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Chapter 26
Population Growth and
Regulation
Lecture Outlines by Gregory Ahearn,
University of North Florida
Copyright © 2011 Pearson Education Inc.
Chapter 26 At a Glance
 26.1 How Does Population Size Change?
 26.2 How Is Population Growth Regulated?
 26.3 How Are Populations Distributed in Space
and Time?
 26.4 How Is the Human Population Changing?
Biology: Life on Earth, 9e
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26.1 How Does Population Size Change?
 A population consists of all the members of a
particular species that live within an ecosystem
 A community is group of interacting populations
 Communities exist within ecosystems, which include
all the living an nonliving components of a defined
geographical area
 The biosphere is the enormous ecosystem that
encompasses all of Earth’s habitable surface
 Ecology is the study of the interrelationships of
organisms with each other and with the nonliving
environment
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26.1 How Does Population Size Change?
 Population size is the outcome of opposing
forces
– Four factors determine whether and how much
the size of a population changes
–Births
–Deaths
–Migration of individuals into the population
(immigration)
–Migration of individuals out of the population
(emigration)
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26.1 How Does Population Size Change?
 Population size is the outcome of opposing
forces (continued)
– Birth and immigration add individuals to a
population
– Death and emigration remove individuals from
the population
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26.1 How Does Population Size Change?
 Population size is the outcome of opposing
forces (continued)
– A simple equation describes the change in
population size within a given time period:
–Change in population size = (births – deaths)
+ (immigrants – emigrants)
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Population Change
births
deaths
immigration
emigration
(births  deaths)  (immigrants  emigrants)
 change in population size
Biology: Life on Earth, 9e
Fig. 26-1
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26.1 How Does Population Size Change?
 Population size is the outcome of opposing forces
(continued)
– Two opposing forces that determine birth and death rates
are biotic potential and environmental resistance
– Biotic potential is the theoretical maximum rate at
which a population could increase, assuming a
maximum birth rate and minimal death rate
– Environmental resistance refers to the curbs on
population growth that are set by the living and
nonliving environment
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26.1 How Does Population Size Change?
 Population size is the outcome of opposing forces
(continued)
– Examples of environmental resistance include:
– Interactions among species, such as competition,
predation, and parasitism
– The always-limited availability of nutrients, energy,
and space
– Natural events, such as storms, fires, freezing
weather, floods, and droughts
 In nature, the interaction between biotic potential and
environmental resistance usually results in a balance
between the size of a population and the resources
available to support it
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26.1 How Does Population Size Change?
 Biotic potential can produce exponential growth
– Evolutionarily successful organisms possess
traits that make them well adapted to their
environment
– These organisms pass these inherited traits on
to as many healthy offspring as possible
– If environmental resistance is reduced,
populations can grow extremely rapidly
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26.1 How Does Population Size Change?
 Population growth is a function of the birth rate,
the death rate, and population size
– The growth rate (r) of a population, also called
the rate of natural increase, is the change in the
population size per individual per unit of time
– Growth rate is expressed by the equation:
–r (growth rate) = b (birth rate) – d (death rate)
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26.1 How Does Population Size Change?
 The growth rate of a population
– Birth rate (b) is the number of births per individual during
a specific unit of time, such as a month or a year
– For example, if there are 150 births among 1,000
individuals in a year, b = 0.15
– Death rate (d) is the number of deaths per individuals
during a specific unit of time
– For example, if there are 50 deaths among 1,000
individuals in a year, d = 0.05
– If the birth rate exceeds the death rate, the population will
grow
– If the death rate exceeds the birth rate, the population will
decline
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26.1 How Does Population Size Change?
 The growth rate of a population (continued)
– The growth rate of this population is therefore:
–r (growth rate) = 0.15 (birth rate) – 0.05 (death
rate) = 0.1 = 10% per year
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26.1 How Does Population Size Change?
 The growth rate of a population (continued)
– Population growth (G), which is the number of
individuals added to a population in a given time
period, can be calculated by multiplying growth
rate (r) by the original population size (N)
–Population growth (G) = r (growth rate) x N
(population size)
– In the previous example, population growth (rN)
= 0.1 x 1,000 = 100, so the population has grown
by one hundred individuals in the first year
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26.1 How Does Population Size Change?
