation
• Predation may maintain prey populations
near carrying capacity
– “Surplus" animals are weakened or more
exposed
Predation
• Predation can also maintain prey
populations well below carrying capacity
– Example: the cactus moth used to control
exotic prickly pear in Australia
Parasitism
• Parasitism involves a parasite living on
or in a host organism, feeding on it but
not generally killing it
– Examples: bacterium causing Lyme disease,
some fungi, intestinal worms, ticks, and some
protists
Parasitism
• While parasites seldom directly kill their
hosts, they may weaken them enough that
death due to other causes is more likely
• Parasites spread more readily in large
populations
Competition for Resources
• Competition
– Describes the interaction among individuals
who attempt to utilize a resource that is
limited relative to the demand for it
Competition for Resources
• Competition intensifies as populations
grow and near carrying capacity
• For two organisms to compete, they must
share the same resource(s)
Competition for Resources
• Competition may be divided into two
groups based on the species identity of
the competitors
– Interspecific competition is between
individuals of different species
– Intraspecific competition is between
individuals of the same species
Competition for Resources
• Intense local competition may drive
organisms to emigrate
– Example: swarming in locusts
Factors Interact
• The size of a population at any given time
is the result of complex interactions
between density-independent and densitydependent forms of environmental
resistance
• Logistic growth = slowed by populationlimiting factors
– It tends to level off at
carrying capacity
– Carrying capacity =
the maximum
population size
that an environment
can support at a
particular time
with no degradation
to the habitat
Figure 35.3B
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• The equation G = rN(K - N)/K describes a
logistic growth curve
– K = carrying capacity
– The term
(K - N)/K
accounts
for the
leveling
off of the
curve
Figure 35.3C
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• The logistic growth model predicts that
– a population's growth rate will be low when
the population size is either small or large
– a population’s growth rate will be highest
when the population is at an intermediate
level relative to the carrying capacity
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35.4 Multiple factors may limit population growth
Review:
• The regulation of growth in a natural
population is determined by several factors
– limited food supply (competition)
– the buildup of toxic wastes
– increased disease (parasitism)
– predation
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LIFE HISTORIES AND THEIR EVOLUTION
35.6 Life tables track mortality and survivorship in
populations
• Life tables and survivorship curves predict an
individual's statistical chance of dying or
surviving during each interval in its life
• Life tables predict how long, on average, an
individual of a given age can expect to live
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– This table was compiled using 1995 data from
the U.S. Centers for Disease Control
Table 35.6
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• Survivorship curves plot the proportion of
individuals alive at each age
• Three types of survivorship curves reflect
important species differences in life history
Figure 35.6
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Survivorship in Populations
• "Late loss" curves: seen in many animals
with few offspring that receive substantial
parental care; are convex in shape, with low
mortality until individuals reach old age
– Examples: humans and many large mammals
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Survivorship in Populations
• "Constant loss" curves: an approximate
straight line, indicates an equal chance of
dying at any age
– Example: some bird species
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Survivorship in Populations
• "Early loss" curves: high early mortality as
most offspring fail to become established; are
concave in shape
– Typical of most plants and many animals that
do not receive parental care
– Examples: most invertebrates and fish
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35.7 Evolution shapes life histories
• An organism's life history is the series of events
from birth through reproduction to death
• Life history traits (which influence the biotic
potential) includes
– the age at which reproduction first occurs
– the frequency of reproduction
– the number of offspring
– the amount of parental care given
– the energy cost of reproduction
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Biotic Potential
(1) The age at which the organism first
reproduces
– Populations that have their offspring earlier
in life tend to grow at a faster rate
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Biotic Potential
(2) The frequency at which reproduction
occurs
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Biotic Potential
(3) The average number of offspring produced
each time
(4) The length of the organism's reproductive
life span
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Biotic Potential
(5) The death rate of individuals
– Increased death rates can slow the rate of
population growth significantly
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• The effects of
predation on
life history
traits of
guppies has
been tested
by field
experiments
for several
years
Experimental
transplant of
guppies
Predator: Killifish;
preys mainly on
small guppies
Guppies:
Larger at
sexual maturity
than those in “pike-cichlid”
pools
Predator: Pike-cichlid;
preys mainly on large
