biology 102 week 9
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Biology 102 Week 9. Population Ecology. Overview: Earth’s Fluctuating Populations. To understand human population growth, we must consider general principles of population ecology. - PowerPoint PPT PresentationTRANSCRIPT
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PowerPoint Lectures for Biology, Seventh Edition
Neil Campbell and Jane Reece
Biology 102 Week 9
Population Ecology
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Overview: Earth’s Fluctuating Populations
• To understand human population growth, we must consider general principles of population ecology
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• Population ecology is the study of populations in relation to environment, including environmental influences on density and distribution, age structure, and population size
• The fur seal population of St. Paul Island, off the coast of Alaska, has experienced dramatic fluctuations in size
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 52.1: Dynamic biological processes influence population density, dispersion, and demography
• A population is a group of individuals of a single species living in the same general area
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Density and Dispersion
• Density is the number of individuals per unit area or volume
• Dispersion is the pattern of spacing among individuals within the boundaries of the population
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Density: A Dynamic Perspective
• Determining the density of natural populations is difficult
• In most cases, it is impractical or impossible to count all individuals in a population
• Density is the result of an interplay between processes that add individuals to a population and those that remove individuals
LE 52-2
Populationsize
Emigration
Deaths
ImmigrationBirths
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Patterns of Dispersion
• Environmental and social factors influence spacing of individuals in a population
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• In a clumped dispersion, individuals aggregate in patches
• A clumped dispersion may be influenced by resource availability and behavior
Video: Flapping Geese (clumped)
LE 52-3a
Clumped. For many animals, such as these wolves, living in groups increases the effectiveness of hunting, spreads the work of protecting and caring for young, and helps exclude other individuals from their territory.
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• A uniform dispersion is one in which individuals are evenly distributed
• It may be influenced by social interactions such as territoriality
Video: Albatross Courtship (uniform)
LE 52-3b
Uniform. Birds nesting on small islands, such as these king penguins on South Georgia Island in the South Atlantic Ocean, often exhibit uniform spacing, maintained by aggressive interactions between neighbors.
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• In a random dispersion, the position of each individual is independent of other individuals
Video: Prokaryotic Flagella (Salmonella typhimurium) (random)
LE 52-3c
Random. Dandelions grow from windblown seeds that land at random and later germinate.
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Demography
• Demography is the study of the vital statistics of a population and how they change over time
• Death rates and birth rates are of particular interest to demographers
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Life Tables
• A life table is an age-specific summary of the survival pattern of a population
• It is best made by following the fate of a cohort
• The life table of Belding’s ground squirrels reveals many things about this population
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Survivorship Curves
• A survivorship curve is a graphic way of representing the data in a life table
• The survivorship curve for Belding’s ground squirrels shows a relatively constant death rate
LE 52-4
Males
Females
10Age (years)
Num
ber o
f sur
vivo
rs (l
og s
cale
)
4 6 80 2
1,000
100
10
1
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• Survivorship curves can be classified into three general types: Type I, Type II, and Type III
LE 52-5
III
II
100Percentage of maximum life span
Num
ber o
f sur
vivo
rs (l
og s
cale
)
0 50
1,000
100
10
1
I
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Reproductive Rates
• A reproductive table, or fertility schedule, is an age-specific summary of the reproductive rates in a population
• It describes reproductive patterns of a population
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 52.2: Life history traits are products of natural selection
• Life history traits are evolutionary outcomes reflected in the development, physiology, and behavior of an organism
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Life History Diversity
• Life histories are very diverse
• Species that exhibit semelparity, or “big-bang” reproduction, reproduce once and die
• Species that exhibit iteroparity, or repeated reproduction, produce offspring repeatedly
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
“Trade-offs” and Life Histories
• Organisms have finite resources, which may lead to trade-offs between survival and reproduction
LE 52-7
FemalePa
rent
s su
rviv
ing
the
follo
win
g w
inte
r (%
)
Normalbrood size
100
80
60
0Reduced
brood sizeEnlarged
brood size
Male
40
20
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• Some plants produce a large number of small seeds, ensuring that at least some of them will grow and eventually reproduce
LE 52-8a
Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds, ensuring that at least some will grow into plants and eventually produce seeds themselves.
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• Other types of plants produce a moderate number of large seeds that provide a large store of energy that will help seedlings become established
LE 52-8b
Some plants, such as this coconut palm, produce a moderate number of very large seeds. The large endosperm provides nutrients for the embryo, an adaptation that helps ensure the success of a relatively large fraction of offspring.
