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Chapter 52Chapter 52
Population EcologyPopulation Ecology
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Population ecology• Study of populations in relation to environment
– Including environmental influences on
• Population density, distribution, demography
• Population growth models
• Regulation of populations
• Human population growth
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Density and Dispersion
• Density - number of individuals within a boundary
Figure 52.2
Births and immigration add individuals to a population.
Births Immigration
PopuIationsize
Emigration
Deaths
Deaths and emigration remove individuals from a population.
Changed by:- births and deaths-immigration and emigration
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Dispersion
• Pattern of spacing among individuals
• Influenced by environmental and social factors
Figure 52.17
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Clumped dispersion
• Based on resource availability and behavior
Figure 52.3a
(a) 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.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Uniform dispersion
• Social interactions, eg. territoriality, competition
Figure 52.3b
(b) 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.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Random dispersion
• Usually homogeneous habitat
– No attractions or repulsions between individuals
Figure 52.3c
(c) Random. Dandelions grow from windblown seeds that land at random and later germinate.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Demography
Describing a population by vital statistics:
- birth rate
- death rate
- sex ratio
- age structure
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Demography
• Life table: age specific summary of survival patterns of a population
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Survivorship Curves
• Graphic way of representing life table
– Belding’s ground squirrels - death rate ~constant
Figure 52.4
1000
100
10
1
Num
ber
of s
urvi
vors
(lo
g sc
ale)
0 2 4 6 8 10
Age (years)
Males
Females
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Survivorship Curves
• Three general types:
– Type I, Type II, and Type III
Figure 52.5
I
II
III
50 10001
10
100
1,000
Percentage of maximum life span
Num
ber
of s
urvi
vors
(lo
g sc
ale)
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Minutes
Months
1 year
4-5 years
5 years
5-11 years
many
years
Reproduction
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Does human reproductive table follow this pattern?
Reproductive table
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SEMELPARITY = breed only once (“big bang”)
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ITEROPARITY = breed more than once
- frequency depends on:
environment – food, etc.
inherent biology
eg. elephants – 22 month gestation - long maternal care period
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Why not do both?
• Organisms have finite resources
Figure 52.7
Researchers in the Netherlands studied the effects of parental caregiving in European kestrels over 5 years. The researchers transferred chicks among nests to produce reduced broods (three or four chicks), normal broods (five or six), and enlarged broods (seven or eight). They then measured the percentage of male and female parent birds that survived the following winter. (Both males and females provide care for chicks.)
EXPERIMENT
The lower survival rates of kestrels with larger broods indicate that caring for more offspring negatively affects survival of the parents.
CONCLUSION
100
80
60
40
20
0Reduced
brood sizeNormal brood
sizeEnlarged
brood size
Par
ents
sur
vivi
ng th
e fo
llow
ing
win
ter
(%)
MaleFemale
RESULTS
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Fecundity = number of offspring per reproductive episode
Varies with species and environment
elephants and whales 1
squirrels 2-6
oak trees thousands of acorns
salmon thousands of eggs
many invertebrates hundreds of thousands of eggs
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Plant parenting
• Many small seeds or fewer large seeds
(a) Most weedy plants, such as this dandelion, grow quickly and produce a large number of seeds.
(b) Some plants, such as this coconut palm, produce a moderate number of very large seeds.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Population growth models
• exponential growth requires an idealized environment
E. Coli growth curve
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Exponential population growth
• J-shaped curve
• Resource limitations not a factor
Figure 52.9
0 5 10 150
500
1,000
1,500
2,000
Number of generations
Pop
ulat
ion
size
(N
)
dNdt
1.0N
dNdt
0.5N
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J-shaped curve
• Is characteristic of some populations that are rebounding
Figure 52.10
1900 1920 1940 1960 1980
Year
0
2,000
4,000
6,000
8,000E
leph
ant p
opul
atio
n
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
“I think I may fairly make two postulata. First, that food is necessary to the existence of man. Secondly, that the passion between the sexes is necessary and will remain nearly in its present state. . . Population, when unchecked, increases in a geometrical ratio: Subsistence increases only in an arithmetical ratio. A slight acquaintance with numbers will show the immensity of the first power in comparison of the second.”
Thomas Malthus, 1798
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Population
Food
Time
Ab
und
ance
or
De
nsity
Malthus (1798) was the first to recognize the potential for populations to grow beyond the resources that can support them.