 If births exceed deaths by a constant percentage,
population growth produces a J-curve
– A patter of continuously accelerating increase in
population size is called exponential growth
– Exponential growth occurs when, over a given period
of time, a population grows by a fixed percentage of
its size at the beginning of that time period
– Thus, an increasing number of individuals is added to
the population during each succeeding time period,
causing the population to grow at an accelerating
pace
– When graphed against time, a shape called a J-curve
is produced
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26.1 How Does Population Size Change?
 If births exceed deaths by a constant
percentage, population growth produces a Jcurve (continued)
– This high biotic potential evolved because it
helps ensure that, in a world filled with forces of
environmental resistance, some offspring survive
to reproduce
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26.1 How Does Population Size Change?
 Several factors influence biotic potential
– The age at which the organism first reproduces
– The frequency at which reproduction occurs
– The average number of offspring produced each
time
– The length of the organism’s reproductive life
span
– The death rate of individuals
– Increased death rates can slow the rate of
population growth significantly
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26.1 How Does Population Size Change?
 The age at which an organism first reproduces affects
the size of the future population
– For example, consider two populations of golden eagles
that are followed for 30 years
– Individuals in one population begin reproducing at the
age of 4 years
– Individuals in the other population begin reproducing
at 6 years
– Both populations will follow a J-shaped population growth
curve, but more individuals will be added to the earlier
reproducing population, resulting in a steeper increase in
population numbers
– At 30 years, the earlier reproducing population would be
10 times the size of the other population
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Exponential Growth Curves are J-Shaped
reproduce at 4 years (pop. 1)
reproduce at 6 years (pop. 2)
At 24 years,
this population
has 2,504 eagles
Number Number
of
of
Time eagles eagles
(years) (pop. 1) (pop. 2)
0
2
2
6
8
4
12
52
18
18
362
86
24
2,504
392
30
17,314
1,764
At 24 years,
this population
has 392 eagles
Fig. 26-2
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26.1 How Does Population Size Change?
 The death rate affects population size
– As long as birth rate exceeds death rate,
population size will follow a J-shaped rate of
increase
– However, the time for each population to reach a
specific number of individuals will depend upon
the magnitude of the death rate
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26.1 How Does Population Size Change?
 The death rate affects population size
(continued)
– An example would be to examine three
hypothetical bacteria populations with different
death rates
–In the bacterial colony with no deaths, it takes
3.5 hours to produce 1,500 bacteria
–In the colony with a 10% death rate, it takes 4
hours to generate 1,500 bacteria
–In the colony with a 25% death rate, it takes
5.5 hours to generate 1,500 bacteria
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The Effect of Death Rates on Population Growth
no deaths
10% death rate
25% death rate
It takes about 4
hours to produce
1,500 bacteria
It takes
about 5.5
hours to
produce
1,500
bacteria
It takes about 3.5
hours to produce
1,500 bacteria
Fig. 26-3
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26.2 How Is Population Growth Regulated?
 Exponential growth only occurs under special
conditions
– Exponential growth cannot continue indefinitely
– All populations that exhibit exponential growth
must eventually stabilize or crash
– Exponential growth can be observed in
populations that undergo boom-and-bust
cycles, in which periods of rapid population
growth are followed by a sudden, massive die-off
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26.2 How Is Population Growth Regulated?
 Exponential growth only occurs under special
conditions (continued)
– Boom-and-bust cycles can be seen in short-lived, rapidly
reproducing species, such as microbes and insects
– Seasonal populations are linked to changes in rainfall,
temperature, or nutrient availability
– Ideal conditions encourage rapid growth; deteriorating
conditions encourage massive die-off
– For example, each year, photosynthetic bacteria in a
lake may exhibit exponential growth when conditions
are ideal, but crash when they have depleted their
nutrient supply
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Boom-and-Bust Population Cycles
Nutrients are depleted, and
water temperature falls
Favorable growth
conditions occur
“boom”
“bust”
(a) A boom-and-bust cycle in photosynthetic bacteria
Fig. 26-4a
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26.2 How Is Population Growth Regulated?
 Boom-and-bust cycles can be seen in short
lived, rapidly reproducing species (continued)
– Complex factors produce four-year cycles for
small rodents, such as lemmings
–Lemming populations may grow until lack of
food, large migrations, and predators and
starvation cause sudden high mortality
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Boom-and-Bust Population Cycles
(b) Boom-and-bust cycles in a lemming population in Alaska
Fig. 26-4b
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26.2 How Is Population Growth Regulated?
