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CAMPBELL BIOLOGY IN FOCUS
Urry • Cain • Wasserman • Minorsky • Jackson • Reece
40
Population Ecology
and the Distribution
of Organisms
Lecture Presentations by
Kathleen Fitzpatrick and Nicole Tunbridge
© 2014 Pearson Education, Inc.
Overview: Discovering Ecology
 Ecology is the scientific study of the interactions
between organisms and the environment
 These interactions determine the distribution of
organisms and their abundance
 Modern ecology includes observation and
experimentation
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Figure 40.2
Global ecology
Landscape ecology
Ecosystem ecology
Community ecology
Population ecology
Organismal ecology
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 Global ecology is concerned with the biosphere,
or global ecosystem, which is the sum of all the
planet’s ecosystems
 Global ecology examines the influence of energy
and materials on organisms across the biosphere
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 Landscape ecology focuses on the exchanges of
energy, materials, and organisms across multiple
ecosystems
 A landscape (or seascape) is a mosaic of
connected ecosystems
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 Ecosystem ecology emphasizes energy flow and
chemical cycling among the various biotic and
abiotic components
 An ecosystem is the community of organisms in an
area and the physical factors with which they interact
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 Community ecology deals with the whole array of
interacting species in a community
 A community is a group of populations of different
species in an area
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 Population ecology focuses on factors affecting
population size over time
 A population is a group of individuals of the same
species living in an area
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 Organismal ecology studies how an organism’s
structure, physiology, and (for animals) behavior
meet environmental challenges
 Organismal ecology includes physiological,
evolutionary, and behavioral ecology
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Differentiate between biotic and abiotic factors.
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 Abiotic factors are the nonliving chemical and
physical attributes of the environment
 Biotic factors are the other organisms that make up
the living component of the environment
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Concept 40.1: Earth’s climate influences the
structure and distribution of terrestrial biomes
 The long-term prevailing weather conditions in an
area constitute its climate
 Four major abiotic components of climate are
temperature, precipitation, sunlight, and wind
 Macroclimate consists of patterns on the global,
regional, and landscape level
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Describe factors that affect
global climate patterns.
(i.e. temperature, precipitation, sunlight, and
wind)
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Figure 40.3a
Atmosphere
90N (North Pole)
Low angle of incoming sunlight
23.5N (Tropic of
Cancer)
Sun overhead at equinoxes
0 (Equator)
23.5S (Tropic of
Capricorn)
Low angle of incoming sunlight
90S (South Pole)
Latitudinal variation in sunlight intensity
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 Global air circulation and precipitation patterns
are initiated by intense solar radiation near the
equator
 Warm, wet air rising near the equator creates
precipitation in the tropics
 Dry air descending at 30 north and south latitudes
causes desert conditions
 This pattern of precipitation and drying is repeated at
the 60 north and south latitudes and the poles
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 Variation in the speed of Earth’s rotation at different
latitudes results in the major wind patterns
 Trade winds blow east to west in the tropics
 Westerlies blow west to east in temperate zones
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Figure 40.3ba
66.5N (Arctic Circle)
60N
30N
Westerlies
Northeast trades
0
Southeast trades
30S
Westerlies
60S
66.5S (Antarctic Circle)
Global air circulation and precipitation patterns
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Figure 40.3bb
30N
Descending
dry air
absorbs
moisture.
Ascending
moist air
releases
moisture.
0
Global air circulation and precipitation patterns
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Regional Effects on Climate
 Climate (in a particular area of the biosphere) is
affected by seasonality, large bodies of water, and
mountains
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Define a biome.
How can you predict what type of biome an
abiotic factor may be found in?
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Climate and Terrestrial Biomes
 Biomes are major life zones characterized by
vegetation type (terrestrial biomes) or physical
environment (aquatic biomes)
 Climate is very important in determining why
terrestrial biomes are found in certain areas
 Climate affects the latitudinal patterns of terrestrial
biomes
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Figure 40.7
30N
Tropic of
Cancer
Equator
Tropic of Capricorn
30S
Tropical forest
Savanna
Desert
Chaparral
Temperate grassland
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Temperate broadleaf forest
Northern coniferous forest
Tundra
High mountains
Polar ice
 A climograph plots the temperature and
precipitation in a region
 Biomes are affected not just by average temperature
and precipitation, but also by the pattern of
temperature and precipitation through the year
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Figure 40.8
Annual mean temperature (C)
Desert
Temperate grassland
Tropical forest
30
Temperate
broadleaf
forest
15
Northern
coniferous
forest
0
Arctic and
alpine
tundra
15
0
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200
100
400
300
Annual mean precipitation (cm)
How might catastrophic events
affect biomes?