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Potential population growth
Resources
Time
Ab
und
ance
or
De
nsity
Actual population growth
Does it really happen?
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Potential population growth
Resources
Time
Ab
und
ance
or
De
nsity
Darwin (Campbell, p. 435)
1. Species have potential to increase population size exponentially
2. Most do not; resources are limited.
3. More individuals are produced than the environment can support.
4. This leads to a struggle for existence, with only a fraction of offspring surviving each generation.
5. Individuals vary, variation is heritable, and survival and reproduction depend on hereditary constitution
6. Unequal ability to survive and reproduce leads to change in a population
Actual population growth
Population and evolutionary change
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U.S. 295,995,954World 6,438,377,20214:44 GMT (EST+5) Apr 29, 2005
What about humans?
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Logistic growth model
• Includes concept of carrying capacity (K)
– maximum population size the environment can support
• Exponential growth cannot be sustained
• This model limits growth by incorporating carrying capacity
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Logistic model
• Produces a sigmoid (S-shaped) curve
Figure 52.12
dNdt
1.0N Exponential growth
Logistic growth
dNdt
1.0N1,500 N
1,500
K 1,500
0 5 10 150
500
1,000
1,500
2,000
Number of generations
Pop
ulat
ion
size
(N
)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 52.13a
800
600
400
200
0
Time (days)0 5 10 15
(a) A Paramecium population in the lab. The growth of Paramecium aurelia in small cultures (black dots) closely approximates logistic growth (red curve) if the experimenter maintains a constant environment.
1,000
Nu
mb
er
of
Pa
ram
eci
um
/ml
Real Populations
• Laboratory populations of paramecia
– S-shaped curve
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Real populations
• Some overshoot K then stabilize
Figure 52.13b
180
150
0
120
90
60
30
Time (days)
0 16014012080 100604020
Nu
mb
er
of
Da
ph
nia
/50
m
l
(b) A Daphnia population in the lab. The growth of a population of Daphnia in a small laboratory culture (black dots) does not correspond well to the logistic model (red curve). This population overshoots the carrying capacity of its artificial environment and then settles down to an approximately stable population size.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Some populations
– Fluctuate greatly around K. Why?
Figure 52.13c
0
80
60
40
20
1975 1980 1985 1990 1995 2000
Time (years)
Nu
mb
er
of
fem
ale
s
(c) A song sparrow population in its natural habitat. The population of female song sparrows nesting on Mandarte Island, British Columbia, is periodically reduced by severe winter weather, and population growth is not well described by the logistic model.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Life history traits
• K-selection, or density-dependent selection
– Selects for traits sensitive to population density
• r-selection, or density-independent selection
– Selects for traits that maximize reproduction
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Populations regulated by:
• complex interaction of biotic and abiotic influences
• So –
– What environmental factors stop a population from growing?
– Why do some populations show radical fluctuations in size over time
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Population Change
• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Population Regulation
• Density-dependent birth and death rates
– Are an example of negative feedback that regulates population growth
– Are affected by many different mechanisms
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Competition for Resources
• Increasing population density
– Intensifies intraspecific competition
Figure 52.15a,b
100 100
100
0
1,000
10,000
Ave
rag
e n
um
be
r o
f se
ed
s p
er
rep
rod
uci
ng
ind
ivid
ua
l (lo
g s
cale
)
Ave
rag
e c
lutc
h s
ize
Seeds planted per m2 Density of females
0 7010 20 30 40 50 60 802.8
3.0
3.2
3.4
3.6
3.8
4.0
(a) Plantain. The number of seeds produced by plantain (Plantago major) decreases as density increases.