 Exponential growth occurs when environmental
resistance is reduced
– In populations that do not experience boom-and-bust
cycles, exponential growth may occur temporarily under
special circumstances such as:
– When food supply is increased
– When population-controlling factors, such as
predators, are reduced
– For example, the whooping crane population has grown
exponentially since they were first protected from hunting
and human disturbance in 1940
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Exponential Growth of Wild Whooping Cranes
Fig. 26-5
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26.2 How Is Population Growth Regulated?
 Exponential growth can occur when individuals
invade a new habitat
– Invasive species are organisms with a high
biotic potential that are introduced into
ecosystems where they did not evolve and
where they encounter little environmental
resistance
– When they are introduced into a new ecosystem,
population numbers may explode due to a lack
of natural predators
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26.2 How Is Population Growth Regulated?
 Environmental resistance limits population
growth
– Many populations that exhibit exponential growth
eventually stabilize
–As resources become depleted, reproduction
slows and the growth rate would eventually
drop to zero, causing the population size to
remain constant
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26.2 How Is Population Growth Regulated?
 Environmental resistance limits population growth
(continued)
– This growth pattern, where populations increase to the
maximum number sustainable by their environment and
then stabilize, is called logistic population growth
– The maximum population size that can be sustained by
an ecosystem for an extended time without damage to
the ecosystem is called its carrying capacity (K)
– When logistic growth is plotted, it results in an S-shaped
growth curve, or S-curve
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Author Animation: Population Growth and
Regulation
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Author Animation: Logistic Growth
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The S-Curve of Logistic Population Growth
Growth
rate slows
Growth stops and the
population stabilizes close
to the carrying capacity
Population
grows rapidly
(a) An S-shaped growth curve stabilizes at carrying capacity
Fig. 26-6a
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26.2 How Is Population Growth Regulated?
 Environmental resistance limits population
growth (continued)
– If a population far exceeds the carrying capacity
of its environment, excess demands placed on
the ecosystem are likely to destroy crucial
resources
– This can permanently and severely reduce
carrying capacity, causing the population to
decline to a fraction of its former size or
disappear entirely
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The S-Curve of Logistic Population Growth
The population
overshoots the
carrying
capacity; the
environment
is damaged
Low damage; resources
recover, and the
population fluctuates
Extreme
damage; the
population
dies out
High damage; the
carrying capacity is
permanently lowered
(b) Consequences of exceeding carrying capacity
Fig. 26-6b
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26.2 How Is Population Growth Regulated?
 Environmental resistance limits population
growth (continued)
– For example, when reindeer were introduced
onto an island with no large predators, their
population increased rapidly, seriously
overgrazing the vegetation they relied on for food
– As a result, the reindeer population plummeted
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The Effects of Exceeding Carrying Capacity
exponential
growth
population
crash
Fig. 26-7
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26.2 How Is Population Growth Regulated?
 Environmental resistance limits population
growth (continued)
– Logistic population growth can occur in nature
when a species moves into a new habitat
–For example, new barnacle settlers along a
rocky coast may find ideal conditions that
allow their population to grow exponentially
–As population density increases, however,
individuals begin to compete for space,
energy, and nutrients
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number of barnacles (per cm2)
A Logistic Curve in Nature
time (weeks)
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Fig. 26-8
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26.2 How Is Population Growth Regulated?
 Environmental resistance limits population
growth (continued)
– These forms of environmental resistance can
reduce the reproductive rate and increase the
death rate, as has been demonstrated in
laboratory populations of fruit flies
–In response to crowding, the fruit flies show a
decrease in both reproductive rate and life
span
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Density-Dependent Environmental Resistance
days
offspring per day
life span
population density
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Fig. 26-9
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26.2 How Is Population Growth Regulated?
 Environmental resistance limits population
growth (continued)
– As environmental resistance increases,
population growth slows and eventually stops
– In nature, conditions are never completely
stable, so both carrying capacity 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
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26.2 How Is Population Growth Regulated?
