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Population Ecology
Chapter 52
Study of populations in relation to
environment
– Environmental influences on:
• population density and distribution
• age structure
• variations in population size
Definition of Population:
• Group of individuals of a single species living in the
same general area
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
Density: A Dynamic Perspective
• Determining the density of natural populations is
possible, but difficult to accomplish
• In most cases it is impractical or impossible to count all
individuals in a population
– How do wildlife biologists approximate populations?
Estimating Wildlife Population Size
Defined Populations
Undefined Populations
• Density is the result of a dynamic interaction of
processes that add individuals to a population and those
that remove individuals from it
Births and immigration add
individuals to a population.
Births
Immigration
How do these factors
Contribute to Population Size??
PopuIation
size
•
•
•
•
Emigration
Deaths
Deaths and emigration
remove individuals from a
population.
Figure 52.2
Births
Deaths
Immigration
Emigration
Patterns of Dispersion
• Environmental and social factors influence the
spacing of individuals in a population
Clumped Dispersion
– Individuals aggregate in patches
– May be influenced by resource availability and
behavior
Uniform Dispersion
–
–
Individuals are evenly distributed
May be influenced by social interactions such
as territoriality
Random Dispersion
• Position of each individual is independent of other
individuals
(c) Random. Dandelions grow from windblown seeds that
land at random and later germinate.
Life history traits are products of
natural selection
• Life history traits are evolutionary outcomes
– Reflected in the development, physiology, and
behavior of an organism
Semelparity: Big Bang
– Reproduce a single time and die
Figure 52.6
Iteroparity – Repeated Reproduction
–
Produce offspring repeatedly over time
“Trade-offs” and Life Histories
• Organisms have finite resources
–
Which may lead
to trade-offs
between survival
and reproduction
Kestrels:
•
Produce a few eggs?
–
•
Can invest more into
each, ensuring
greater survival
Produce many eggs?
–
Costly but if all
survive, fitness is
better
Male
Female
Parents surviving the following winter (%)
–
100
80
RESULTS
60
40
20
0
Reduced
brood size
Normal brood
size
Enlarged
brood size
CONCLUSION
The lower survival rates of kestrels with larger broods indicate
that caring for more offspring negatively affects survival of the parents.
More is Better?
• Some plants produce a large number of small seeds
– Ensuring that at least some of them will grow
and eventually reproduce
(a) 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.
Figure 52.8a
Fewer is Better?
• Other types of plants produce a moderate number
of large seeds
–
That provide a large store of energy that will
help seedlings become established
(b) 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.
Figure 52.8b
Demography
• Study of the vital statistics of a population
– And how they change over time
• Death rates and birth rates
• Zero population growth
– Occurs when the birth rate equals the death
rate
Exponential Population Growth
Population increase under idealized conditions
No limits on growth
• Under these conditions
– The rate of reproduction is at its maximum, called
the intrinsic rate of increase
Example-understanding growth
Question: I offer you a job for 1 cent/day and your pay will
double every day. You will be hired for 30 days. Will you
take my job offer?
Answer: If you said YES, you will have made $~21 million
dollars for 30 days of work.
How is this possible?????
1ST DAY OF WORK: 1 cent pay/day
30TH DAY OF WORK: ~10.2 million/day
Amount of
Pay/Day
How is this possible?????
# of Days
Exponential Growth Model
*Idealized population in an unlimited
environment
*Very rapid doubling time; steep J curve
*r=N=(b-d)N
t
r=instrinsic rate of growth
dN 
dt rmaxN
Exponential Growth in the Real World
• Characteristic of some populations that are rebounding
8,000
Elephant
population
6,000
4,000
2,000
0
1900
1920
1940
Year
1960
1980
–Cannot be sustained for long in any population
Logistic Population Growth
• A more realistic population model
– Limits growth by incorporating carrying capacity
Logistic Population Growth
• Carrying capacity (K)
– Is the maximum population size the
environment can support
• In the logistic population growth model
– The per capita rate of increase declines as
carrying capacity is reached
Logistic Growth Equation
–
Includes K, the carrying capacity
(K  N)
dN
 rmax N
dt
K
Logistic Population Growth
–
Produces a sigmoid (S-shaped) curve
2,000
dN
dt
Population size (N)
1,500
 1.0N
Exponential
growth
K  1,500
Logistic growth
1,000
dN
dt
 1.0N
1,500  N
1,500
500
(K  N)
dN
 rmax N
dt
K
0
0
5
10
Number of generations
Figure 52.12
15
The Logistic Model and Real Populations
• The growth of laboratory populations of paramecia
– Fits an S-shaped curve
Number of Paramecium/ml
1,000
Figure 52.13a
800
600
400
200
0
0
5
10
Time (days)
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.
Logistic Growth and The Real World
• Some populations overshoot K
Before settling down to a relatively stable
density
Number of Daphnia/50 ml
–
180
150
120
90
60
30
0
0
20
40
60
80
100 120 140 160
Time (days)
Figure 52.13b
(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.
Logistic Growth and the Real World
• Some populations
– Fluctuate greatly around K
Number of females
80
60
40
20
0
1975
1980
1985
1990
1995
2000
Time (years)
Figure 52.13c
(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.
