Transcript Populations

BIOLOGY
A GUIDE TO THE NATURAL WORLD
FOURTH EDITION
DAVID KROGH
An Interactive Living World 1:
Populations in Ecology
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33.1 The Study of Ecology
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The Study of Ecology
• Ecology is the study of the interactions living
things have with each other and with their
environment.
• Ecology is not the same thing as
environmentalism.
• The function of ecology is to describe
interactions that affect the living world, not to
work on behalf of the environment.
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An Ecologist on the Job
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Figure 33.1
Scales of Life
• There are five scales of life that concern
ecology:
–
–
–
–
–
organism
populations
communities
ecosystems
the biosphere
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Scales of Life
• A population is all the members of a single
species that live together in a specified
geographical area.
• A community is all the members of all species
that live in a single area.
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Scales of Life
• An ecosystem is a community and all the
nonliving elements that interact with it.
• The biosphere is the interactive collection of all
the Earth’s ecosystems.
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Scales of Life
organism
(sea lion)
population
(colony)
community
(giant kelp forest)
ecosystem
(Southern California
coast)
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biosphere
(Earth)
Figure 33.2
33.2 Populations: Size and Dynamics
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Populations
• Ecologists employ several means to estimate
the size of populations of living things, among
them counting animal droppings or surveying
bird populations as they migrate.
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Arithmetic and Exponential Growth
• An arithmetical increase occurs when, over a
given interval of time, an unvarying number of
new units is added to a population.
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Arithmetic and Exponential Growth
• An exponential increase occurs when the
number of new units added to a population is
proportional to the number of units that exists.
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Arithmetic and Exponential Growth
exponential growth
of water-flea
population
256,000
arithmetic growth
of car production
25,000
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Figure 33.3
Arithmetic and Exponential Growth
• Populations of living things are capable of
increasing exponentially because living things
are capable of giving rise to more living things.
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Arithmetic and Exponential Growth
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Figure 33.4
Arithmetic and Exponential Growth
• The rapid growth that sometimes characterizes
living populations is referred to as exponential
growth or as the J-shaped growth curve.
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Arithmetic and Exponential Growth
• Populations that initially grow, but whose
growth later levels out, have experienced
logistic growth, sometimes referred to as the Sshaped growth curve.
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Models of Growth for Natural
Populations
Exponential growth
Logistic growth
K
More complex growth
K
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Figure 33.5
Environmental Resistance
• The size of living populations is kept in check
by environmental resistance, defined as all the
forces of the environment that act to limit
population growth.
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Calculating Exponential Growth
• Exponential growth in living populations can be
calculated by subtracting a population’s death
rate from its birth rate, which yields the
population’s growth rate.
• Denoted as r, this rate is also known as the
population’s intrinsic rate of increase.
• It can be thought of as the population’s
potential for growth.
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Intrinsic Rate of Increase (r)
r = 0.06
high intrinsic
rate of
increase
r = 0.02
low intrinsic
rate of
increase
r=0
zero population
growth
r = –0.05
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negative intrinsic
rate of increase
Figure 33.6
How Long Between Generations?
whale
elephant
beaver human
mouse
housefly
Daphnia
Paramecium
E. coli
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Figure 33.7
Carrying Capacity (K)
• Carrying capacity, denoted as K, is the
maximum population density of a given species
that can be sustained within a defined
geographical area over an extended period of
time.
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Population Growth
PLAY
Animation 33.1: Population Growth
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33.3 r-Selected and K-Selected Species
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Reproductive Strategies
• Different species have different reproductive
strategies, meaning characteristics that have the
effect of increasing the number of fertile
offspring they bear.
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K-Selected Species
• Some species are said to be K-selected, or
equilibrium, species.
• These species tend to be physically large, to
experience their environment as relatively
stable, and to lavish a good deal of attention on
relatively few offspring.
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K-Selected Species
• The pressures on K-selected species tend to be
density dependent, meaning that as a
population’s density goes up, factors that limit
the population’s growth assert themselves ever
more strongly.
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r-Selected Species
• Other species are said to be r-selected or
opportunist species.