 Environmental resistance can be classified into
two broad categories
– Density-independent factors, which limit
population size regardless of the population
density
– Density-dependent factors, which increase in
effectiveness as the population density increases
–Nutrients, energy, and space are all densitydependent regulators of population size
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26.2 How Is Population Growth Regulated?
 Density-independent factors limit populations
regardless of their density
– climate and weather
– For example, most insects and annual plant
populations are limited in size by the number of
individuals that can be produced before the first hard
freeze
– Hurricanes, droughts, floods, and fire can have
profound effects on local populations, particularly
small, short-lived species
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26.2 How Is Population Growth Regulated?
– Human activities can also limit the growth of
natural populations
–Pesticides and pollutants can cause drastic
declines in natural populations
–Overhunting has driven some species to
extinction
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26.2 How Is Population Growth Regulated?
 Density-dependent factors become more effective as
population density increases
– Populations of organisms with a life span of more than a
year have evolved adaptations that allow them survive
seasonal changes, such as the onset of winter
– Many mammals develop thick coats and store fat;
some hibernate
– Many birds migrate long distances to find food and a
hospitable climate
– Tree and bushes enter dormancy, dropping leaves
and slowing their metabolic activities
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26.2 How Is Population Growth Regulated?
 Important density-dependent factors limiting
population growth are:
– Predation
– Parasitism
– Competition
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26.2 How Is Population Growth Regulated?
 Predators exert density-dependent controls on
populations
– Predators are organisms that eat other
organisms, called their prey
– Predation becomes important as prey
populations grow because predators eat a
variety of prey, depending upon what is most
abundant and easiest to find
– In this way, predators exert density-dependent
population control over more than one prey
population
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Predators Help Control Prey Populations
Fig. 26-10a
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26.2 How Is Population Growth Regulated?
 Predator populations often grow as their prey
become more abundant
– The number offspring produced is determined by
the abundance of prey
–For example, snowy owls hatch up to 12
chicks when lemmings (their prey) are
abundant, but may not reproduce at all in
years when the lemming population has
crashed
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Predators Help Control Prey Populations
Fig. 26-10b
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26.2 How Is Population Growth Regulated?
 Some predator-prey population cycles are outof-phase when predators cause a dramatic
decline in prey populations, which in turn results
in a decline in the predator population at a
future date
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26.2 How Is Population Growth Regulated?
 Parasites spread more rapidly among dense
populations
– Parasitism involves a parasite living on or in a host
organism, harming it but not generally killing it because
many parasites benefit by having their host remain alive
– Most parasites cannot travel long distances, so they
spread more readily among hosts in dense populations
– Parasites influence population size by weakening their
hosts and making them more susceptible to death from
other causes, such as harsh weather or predators
– Examples of parasites include the bacterium that
causes Lyme disease, some fungi, intestinal worms,
ticks, and some protists
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26.2 How Is Population Growth Regulated?
 Competition for resources helps control
populations
– Competition is defined as the interaction among
individuals who attempt to use the same limited
resource, and this interaction limits population
size in a density-dependent manner
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26.2 How Is Population Growth Regulated?
 There are two major forms of competition:
– Interspecific competition, between individuals
of different species
– Intraspecific competition, between individuals
of the same species
–Because the needs of members of the same
species for resources are almost identical,
intraspecific competition is an important
density-dependent mechanism of population
control
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26.2 How Is Population Growth Regulated?
 Organisms have evolved ways to deal with intraspecific
competition
– Most plants and many insects engage in scramble
competition—a free-for-all scramble as individuals try to beat
others to a limited pool of resources
– For example, gypsy moth females each lay a mass of up to
1,000 eggs on tree trunks in eastern North America
– As the eggs hatch, armies of caterpillars crawl up the tree
– Huge outbreaks of this invasive species can completely
strip large trees of their leaves in a few days
– Competition for food may be so great that most of the
caterpillars die before they can metamorphose into
egg-laying moths
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Scramble Competition
Fig. 26-12
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26.2 How Is Population Growth Regulated?
 Organisms have evolved ways to deal with intraspecific
competition (continued)
– Many animals have evolved contest competitions,
where social or chemical interactions determine access
to important resources
– Territorial species—such as wolves, fish, rabbits, and
songbirds—defend areas that contain important
resources
– Only the best adapted individuals are able to defend
their territories, whereas those without territories may
not reproduce or may become easy prey
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26.2 How Is Population Growth Regulated?