The Logistic Model and Life Histories
• Life history traits favored by natural selection
– May vary with population density and
environmental conditions
Life History and Logistic Growth
• K-selection, or density-dependent selection
– Selects for life history traits that are sensitive to
population density
• Reproduce slowly, small litters
• r-selection, or density-independent selection
– Selects for life history traits that maximize
reproduction
• Reproduce rapidly, large litters
Natural selection (diverse reproductive strategies)
a) Relatively few, large offspring (K selected species)
b) Many, small offspring (r selected species)
(K selected species)
(r selected species)
Human Populations
• No population can grow indefinitely and humans are no
exception
6
4
3
2
The Plague
1
0
Figure 52.22
8000
B.C.
4000
B.C.
3000
B.C.
2000
B.C.
1000
B.C.
0
1000
A.D.
2000
A.D.
Human population (billions)
5
Global Carrying Capacity
• Just how many humans can the biosphere support?
• Carrying capacity of earth is unknown….
http://www.youtube.com/watch?v=9_9SutNmfFk
http://www.youtube.com/watch?v=UUOEcNomakw&feature=rec
-LGOUT-exp_fresh+div-1r-8-HM
http://www.youtube.com/watch?v=4B2xOvKFFz4&feature=relat
ed
Populations Regulated Biotic and
Abiotic Factors
Two general questions we can ask about regulation of
population growth
1. What environmental factors stop a population
from growing?
2. Why do some populations show radical
fluctuations in size over time, while others remain
stable?
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
Density-Dependent Population Regulation
• Density-dependent birth and death rates
– Are an example of negative feedback that
regulates population growth
– Are affected by many different mechanisms
Competition for Resources
• In crowded populations, increasing population density
– Intensifies intraspecific competition for resources
4.0
3.8
Average clutch size
Average number of seeds
per reproducing individual
(log scale)
10,000
1,000
100
3.4
3.2
3.0
2.8
0
0
10
100
Seeds planted per m2
(a) Plantain. The number of seeds
produced by plantain (Plantago major)
decreases as density increases.
Figure 52.15a,b
3.6
0
10
20
30
40
50
60
70
Density of females
(b) Song sparrow. Clutch size in the song sparrow
on Mandarte Island, British Columbia, decreases
as density increases and food is in short supply.
80
Territoriality
• In many vertebrates and some invertebrates
– Territoriality may limit density
Territoriality Example: Cheetas
• Cheetahs are highly territorial
– Using chemical communication to warn other
cheetahs of their boundaries
Figure 52.16
Territoriality: Ocean birds
–
Figure 52.17
Exhibit territoriality in nesting behavior
Health
• Population density
– Can influence the health and survival of
organisms
• In dense populations
– Pathogens can spread more rapidly
Predation
• As a prey population builds up
– Predators may feed preferentially on that
species
Intrinsic Factors
• For some populations
– Intrinsic (physiological) factors appear to
regulate population size
Population Dynamics
• The study of population dynamics
– Focuses on the complex interactions between
biotic and abiotic factors that cause variation
in population size
Fluctuations in Population Size
• Extreme fluctuations in population size
– Are typically more common in invertebrates
than in large mammals
Commercial catch (kg) of
male crabs (log scale)
730,000
100,000
10,000
1950
Figure 52.19
1960
1970
Year
1980
1990
Metapopulations and Immigration
• Metapopulations
– Groups of populations linked by immigration and
emigration
Immigration- Movement Into a Population
• High levels of immigration combined with higher
survival can result in greater stability in
populations
60
Number of breeding females
50
40
Mandarte
island
30
20
10
Small
islands
0
1988
Figure 52.20
1989
1990
Year
1991
Population Cycles
160
Snowshoe hare
120
Lynx
9
80
6
40
3
0
1850
0
1875
1900
Year
• Influenced by complex interactions between biotic and
abiotic factors
1925
Lynx population size
(thousands)
Hare population size
(thousands)
• Many populations undergo regular boom-and-bust cycles
The Global Human Population
• The human population increased relatively slowly
until about 1650 and then began to grow
exponentially
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 rates – High
death rates
• Zero population growth = Low birth rates – Low
death rates
Age Structure
• One important demographic factor in present and
future growth trends
– Is a country’s age structure, the relative
number of individuals at each age
• Age structure is commonly represented in pyramids
Rapid growth
Afghanistan
Male
Female
8 6 4 2 0 2 4 6 8
Percent of population
Figure 52.25
Age
85
80–84
75–79
70–74
65–69
60–64
55–59
50–54
45–49
40–44
35–39
30–34
25–29
20–24
15–19
10–14
5–9
0–4
Slow growth
United States
Female
Male
8 6 4 2 0 2 4 6 8
Percent of population
Age
85
80–84
75–79
70–74
65–69
60–64
55–59
50–54
45–49
40–44
35–39
30–34
25–29
20–24
15–19
10–14
5–9
0–4
Decrease
Italy
Female
Male
8 6 4 2 0 2 4 6 8
Percent of population
Infant Mortality and Life Expectancy
• Infant mortality and life expectancy at birth
50
Life expectancy (years)
Infant mortality (deaths per 1,000 births)
– Vary widely among developed and developing
countries but do not capture the wide range of the
60
80
human condition
40
30
20
40
20
10
0
0
Developed
countries
Figure 52.26
60
Developing
countries
Developed
countries
Developing
countries