copyright © 2009 pearson education, inc. population structure and dynamics
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POPULATION STRUCTURE AND DYNAMICS
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36.1 Population ecology is the study of how and why populations change
Population
– A group of individuals of a single species that occupy the same general area
Individuals in a population
– Rely on the same resources
– Are influenced by the same environmental factors
– Are likely to interact and breed with one another
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A population can be described by the number and distribution of individuals
Population dynamics is the interactions between
– Biotic and abiotic factors
It is the cause of variation in population sizes
– A population increases through birth and immigration
– Death and emigration out of an area decrease the population
36.1 Population ecology is the study of how and why populations change
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36.2 Density and dispersion patterns are important population variables
Population density is the number of individuals of a species per unit area or volume
Examples of population density
– The number of oak trees per square kilometer in a forest
– The number of earthworms per cubic meter in forest soil
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Within a population’s geographic range, local densities may vary greatly
The dispersion pattern of a population refers to the way individuals are spaced within their area
Dispersion patterns can be
– Clumped
– Uniform
– Random
36.2 Density and dispersion patterns are important population variables
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In a clumped pattern individuals are grouped in patches
36.2 Density and dispersion patterns are important population variables
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In a uniform pattern individuals are equally spaced in the environment
36.2 Density and dispersion patterns are important population variables
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In a random pattern of dispersion, the individuals in a population are spaced in an unpredictable way
36.2 Density and dispersion patterns are important population variables
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36. 3 Life tables track survivorship in populations
Life tables track survivorship over the life span of individuals in a population
Survivorship curves plot the proportion of individuals alive at each age
– Type I
– Type II
– Type III
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Percentage of maximum life span
Per
cen
tag
e o
f su
rviv
ors
(lo
g s
cale
)
0 50 1000.1
1
10
100
III
II
I
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36.4 Idealized models predict patterns of population growth
Exponential growth model
– The rate of population increases under ideal conditions
– Calculated using the equation G = rN
– G is the growth rate of the population
– N is the population size
– r is the per capita rate of increase
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Time (months)
Po
pu
lati
on
siz
e (N
)
0 1 2 3 4 5 6 7 8 9 10 11 120
50
100
150
200
250
300
350
400
450
500
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Logistic growth model
– This growth model takes into account limiting factors
– Limiting factors are environmental factors that restrict population growth
– Formula
36.4 Idealized models predict patterns of population growth
(K N)G = rN
K
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Year
Bre
edin
g m
ale
fur
seal
s(t
ho
usa
nd
s)
1915 1925 1935 19450
2
4
6
8
10
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Idealized models describe two kinds of population growth
– Exponential growth
– Logistic growth
36.4 Idealized models predict patterns of population growth
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Time
Nu
mb
er o
f in
div
idu
als
(N)
0
K
G = rN
G = rN(K – N)
K
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36.5 Multiple factors may limit population growth
The logistic growth model
– Population growth slows and ceases as population density increases
– Increasing population density results in a decrease in birth rate, an increase in death rate, or both
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Density of females
Clu
tch
siz
e
0 10 20 30 40 50 60 70 802.8
3.0
3.2
3.4
3.6
3.8
4.0
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Abiotic factors may reduce population size before other limiting factors become important
36.5 Multiple factors may limit population growth
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Apr May Jun Jul Aug Sep Oct Nov Dec
Nu
mb
er o
f ap
hid
s
Exponentialgrowth
Suddendecline
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Most populations fluctuate in numbers
36.5 Multiple factors may limit population growth
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Time (years)
Nu
mb
er o
f fe
mal
es
1975 1980 1985 1990 1995 20000
20
40
60
80
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36.6 Some populations have “boom-and-bust” cycles
Some populations fluctuate in density with regularity
Boom-and-bust cycles
– Food shortages
– Predator-prey interactions
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Lynx
Snowshoe hare
Ly
nx
po
pu
lati
on
siz
e(t
ho
usa
nd
s)
Ha
re p
op
ula
tio
n s
ize
(th
ou
san
ds)
Year1850 1875 1900 1925
0
40
80
120
160
0
3
6
9
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36.7 EVOLUTION CONNECTION: Evolution shapes life histories
Life history
– Series of events from birth to death
R/K selection
– R-selective traits: populations with rapid growth and maturation; little postnatal care; also known as “opportunistic populations”
– K-selective traits: populations with a roughly constant size whose members have low reproductive rates. Lots of postnatal care required. “K” = Kids!
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THE HUMAN POPULATION
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36.10 CONNECTION: Age structures reveal social and economic trends
Age structure diagram
– Reveals a population’s growth trends
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Age1980
Birth years Male Female2005 2030
Birth years Male Female Birth years Male Female80+ before 1900 before 1926 before 1951
1951-551926-301901-190575-7970-74 1906-10 1931-35 1956-60
1961-651936-401911-1565-6960-6455-59
1916-20 1941-45 1966-701971-751946-501921-25
50-54 1926-30 1951-55 1976-801981-851956-601931-3545-49
40-44 1936-40 1961-65 1986-901991-951966-701941-4535-39
30-34 1946-50 1971-75 1996-20002001-051976-801951-5525-29
20-24 1956-60 1981-85 2006-102011-151986-901961-6515-19
10-14 1966-70 1991-95 2016-202021-251996-20001971-755-9
0-4 1976-80 2001-2005 2026-30
12 10 8 6 4 2 0 2 4 6 8 10 12Population in millions
Total population size = 363,811,435Population in millions
Total population size = 295,734,134
12 10 8 6 4 2 0 2 4 6 8 10 1212 10 8 6 4 2 0 2 4 6 8 10 12Population in millions
Total population size = 227,726,463
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You should now be able to
1. Explain the factors that determine the characteristics of a population
2. Describe exponential growth and the factors that produce logistic growth of a population
3. Explain the limiting factors that influence population growth
4. Distinguish between r- and K-strategies
5. Describe and give examples of the different types of life histories