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• In animals, parental care of smaller broods may facilitate survival of offspring
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Concept 52.3: The exponential model describes population growth in an idealized, unlimited environment
• It is useful to study population growth in an idealized situation
• Idealized situations help us understand the capacity of species to increase and the conditions that may facilitate this growth
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Per Capita Rate of Increase
• If immigration and emigration are ignored, a population’s growth rate (per capita increase) equals birth rate minus death rate
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• Zero population growth occurs when the birth rate equals the death rate
• Most ecologists use differential calculus to express population growth as growth rate at a particular instant in time:
dNdt rN
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Exponential Growth
• Exponential population growth is population increase under idealized conditions
• Under these conditions, the rate of reproduction is at its maximum, called the intrinsic rate of increase
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• Equation of exponential population growth:
dNdt rmaxN
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• Exponential population growth results in a J-shaped curve
LE 52-9
Number of generations
Popu
latio
n si
ze (N
)2,000
= 1.0N
1,000
1,500
500
0151050
dNdt
= 0.5NdNdt
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• The J-shaped curve of exponential growth characterizes some rebounding populations
LE 52-10
Year
Elep
hant
pop
ulat
ion
8,000
4,000
6,000
2,000
019801960194019201900
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Concept 52.4: The logistic growth model includes the concept of carrying capacity
• Exponential growth cannot be sustained for long in any population
• A more realistic population model limits growth by incorporating carrying capacity
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• Carrying capacity (K) is the maximum population size the environment can support
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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
• We construct the logistic model by starting with the exponential model and adding an expression that reduces per capita rate of increase as N increases
LE 52-11
Population size (N)
Per c
apita
rate
of i
ncre
ase
(r)
Maximum
Positive
Negative
N = K0
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• The logistic growth equation includes K, the carrying capacity
dNdt (K N)
Krmax N
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• The logistic model of population growth produces a sigmoid (S-shaped) curve
LE 52-12
Number of generations
Popu
latio
n si
ze (N
)
K = 1,5001,500
2,000
1,000
500
1510500
Logistic growth
Exponentialgrowth
= 1.0NdNdt
= 1.0NdNdt
1,500 – N1,500
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The Logistic Model and Real Populations
• The growth of laboratory populations of paramecia fits an S-shaped curve
LE 52-13a
Time (days)
Num
ber o
f Par
amec
ium
/mL
1,000
0
400
5
200
100
15
800
600
A Paramecium population in the lab
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• Some populations overshoot K before settling down to a relatively stable density
LE 52-13b
Time (days)
Num
ber o
f Dap
hnia
/50
mL
180
0
90
20
60
400
60
150
120
A Daphnia population in the lab
30
80 100 120 140 160
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• Some populations fluctuate greatly around K
LE 52-13c
Time (years)
Num
ber o
f fem
ales
80
1975 1980
40
19850
1990
60
A song sparrow population in its natural habitat
20
1995 2000
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• The logistic model fits few real populations but is useful for estimating possible growth
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The Logistic Model and Life Histories
• Life history traits favored by natural selection may vary with population density and environmental conditions
• 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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• The concepts of K-selection and r-selection are somewhat controversial and have been criticized by ecologists as oversimplifications
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Concept 52.5: Populations are regulated by a complex interaction of biotic and abiotic influences
• There are two general questions about regulation of population growth:
– What environmental factors stop a population from growing?
– Why do some populations show radical fluctuations in size over time, while others remain stable?
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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
LE 52-14
Population density
Equilibriumdensity
Density-independentbirth rate
Density-dependentdeath rate
Population density
Equilibriumdensity
Density-independentdeath rate
Density-dependentbirth rate
Population density
Equilibriumdensity
Density-dependentdeath rate
Density-dependentbirth rate
per c
a pi ta
Birt
h or
dea
th ra
te
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Density-Dependent Population Regulation
• Density-dependent birth and death rates are an example of negative feedback that regulates population growth
• They are affected by many factors, such as competition for resources, territoriality, health, predation, toxic wastes, and intrinsic factors
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Competition for Resources
• In crowded populations, increasing population density intensifies intraspecific competition for resources
LE 52-15
10,000
Ave
rage
num
ber o
f see
dspe
r rep
rodu
cing
indi
vidu
al(lo
g sc
ale)
1,000
100
100101Plants per m2 (log scale)
Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases.
Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply.