 Organisms have evolved ways to deal with
intraspecific competition (continued)
– As population densities increase and competition
becomes more intense, some animals react by
emigrating
– Large numbers leave their homes to colonize
new areas; many die in the quest
–For example, locusts emigrate, consuming
vegetation in their path
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Emigration
Fig. 26-13
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26.2 How Is Population Growth Regulated?
 Density-independent and density-dependent
factors interact to regulate population size
– The size of a population at any given time is the
result of complex interactions between densityindependent and density-dependent forms of
environmental resistance
–For example, a caribou weakened by hunger
(density-dependent) and attacked by parasites
(density-dependent) is more likely to be killed
by an exceptionally cold winter (densityindependent)
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26.3 How Are Populations Distributed in Space and
Time?
 Populations exhibit different spatial distributions
– There are three major types of spatial
distributions
–Clumped
–Uniform
–Random
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26.3 How Are Populations Distributed in Space and
Time?
 Populations who live in groups exhibit clumped
distribution
– Examples include elephant herds, wolf packs, prides of
lions, flocks of birds, and schools of fish
– Advantages of clumped distributions include:
– Many eyes that can search for localized food sources
– Movement of the group (e.g., schools of fish or flocks
of birds) can confuse predators by their sheer
numbers
– Predators, in turn, may hunt in groups, cooperating to
bring down larger prey
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Clumped Distribution
Fig. 26-14a
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26.3 How Are Populations Distributed in Space and
Time?
 Organisms with a uniform distribution maintain a
relatively constant distance between individuals
– This is common among territorial animals defending
scarce resources or breeding territories
– An example among plants is desert creosote bushes,
which are spaced evenly resulting from competition
among their root systems for water and nutrients
– An advantage of uniform distribution is that it helps
ensure adequate resources for each individual
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Uniform Distribution
Fig. 26-14b
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26.3 How Are Populations Distributed in Space and
Time?
 Organisms with a random distribution are
rare, exhibited by individuals that do not form
social groups
– Random distribution occurs when resources are
not scarce enough to require territorial spacing
–Examples include trees and other plants in
rain forests
– There are probably no vertebrate species that
maintain a random distribution throughout the
year; most interact socially, at least during the
breeding season
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Random Distribution
Fig. 26-14c
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26.3 How Are Populations Distributed in Space and
Time?
 Survivorship in populations follows three basic
patterns
– Late-loss populations
– Constant-loss populations
– Early-loss populations
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26.3 How Are Populations Distributed in Space and
Time?
 Survivorship in populations follows three basic
patterns (continued)
– Survivorship describes the pattern of survival in
a population
– Survivorship tables track groups of organisms
born at the same time throughout their life span,
and record how many continue to survive in each
succeeding year
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26.3 How Are Populations Distributed in Space and
Time?
 Survivorship in populations follows three basic
patterns (continued)
– A survivorship curve for a population can be
produced by graphing survivorship table data
–The Y-axis logs the number of individuals
surviving to a particular age out of an initial
population size born at a specific time
–The X-axis plots increasing age categories
after a specific birth date
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26.3 How Are Populations Distributed in Space and
Time?
 Late-loss populations produce convex
survivorship curves
– These populations have relatively low juvenile
death rates; many or most individuals survive to
old age
– Late-loss curves are seen in many long-lived
animals, who produce few offspring that receive
substantial parental care during early life
–Examples include humans and many large
mammals, such as elephants and mountain
sheep
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26.3 How Are Populations Distributed in Space and
Time?
 Constant-loss populations produce relatively
straight lines
– In these populations, individuals have an equal
chance of dying at any time during their life span
–Examples include some bird species, such as
gulls and the American robin, as well as
organisms that reproduce asexually, such as
Hydra and bacteria
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26.3 How Are Populations Distributed in Space and
Time?