• These species tend to be physically small, to
experience their environment as relatively
unstable, and to give little or no attention to the
numerous offspring they produce.
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r-Selected Species
• The pressures on r-selected species tend to be
density independent, meaning pressures that are
unrelated to the population’s density.
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K-Selected and r-Selected Species
K-selected
equilibrium species
r-selected
opportunist species
K
r
Population size:
• limited by carrying capacity (K)
• density dependent
• relatively stable
Population size:
• limited by reproductive rate (r)
• density independent
• relatively unstable
Organisms:
• larger, long lived
• produce fewer offspring
• provide greater care for offspring
Organisms:
• smaller, short lived
• produce many offspring
• provide no care for offspring
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Figure 33.8
Survivorship Curves
• Survivorship curves describe how soon species
members tend to die within the species’ life
span.
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Survivorship Curves
• There are three idealized types of survivorship
curves:
– Late loss (type I)
– Constant loss (type II)
– Early loss (type III)
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Survivorship Curves
• Members of late-loss species tend to survive
into old age.
• Members of constant-loss species tend to die
off at a nearly constant rate throughout their
lifespan.
• Members of early-loss species tend to have
high death rates early in life, with these rates
leveling out thereafter.
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Survivorship Curves
Most die at
young age
Most live
until old age
Die at
all ages
K-selected
species
r-selected
species
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Figure 33.9
33.4 Thinking About Human Populations
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Life Tables
• Survivorship curves are created from life tables,
which set forth the probabilities of a member of
a species being alive after given intervals of
time.
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Life Table for the U.S.A.
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Table 33.1
Population Pyramids
• An important step in calculating the future
growth of human populations is to learn what
proportion of the population is at or under
reproductive age.
• A population pyramid displays this proportion.
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Population Pyramids
• Populations whose pyramids are heavily
weighted toward younger age groups are likely
to experience relatively large growth.
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Population Pyramids
Kenya: 2006
males
United States: 2006
females
males
females
Kenya: 2050
males
females
reproductive
ages
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Figure 33.10
Human Population Increase
• Following decades of explosive increase, the
world’s human population is projected to
stabilize in the coming decades, going from
about 6.5 billion now to a maximum of about
9.2 billion just past mid-century.
• This stabilization is being brought about by a
decrease in the total fertility rate, informally
defined as the number of children born, on
average, to each woman in a population.
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Human Population Increase
Old Stone Age
beginning of
Agricultural Revolution
New Stone Age
Bronze
Age
Middle Modern
Ages
Times
Iron Age
beginning of
Industrial Revolution
beginning of
agriculturally based
urban societies
bubonic plague
fall of Roman Empire
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Figure 33.11
Big Changes in Fertility
1960–1965
2000–2005
7.3
6.4
6.2
5.7
5.6
2.1
5.7
2.2
1.8
2.0
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1.9
2.0
Figure 33.12
The World’s Human Population
• The global reduction in fertility masks
enormous, ongoing differences between fertility
in more-developed and less-developed
countries.
• Fertility in less-developed countries tends to be
much higher than that in more-developed
countries.
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The World’s Human Population
• The fertility in most European nations is now so
low that the continent’s population stands to
shrink significantly by mid-century.
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The World’s Human Population
• The population of the United States, however,
is projected to grow significantly during this
same period.
• The primary factors bringing the U.S. increase
about are immigration and a high total fertility
rate relative to other developed nations.
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The World’s Human Population
• Some scientists believe that there is no greater
single threat to the environment than the
continued growth of the human population.
• Others argue that a more important concern is
the use of natural resources per person.
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Per Capita and Total Carbon Emissions
1997 5.66
1997 1.52
2003 5.43
2003 1.58
1997 0.90
1997 0.72
2003 0.86
2003 1.13
U.S. per capita CO2 emissions far exceed those in
China, but China’s per capita emissions grew by
19% between 1997 and 2003 . . .
. . . When coupled with China’s large population,
this per capita growth meant that China’s total CO2
emissions went from 59 percent of those in U.S.
in 1997 to 72 percent of those in U.S. in 2003.
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Figure 33.13