Ave
rage
clu
tch
size
2.880
Females per unit area
3.0
3.8
4.0
3.4
3.6
3.2
60 705030 40200 10
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Territoriality
• In many vertebrates and some invertebrates, territoriality may limit density
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• Cheetahs are highly territorial, using chemical communication to warn other cheetahs of their boundaries
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• Oceanic birds exhibit territoriality in nesting behavior
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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Health
• Population density can influence the health and survival of organisms
• In dense populations, pathogens can spread more rapidly
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Predation
• As a prey population builds up, predators may feed preferentially on that species
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Toxic Wastes
• Accumulation of toxic wastes can contribute to density-dependent regulation of population size
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Intrinsic Factors
• For some populations, intrinsic (physiological) factors appear to regulate population size
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Population Dynamics
• The study of population dynamics focuses on the complex interactions between biotic and abiotic factors that cause variation in population size
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Stability and Fluctuation
• Long-term population studies have challenged the hypothesis that populations of large mammals are relatively stable over time
LE 52-18
1960Year
Moo
se p
opul
atio
n si
ze2,500
Steady decline probably caused largely by wolf predation
2,000
1,500
1,000
500
01970 1980 1990 2000
Dramatic collapse caused by severe winter weather and food shortage, leading to starvation of more than 75% of the population
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• Extreme fluctuations in population size are typically more common in invertebrates than in large mammals
LE 52-19
1960Year
Com
mer
cial
cat
ch (k
g) o
fm
ale
crab
s (lo
g sc
ale)
730,000
100,000
10,000
1970 1980 19901950
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Metapopulations and Immigration
• Metapopulations are groups of populations linked by immigration and emigration
• High levels of immigration combined with higher survival can result in greater stability in populations
LE 52-20
1988Year
Num
ber o
f bre
edin
g fe
mal
es
60
1989 1990 1991
Smallislands
MandarteIsland
50
40
30
20
10
0
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• Many populations undergo boom-and-bust cycles
• Boom-and-bust cycles are influenced by complex interactions between biotic and abiotic factors
LE 52-21
Year
Har
e po
pula
tion
size
(thou
sand
s)
1850
Snowshoe hare
01875 1900 1925
40
80
120
160
Lynx
pop
ulat
ion
size
(thou
sand
s)
Lynx
0
3
6
9
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Concept 52.6: Human population growth has slowed after centuries of exponential increase
• No population can grow indefinitely, and humans are no exception
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The Global Human Population
• The human population increased relatively slowly until about 1650 and then began to grow exponentially
LE 52-22
8000B.C.
Hum
an p
opul
atio
n (b
illio
ns)6
5
4
3
2
1
04000B.C.
3000B.C.
2000B.C.
1000B.C.
The Plague
0 1000A.D.
2000A.D.
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• Though the global population is still growing, the rate of growth began to slow about 40 years ago
LE 52-23
Ann
ual p
erce
nt in
crea
se
2.2
2
1.8
1.6
1.4
1.2
1
2003
2050Year
2025200019751950
0.8
0.6
0.4
0.2
0
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Regional Patterns of Population Change
• To maintain population stability, a regional human population can exist in one of two configurations:
– Zero population growth = High birth rate – High death rate
– Zero population growth =Low birth rate – Low death rate
• The demographic transition is the move from the first state toward the second state
LE 52-24
Birt
h or
dea
th ra
te p
er 1
,000
peo
ple
50
40
30
20
10 Sweden
2050Year
20001900 195018500
18001750
Birth rateDeath rate
MexicoBirth rateDeath rate
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• The demographic transition is associated with various factors in developed and developing countries
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Age Structure
• One important demographic factor in present and future growth trends is a country’s age structure
• Age structure is the relative number of individuals at each age
• It is commonly represented in pyramids
LE 52-25
Rapid growthAfghanistan
AgeMale
Percent of population
Female
8 6 4 2 2 4 6 80
45–4940–4435–3930–3425–2920–2415–1910–14
5–90–4
85+80–8475–7970–7465–6960–6455–5950–54
Slow growthUnited States
AgeMale
Percent of population
Female
6 4 2 2 4 6 80
45–4940–4435–3930–3425–2920–2415–1910–14
5–90–4
85+80–8475–7970–7465–6960–6455–5950–54
8
Decrease Italy
Male
Percent of population
Female
6 4 2 2 4 6 808
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• Age structure diagrams can predict a population’s growth trends
• They can illuminate social conditions and help us plan for the future
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Infant Mortality and Life Expectancy
• Infant mortality and life expectancy at birth vary greatly among developed and developing countries but do not capture the wide range of the human condition
LE 52-26
Infa
nt m
orta
lity
(dea
ths
per 1
,000
birt
hs)
50
40
30
20
10
0Developedcountries
60
Developingcountries
Life
exp
ecta
ncy
(yea
rs)
80
40
20
0Developedcountries
60
Developingcountries
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Global Carrying Capacity
• How many humans can the biosphere support?
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Estimates of Carrying Capacity
• The carrying capacity of Earth for humans is uncertain
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Ecological Footprint
• The ecological footprint concept summarizes the aggregate land and water area needed to sustain the people of a nation
• It is one measure of how close we are to the carrying capacity of Earth
• Countries vary greatly in footprint size and available ecological capacity
LE 52-27
Ecol
ogic
al fo
otpr
int (
ha p
er p
erso
n)14
12
10
8
6
4
16
0
2
02 4 6 8 10 12 14 16
Available ecological capacity(ha per person)
New Zealand
AustraliaCanada
Sweden
WorldChina
India
SpainUK
Japan
Germany
Norway
USA
Netherlands
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• At more than 6 billion people, the world is already in ecological deficit