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 Natural and human-caused disturbances alter the
distribution of biomes
 A disturbance is an event that changes a
community by removing organisms and altering
resource availability
 For example, frequent fires kill woody plants
preventing woodlands from establishing
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General Features of Terrestrial Biomes
 Terrestrial biomes are often named for major
physical or climatic factors and for vegetation
 Terrestrial biomes usually grade into each other,
without sharp boundaries
 The area of intergradation, called an ecotone, may
be wide or narrow
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Review the biomes and know the relative
latitudinal location, average yearly
temperature, average amounts of yearly
precipitation, major vegetation in the area and
distribution, relative biodiversity.
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How might aquatic biomes differ from terrestrial?
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Concept 40.2: Aquatic biomes are diverse and
dynamic systems that cover most of Earth
 Aquatic biomes account for the largest part of the
biosphere in terms of area
 They show less latitudinal variation than terrestrial
biomes
 Marine biomes have salt concentrations of about 3%
 The largest marine biome is made of oceans, which
cover about 75% of Earth’s surface and have an
enormous impact on the biosphere
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 Freshwater biomes have salt concentrations of less
than 0.1%
 Freshwater biomes are closely linked to soils and
the biotic components of the surrounding terrestrial
biome
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Describe evolutionary adaptations organisms
living in freshwater biomes would have to
evolve that organisms in marine biomes would
not have to content with.
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 Aquatic biomes can be characterized by their
physical and chemical environment, geological
features, photosynthetic organisms, and
heterotrophs
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Generate a table of the aquatic biomes, be sure to
include key characteristics on the physical and
chemical environment, geological features,
photosynthetic organisms, and heterotrophs for
each.
Wetlands, Estuaries, Lakes, Streams and
Rivers, Intertidal Zone, Coral Reefs, Oceanic
Pelagic Zone, Marine Benthic Zone
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Zonation in Aquatic Biomes
 Many aquatic biomes are stratified into zones or
layers defined by light penetration, temperature,
and depth
Review the zonation and
describe the factors that
determine what organisms
will live in different zones.
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Describe the biotic factors that will affect the
movement of organisms into and out of a
particular biome?
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Concept 40.3: Interactions between organisms and
the environment limit the distribution of species
 Species distributions are the result of ecological and
evolutionary interactions through time
 Ecological time is the minute-to-minute time frame of
interactions between organisms and the environment
 Evolutionary time spans many generations and
captures adaptation through natural selection
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Dispersal and Distribution
 Dispersal is the movement of individuals away from
centers of high population density or from their area
of origin
Why should we care
about dispersal
patterns?
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 Transplants indicate if dispersal is a key factor
limiting species distributions
 Transplants include organisms that are intentionally
or accidentally relocated from their original
distribution
 If a transplant is successful, it indicates that the
species’ potential range is larger than its actual
range
 Species transplants can disrupt the communities or
ecosystems to which they have been introduced
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Biotic Factors
 Biotic factors that affect the distribution of
organisms may include
 Predation
 Herbivory (Interpret the graph, next slide)
 Mutualism
 Parasitism
 Competition
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Figure 40.13
Results
Seaweed cover (%)
100
80
Both limpets and urchins
removed
Sea urchin
Only urchins
removed
60
Limpet
40
Only limpets removed
Control (both urchins
and limpets present)
20
0
August
1982
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February
1983
August
1983
February
1984
Describe how the following abiotic
factors can affect the distribution of
biotic factors in a particular biome,
with examples.
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Abiotic Factors
 Abiotic factors affecting the distribution of
organisms include
 Temperature
 Water and oxygen
 Salinity
 Sunlight
 Rocks and soil
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Population Ecology
Density
Distribution
Size
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Concept 40.4: Dynamic biological processes
influence population density, dispersion, and
demographics
 Population ecology explores how biotic and abiotic
factors influence density, distribution, and size of
populations
 A population is a group of individuals of a single
species living in the same general area
 Populations are described by their boundaries
and size
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 Density is the result of an interplay between
processes that add individuals to a population
and those that remove individuals
 Additions occur through birth and immigration,
the influx of new individuals from other areas
 Removal of individuals occurs through death and
emigration, the movement of individuals out of a
population
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Figure 40.15
(a) Clumped
(b) Uniform
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(c) Random
How is population size controlled?
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Concept 40.5: The exponential and logistic models
describe the growth of populations
 Unlimited growth occurs under ideal conditions; in
nature, growth is limited by various factors
 Ecologists study growth in both idealized and
realistic conditions
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Per Capita Rate of Increase
 Change in population size can be defined by the
equation
Change in
Immigrants
Emigrants
population  Births  entering − Deaths − leaving
size
population
population
 If immigration and emigration are ignored, a
population’s growth rate (per capita increase)
equals birth rate minus death rate
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 The population growth rate can be expressed
mathematically as
ΔN
 B −D
Δt
where N is the change in population size, t is the
time interval, B is the number of births, and D is the
number of deaths
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 Births and deaths can be expressed as the average
number of births and deaths per individual during the
specified time interval
B  bN
D  mN
where b is the annual per capita birth rate, m (for
mortality) is the per capita death rate, and N is
population size
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 The population growth equation can be revised
ΔN
 bN −mN
Δt
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 The per capita rate of increase (r) is given by
rb−m
 Zero population growth (ZPG) occurs when the
birth rate equals the death rate (r  0)
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 Change in population size can now be written as
ΔN
 rN
Δt
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 Instantaneous growth rate can be expressed as
dN
 rinstN
dt
where rinst is the instantaneous per capita rate
of increase
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Exponential Growth
 Exponential population growth is population
increase under idealized conditions
 Under these conditions, the rate of increase is at its
maximum, denoted as rmax
 The equation of exponential population growth is
dN
 rmaxN
dt
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 Exponential population growth results in a
J-shaped curve
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Figure 40.17
2,000
Population size (N)
dN
 1.0N
dt
1,500
dN
 0.5N
dt
1,000
500
0
0
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5
10
Number of generations
15
Why is exponential growth
considered to be under ideal
conditions.