 Early-loss populations produce concave survivorship
curves
– These curves are characteristic of organisms that
produce large numbers of offspring that receive little or
no parental care
– Many of these species engage in scramble competition
early in life
– The death rate is high among the young, but those that
reach adulthood have a reasonable chance to survive to
old age
– Examples include most invertebrates, most plants,
and many fish
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Survivorship Tables and Survivorship Curves
Age
Number
of
survivors
0 (birth)
100,000
10
99,124
20
98,713
30
97,754
40
96,489
50
93,698
60
87,967
70
76,241
80
54,117
90
22,312
100
2,523
(a) A survivorship
table
late loss
(human)
constant loss
(American robin)
early loss
(dandelion)
(b) Survivorship curves
Fig. 26-15
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 Demographers track changes in human populations
– Demography is the study of the changing human
population
– Demographers measure human populations, track
population changes in different countries and regions,
and make comparisons between developed and
developing countries
– Demographic data are used to formulate policies in
public health, housing, education, employment,
immigration, and environmental protection
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 The human population continues to grow rapidly
– In the last few centuries, the human population has
grown at nearly an exponential rate following a J-shaped
growth curve
– Over the last decade, however, the human population
has been growing at a relatively constant rate,
suggesting that it may not longer be growing
exponentially
– However, Earth’s human population grows by 75
million people each year
– Are we entering the final bend of the S-shaped growth
curve?
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Author Animation: Human Population Growth
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Human Population Growth
Billions
Time to add
each billion
(years)
1804
1
All of human
history
1927
1960
1975
1987
1999
2012
2
3
4
5
6
7*
123
33
14
13
12
13
1999
1987
1975
year
Date
2012*
2008
1960
*projected
1927
Bubonic
plague
Technical advances
Agricultural advances
1804
Industrial and
medical
advances
Fig. 26-16
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 A series of advances have increased Earth’s
carrying capacity to support people
– Humans have manipulated the environment to
increase the Earth’s carrying capacity
– Several technological advances have greatly
influenced the human ability to make resources
available
–Technical advances
–Agricultural advances
–Industrial and medical advances
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 Technical advances
– Early humans discovered fire, invented tools and
weapons, built shelters, and designed protective
clothing
–Tools and weapons allowed them to hunt
more effectively to increase their food supply
–Shelter and clothing increased the habitable
areas of the globe
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 Agricultural advances
– Around 8000 B.C., animals and plants were
domesticated, providing a larger and more stable
food supply
– This resulted in a longer life span and more
childbearing years, although disease continued
to restrain population growth
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
26.5 How Is the Human Population Changing?
 Industrial and medical advances
– Beginning in England in the mid-eighteenth
century, medical and industrial advances
permitted a population explosion
–Industrial advances allowed fewer people to
produce more food
–Medical advances decreased the death rate
from infectious disease
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 The demographic transition explains trends in
population size
– In developed countries, people benefit from a relatively
high standard of living, with access to modern technology
and medical care, including contraception
– Examples include Australia, New Zealand, Japan, and
countries in North America and Europe
 Most of Earth’s people, however, live in developing
countries, which lack these advantages
– Examples include countries in Central and South
America, Africa, and much of Asia
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 The rate of population growth in countries that are now
developed has changed over time in predictable
stages, in a pattern called demographic transition
– Pre-industrial stage: The population was relatively small
and stable, with high birth rates and high death rates
– Transitional stage: Food production increased and health
care improved, which caused death rates to fall; because
birth rates remained high, there was an explosive
population increase
– Industrial stage: Birth rates fell as contraceptives were
more available, and as people moved from farms to
cities, where children were less important as a source of
labor
– Post-industrial stage: Populations are relatively stable,
with low birth and death rates
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
The Demographic Transition
Pre-industrial
Stage
birth rate
death rate
population
size
Birth and death
rates are high
Population
grows rapidly
Transitional
Stage
Industrial
Stage
Post-industrial
Stage
Population
stabilizes
Population
growth slows
Birth rate
remains high
natural rate
of population
increase
Birth rate
declines
Birth and death
rates are low
Population
remains low
Death rate
declines
Fig. 26-17
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 The demographic transition explains trends in
population size (continued)
– A population will eventually stabilize if parents
have just the number of children to replace
themselves, called replacement-level fertility
(RLF)
–Because not all children survive to maturity,
RLF is 2.1 per woman
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 World population growth is unevenly distributed
– Many developing countries still have rapidly