(b) Song sparrow. Clutch size in the song sparrow on Mandarte Island, British Columbia, decreases as density increases and food is in short supply.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Territoriality
• Cheetahs mark boundaries
Figure 52.16
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Population density
• Also influenced by
– Disease
– Predation
– Toxic wastes
– Intrinsic factors eg. stress
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Population Cycles
• Many populations
– Undergo regular boom-and-bust cycles
Figure 52.21 Year1850 1875 1900 1925
0
40
80
120
160
0
3
6
9
Lynx
pop
ulat
ion
siz
e (t
hous
and
s)
Har
e po
pula
tion
size
(t
hous
and
s)
Lynx
Snowshoe hare
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Meadow Vole (Field Mouse)
Studies on Voles in Mission Valley, Western Montana (1999-2003)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Number of Voles on 29 Sites
0
2040
6080
100
1998 1999 2000 2001 2002 2003 2004
Year
Nu
mb
er
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Number of Voles on 29 Sites
0
2040
6080
100
1998 1999 2000 2001 2002 2003 2004
Year
Nu
mb
er
N = 77
N = 44
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Number of Voles on 29 Sites
0
200
400
600
1998 1999 2000 2001 2002 2003 2004
Year
Nu
mb
er
N = 533
N = 44
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Number of Voles on 29 Sites
0
200
400
600
1998 1999 2000 2001 2002 2003 2004
Year
Nu
mb
er
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Number of Voles on 29 Sites
0
200
400
600
1998 1999 2000 2001 2002 2003 2004
Year
Nu
mb
er
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Number of voles per site in 2001 (year of peak abundance)
0
10
20
30
40
50
60
Sites
Nu
mb
er
of
vo
les
/sit
e
Biological organisms are not evenly distributed in nature: Abundance varies in Time AND Space
Number of Voles on 29 Sites
0
200
400
600
1998 1999 2000 2001 2002 2003 2004
Year
Nu
mb
er
What are interesting questions?
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
What causes changes in numbers of biological organisms over time and space?
What are the mechanisms of change?
What are the biological consequences of changes in numbers (so what?)?
Why do patterns and mechanisms differ among species and areas?
Nutrients in rodent biomass, reduced plant growth, or impacts of grazing on plants
Predator response (individual and population)
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Vole predators in western Montana grasslands
•Grizzly bears
•Coyotes
•Foxes
•Weasels
•Hawks (various)
•Owls (various)
•Skunks
•Ravens
•Gulls
•Snakes (various)
Love to eat them mousies, mousies what I love to eat.
Bite they little heads off, nibble on they tiny
feet
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Number of Voles on 29 Sites
0
200
400
600
1998 1999 2000 2001 2002 2003 2004
Year
Nu
mb
er
A Predator’s View of Vole Populations
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Vole predators in western Montana grasslands
•Grizzly bears
•Coyotes
•Foxes
•Weasels
•Hawks (various)
•Owls (various)
•Skunks*
•Ravens*
•Gulls*
•Snakes* (various)
*Significant nest predators when other prey unavailable
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Prey (voles) abundant
Nest predation low
Bird reproductive success high
Predator populations grow
Predator populations remain highNest predation highBird reproductive success low
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Human population growth
• - has slowed after centuries of exponential increase
• No population can grow indefinitely
– And humans are no exception
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Global Human Population
• The human population
– Increased relatively slowly until about 1650 and then began to grow exponentially
Figure 52.22
8000 B.C.
4000 B.C.
3000 B.C.
2000 B.C.
1000 B.C.
1000 A.D.
0
The Plague Hum
an
pop
ulat
ion
(bill
ions
)
2000 A.D.
0
1
2
3
4
5
6
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Still growing but….
• The rate of growth began to slow approximately 40 years ago
Figure 52.231950 1975 2000 2025 2050
Year
2003
Per
cent
incr
ease
2.2
2
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
1.8
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Age structure• represented in pyramids
Figure 52.25
Rapid growth Afghanistan
Slow growth United States
Decrease Italy
Male Female Male Female Male FemaleAge Age
8 6 4 2 0 2 4 6 8 8 6 4 2 0 2 4 6 8 8 6 4 2 0 2 4 6 8Percent of population Percent of population Percent of population
80–8485
75–7970–7465–6960–6455–5950–5445–4940–4435–3930–34
20–2425–29
10–145–90–4
15–19
80–8485
75–7970–7465–6960–6455–5950–5445–4940–4435–3930–34
20–2425–29
10–145–90–4
15–19
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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
– Is one measure of how close we are to the carrying capacity of Earth
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Ecological footprints• Countries vary greatly in footprint size and available
ecological capacity
Figure 52.27
16
14
12
10
8
6
4
2
00 2 4 6 8 10 12 14 16
New Zealand
AustraliaCanada
Sweden
WorldChina
India
Available ecological capacity (ha per person)
SpainUK
Japan
GermanyNetherlands
Norway
USA
Eco
log
ica
l foo
tprin
t (h
a pe
r pe
rson
)