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 The J-shaped curve of exponential growth
characterizes populations in new environments or
rebounding populations
 For example, the elephant population in Kruger
National Park, South Africa, grew exponentially after
hunting was banned
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Conditions are never ideal.
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Carrying Capacity
 Exponential growth cannot be sustained for long in
any population
 A more realistic population model limits growth by
incorporating carrying capacity
 Carrying capacity (K) is the maximum population
size the environment can support
 Carrying capacity varies with the abundance of
limiting resources
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The Logistic Growth Model
 In the logistic population growth model, the per
capita rate of increase declines as carrying capacity
is reached
 The logistic model starts with the exponential model
and adds an expression that reduces per capita rate
of increase as N approaches K
dN
(K −N)
 rmaxN
dt
K
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Figure 40.19
Exponential
growth
dN
 1.0N
dt
Population size (N)
2,000
1,500
K  1,500
1,000
Logistic growth
(1,500  N)
dN
 1.0N
1,500
dt
Population growth
begins slowing here.
500
0
0
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10
5
Number of generations
15
The Logistic Model and Real Populations
 The growth of many laboratory populations,
including paramecia, fits an S-shaped curve when
resources are limited
 These organisms are grown in a constant
environment lacking predators and competitors
Animation: Population Ecology
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Number of Daphnia/50 mL
Number of Paramecium/mL
Figure 40.20
1,000
800
600
400
200
0
0
5
10
Time (days)
(a) A Paramecium population
in the lab
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15
180
150
120
90
60
30
0
0
20
40
60 80 100 120 140 160
Time (days)
(b) A Daphnia (water flea) population in the lab
 Some populations overshoot K before settling down
to a relatively stable density
 Some populations fluctuate greatly and make it
difficult to define K
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 The logistic model fits few real populations but is
useful for estimating possible growth
 Conservation biologists can use the model to
estimate the critical size below which populations
may become extinct
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Population dynamics
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Concept 40.6: Population dynamics are influenced
strongly by life history traits and population
density
 An organism’s life history comprises the traits that
affect its schedule of reproduction and survival
 The age at which reproduction begins
 How often the organism reproduces
 How many offspring are produced during each
reproductive cycle
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“Trade-offs” and Life Histories
 Organisms have finite resources, which may lead to
trade-offs between survival and reproduction
 Selective pressures influence the trade-off
between the number and size of offspring
 Some plants produce a large number of small
seeds, ensuring that at least some of them will grow
and eventually reproduce
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 K-selection, or density-dependent selection,
selects for life history traits that are sensitive to
population density
 r-selection, or density-independent selection,
selects for life history traits that maximize
reproduction
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How would you differentiate between densityindependent and density-dependent population
changes?
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Population Change and Population Density
 In density-independent populations, birth rate and
death rate do not change with population density
 In density-dependent populations, birth rates fall
and death rates rise with population density
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Figure 40.22
Birth or death rate
per capita
When population
density is low, b  m. As
a result, the population
grows until the density
reaches Q.
When population
density is high, m  b,
and the population
shrinks until the
density reaches Q.
Equilibrium density (Q)
Density-independent
death rate (m)
Density-dependent
birth rate (b)
Population density
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What are some factors that will
negatively affect the population
size in a density-dependent
situation?
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Mechanisms of Density-Dependent Population
Regulation
 Density-dependent birth and death rates are an
example of negative feedback that regulates
population growth
 Density-dependent birth and death rates are
affected by many factors, such as competition for
resources, territoriality, disease, predation, toxic
wastes, and intrinsic factors
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 Competition for resources occurs in crowded
populations; increasing population density intensifies
competition for resources and results in a lower
birth rate
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 Toxic wastes produced by a population can
accumulate in the environment, contributing to
density-dependent regulation of population size
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 Predation may increase with increasing population
size due to predator preference for abundant prey
species
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 Territoriality can limit population density
when space becomes a limited resource
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 Disease transmission rates may increase
with increasing population density
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 Intrinsic factors (for example, physiological
factors like hormonal changes) appear to regulate
population size
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