growing populations, as birth rates vastly exceed
death rates
–This results from low incomes and the need
for many children to raise family income or
produce food
–In these countries, knowledge of, and access
to, contraception are limited
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 World population growth is unevenly distributed
(continued)
– In spite of the population reduction of some
developing countries, zero population growth will
not be achieved globally
–The U.N. predicts a global human population
of over 9 billion, and growing, by the year
2050
–7.9 billion of those people will live in
developing countries
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Historical and Projected World Population
2009: 6.8 billion
developing countries
developed countries
Fig. 26-18
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 The current age structure of a population
predicts its future growth
– Age structure diagrams show the distribution of
human populations by age and gender
– Age structure can be shown graphically
–Age is shown on the vertical axis
–The number of individuals in each age group
is shown on the horizontal axis, with males
and females placed on opposite sides
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 The current age structure of a population
predicts its future growth (continued)
– All age-structure diagrams peak at the maximum
life span, but the shape below the peak reveals if
the population follows one of three basic agestructure patterns
–A growing population
–A stable population
–A shrinking population
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 A growing population
– If adults of reproductive age (15 to 44 years) are
having more children than are needed to replace
themselves, the population is above RLF and is
expanding
–The age-structure diagram will be roughly
triangular
–An example of a country with a growing
population is Mexico
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Age-Structure Diagram
Mexico 2009
male
(a) Population pyramid for Mexico
Biology: Life on Earth, 9e
female
Fig. 26-19a
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 A stable population
– If adults of reproductive age have just the
number of children needed to replace
themselves, the population is at RLF and is
stable
–The age-structure diagram will have relatively
straight sides
–An example of a country with a stable
population is Sweden
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Age-Structure Diagram
Sweden 2009
male
(b) Population pyramid for Sweden
Biology: Life on Earth, 9e
female
Fig. 26-19b
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 A shrinking population
– If adults of reproductive age have fewer children
than are needed to replace themselves, the
population is below RLF and is shrinking
–The age-structure diagram will be narrow a
the base
–An example of a country with a shrinking
population is Italy
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Age-Structure Diagram
Italy 2009
male
(c) Population pyramid for Italy
Biology: Life on Earth, 9e
female
Fig. 26-19c
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 Average-age structure diagrams have been
plotted for developed and developing countries
for 2009, with predictions for 2050
– These diagrams reveal that even if developing
countries were to achieve RLF immediately, their
population increases would continue for decades
–A large population of children today create a
momentum for future growth as they enter
their reproductive years
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Age-Structure Diagrams of Developed Countries
2009
male
2050
female
75
60–74
postreproductive (45–79 yr)
45–59
30–44
reproductive (15–44 yr)
15–29
0–14
prereproductive (0–14 yr)
(a) Developed countries
Fig. 26-20a
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Age-Structure Diagrams of Developing Countries
2009
male
2050
female
75
60–74
45–59
30–44
15–29
0–14
(b) Developing countries
Fig. 26-20b
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 Fertility in Europe is below replacement level
– A comparison of growth rates for various world
regions shows Europe as the only one with an
average rate of change in population that is
negative
–The average fertility rate is 1.5, which is
substantially below RLF
–Concerns about the availability of future
workers and taxpayers have prompted several
countries to offer incentives for couples to
have children at an earlier age
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
Population Change by World Regions
World average: 1.2%
Developing countries average: 1.5%
Africa: 2.4%
Latin America/Caribbean: 1.5%
Asia (excluding China): 1.5%
China: 0.5%
Developed
countries average: 0.2%
N. America: 0.6%
Europe: 0.0%
Fig. 26-21
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 The U.S. population is growing rapidly
– With a population of over 307 million and a
growth rate of about 1% per year, the U.S.
population is the fastest growing of all developed
countries
–The U.S. fertility rate is about 2.0—actually
below RLF
–However, immigration is adding rapidly to the
population, because the fertility rate of new
immigrants is above RLF
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.
U.S. Population Growth
325
300
275
U.S. population (in millions)
250
225
200
175
150
125
100
75
50
25
0
1800
Biology: Life on Earth, 9e
1850
1900
year
1950
2000
Fig. 26-22
Copyright © 2011 Pearson Education Inc.
26.4 How Is the Human Population Changing?
 Rapid population growth in the U.S. may have
serious implications for the environment of the
U.S. and the Earth
– Americans consume far more resources and
produce far more pollution than the global
average
– The spread of housing, commercial
establishments, and energy-extracting
enterprises degrades and destroys natural
habitats, reducing the carrying capacity for nonhuman life of ecosystems in the United states
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.