guppies
Guppies: Smaller at
sexual maturity than
those in “killifish” pools
Figure 35.7A
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• In nature, every population has a particular life
history adapted to its environment
• The agave illustrates
what ecologists call
"big-bang
reproduction"
– It is able to store
nutrients until
environmental
conditions favor
reproductive success
Figure 35.7B
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• Natural selection favors a combination of life
history traits that maximizes an individual's
output of viable, fertile offspring
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• Selection for life history traits that maximize
reproductive success in uncrowded,
unpredictable environments is called
r-selection
– Such populations maximize r, the intrinsic rate
of increase
– Individuals of these populations mature early
and produce a large number of offspring at a
time
– Many insect and weed species exhibit
r-selection
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• Selection for life history traits that maximize
reproductive success in populations that live at
densities close to the carrying capacity (K) of
their environment is called K-selection
– Individuals mature and reproduce at a later
age and produce a few, well-cared-for
offspring
– Mammals exhibit K-selection
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THE HUMAN POPULATION
35.8 Connection: The human population has been
growing exponentially for centuries
• The human population as a whole has doubled
three times in the last three centuries
• The human population now stands at about 6.1
billion and may reach 9.3 billion by the year
2050
• Most of the increase is due to improved health
and technology
– These have affected death rates
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• The history of human population growth
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• The ecological footprint represents the amount
of productive land needed to support a nation’s
resource needs
• The ecological capacity of the world may
already be smaller than its ecological
footprint
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• Ecological footprint in relation to ecological
capacity
Figure 35.8B
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• The exponential growth of the human
population is probably the greatest crisis ever
faced by life on Earth
Figure 35.8C
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35.9 Birth and death rates and age structure affect
population growth
• Population stability is achieved when there is
zero population growth
– Zero population growth is when birth rates
equal death rates
• There are two possible ways to reach zero
population growth (ZPG)
– ZPG = High birth rates - high death rates
– ZPG = Low birth rates - low death rates
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• The demographic transition is the shift from
high birth and death rates to low birth and
death rates
– During this
transition,
populations
may grow
rapidly until
birth rates
decline
Figure 35.9A
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• The age structure of a population is the
proportion of individuals in different agegroups
– Age structure affects population growth
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RAPID GROWTH
SLOW GROWTH
ZERO GROWTH/DECREASE
Kenya
United States
Italy
Male
Female
Male
Female
Ages 45+
Ages 45+
Ages 15–44
Ages 15–44
Under
15
Percent of population
Male
Female
Under
15
Percent of population
Percent of population
Figure 35.9B
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• Age-structure diagrams not only reveal a
population's growth trends
– They also indicate social conditions
• Increasing the status and education of
women may help to reduce family size
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35.10 Connection: Principles of population ecology
have practical applications
• Principles of population ecology may be used
to
– manage wildlife, fisheries, and forests for
sustainable yield
– reverse the decline of threatened or
endangered species
– reduce pest populations
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• Renewable resource management is the
harvesting of crops without damaging the
resource
– However, human economic and political
pressures often outweigh ecological concerns
– There is frequently insufficient scientific
information
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• The collapse of the northern 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
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• Collapse of northern cod fishery
Figure 35.10A
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• For species that are in decline or facing
extinction, resource managers try to increase
population size
• Carrying capacity is usually increased by
providing additional habitat or improving
the quality of existing habitat
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• Endangered species
often have subtle
habitat requirements
– The red-cockaded
woodpecker was
recently recovered
from near-extinction
by protecting its pine
habitat and using
controlled fires to
reduce undergrowth
Figure 35.10B
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• Integrated pest management (IPM) uses a
combination of biological, chemical, and
cultural methods to control agricultural pests
• IPM relies on knowledge of
– the population ecology of the pest
– its associated predators and parasites
– crop growth dynamics
• One objective of IPM is to minimize
environmental and health risks by relying on
natural biological control when possible
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