Population Ecology

1
Chapter 52

• Population Ecology

2

• Population ecology is the study of populations in
  relation to environment, including environmental
  influences on density and distribution, age
  structure, and population size

• A population is a group of individuals of a
  single species living in the same general area

3
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

Immigration
Births

• Determining the density of natural populations is
  difficult (mark recature)
• 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

Population size
Deaths
Emigration
4
Patterns of Dispersion

• Environmental and social factors influence
  spacing of individuals in a population

• In a clumped dispersion, individuals aggregate in
  patches
• A clumped dispersion may be influenced by
  resource availability and behavior

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.

• A uniform dispersion is one in which individuals
  are evenly distributed
• It may be influenced by social interactions such
  as territoriality

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.

• In a random dispersion, the position of each
  individual is independent of other individuals

Random. Dandelions grow from windblown seeds that
land at random and later germinate.
5
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
• Life tables, survivorship curves and reproductive
  rates help demographers show change in
  populations over time

6
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 Beldings ground squirrels
  reveals many things about this population

7
Survivorship Curves

• A survivorship curve is a graphic way of
  representing the data in a life table
• The survivorship curve for Beldings ground
  squirrels shows a relatively constant death rate

1,000
Females
Plots of the individuals that are alive at the
start of each year
100
Males
Number of survivors (log scale)
10
1
0
2
8
10
4
6
Age (years)
8
LE 52-5
Survivorship curves can be classified into three
general types Type I, Type II, and Type III
1,000
I
100
II
Number of survivors (log scale)
10
III
1
100
0
50
Percentage of maximum life span
9
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

10
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

11
Life History Diversity

• Life histories are very diverse (reproductive age
  varies)
• Allocation of limited resources
• Number of reproductive episodes per lifetime
• Species that exhibit semelparity, or big-bang
  reproduction, reproduce once and die
• Agave and salmon
• Species that exhibit iteroparity, or repeated
  reproduction, produce offspring repeatedly
• Lizzards
• IF survival rate is LOW (unpredictable
  environments) semelparity is favored (increases
  survivorship)
• IF survival rate is HIGH (more stable
  environments) Iteroparity is favored

12
LE 52-8a
Selective pressures mandate trade-offs between
investment in reproduction and survival
Some plants produce a large number of small
seeds, ensuring that at least some of them will
grow and eventually reproduce
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.
In animals, parental care of smaller broods may
facilitate survival of offspring
13
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

14
Per Capita Rate of Increase

• If immigration and emigration are ignored, a
  populations growth rate (per capita increase)
  equals birth rate minus death rate

• Zero population growth (ZPG) 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

N population size r increase in growth rate K
carrying capacity t time interval
15
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

• Equation of exponential population growth

N population size rmax intrinsic rate of
increase K carrying capacity t time interval

• Exponential population growth results in a
  J-shaped curve
• When r is greater than 0, populations are growing
  exponenetially

16
2,000
dN
1.0N
dt
1,500
dN
0.5N
dt
Population size (N)
1,000
500
0
15
10
5
0
Number of generations
17
The J-shaped curve of exponential growth
characterizes some rebounding populations
8,000
6,000
Elephant population
4,000
2,000
0
1980
1960
1940
1920
1900
Year
18
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

19
The Logistic Growth Model

• 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
• 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

• The logistic growth equation includes K, the
  carrying capacity

When r is equal/less than 0, populations are
growing logistically
20
The logistic model of population growth produces
a sigmoid (S-shaped) curve
2,000
dN
1.0N
Exponential growth
dt
1,500
K 1,500
Logistic growth
Population size (N)
1,000
dN
1,500 N
1.0N
dt
1,500
500
0
15
10
5
0
Number of generations
21
The Logistic Model and Real Populations
1,000
The growth of laboratory populations of paramecia
fits an S-shaped curve
800
600
Number of Paramecium/mL
400
200
0
5
10
15
0
Time (days)
A Paramecium population in the lab
22
Some populations overshoot K before settling down
to a relatively stable density
180
150
120
Number of Daphnia/50 mL
90
60
30
0
0
20
40
60
80
100
120
140
160
Time (days)
A Daphnia population in the lab
23
The Logistic Model and Life Histories

• The logistic model fits few real populations but
  is useful for estimating possible growth

• 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
• Sickness/disease, competition, territoriality,
  health
• birth rates fall and death rates rise with
  population density
• Density-dependent birth and death rates are an
  example of negative feedback that regulates
  population growth
• r-selection, or density-independent selection,
  selects for life history traits that maximize
  reproduction
• Natural disasters
• birth rate and death rate do not change with
  population density

24
LE 52-15
Competition for Resources
In crowded populations, increasing population
density intensifies intraspecific competition for
resources
4.0
10,000
3.8
3.6
Average number of seeds per reproducing
individual (log scale)
1,000
Average clutch size
3.4
3.2
3.0
100
2.8
60
0
10
100
10
1
80
70
50
30
40
20
Plants per m2 (log scale)
Females per unit area
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.
25
Territoriality (density dependent, k-selection)

• Cheetahs are highly territorial, using chemical
  communication to warn other cheetahs of their
  boundaries

• In many vertebrates and some invertebrates,
  territoriality may limit density

• Oceanic birds exhibit territoriality in nesting
  behavior

26
Health (density dependent, k-selection)

• Population density can influence the health and
  survival of organisms
• In dense populations, pathogens can spread more
  rapidly

Predation (density dependent, k-selection)

• As a prey population builds up, predators may
  feed preferentially on that species

Toxic Wastes (density dependent, k-selection)

• Accumulation of toxic wastes can contribute to
  density-dependent regulation of population size

Intrinsic Factors (density dependent,
k-selection)

• For some populations, intrinsic (physiological)
  factors appear to regulate population size

27
Population Dynamics

• The study of population dynamics focuses on the
  complex interactions between biotic and abiotic
  factors that cause variation in population size

28
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

Song sparrow populations on a cluster of small
islands make up a metapopulation. Immigration
keeps the linked populations more stable than the
isolated population on the larger island.
60
50
Mandarte Island
40
30
Number of breeding females
20
Small islands
10
0
1991
1988
1989
1990
Year
29
LE 52-21
Many populations undergo boom-and-bust
cycles Boom-and-bust cycles are influenced by
complex interactions between biotic and abiotic
factors
Snowshoe hare
160
120
9
Lynx
Lynx population size (thousands)
Hare population size (thousands)
80
6
40
3
0
0
1850
1875
1900
1925
Year
30
Concept 52.6 Human population growth has slowed
after centuries of exponential increase

• No population can grow indefinitely, and humans
  are no exception

31
The Global Human Population
The human population increased relatively slowly
until about 1650 and then began to grow
exponentially
6
5
4
Human population (billions)
3
2
The Plague
1
0
2000 B.C.
8000 B.C.
4000 B.C.
3000 B.C.
1000 B.C.
0
1000 A.D.
2000 A.D.
32
LE 52-23
Though the global population is still growing,
the rate of growth began to slow about 40 years
ago
2.2
2
1.8
1.6
2003
1.4
Annual percent increase
1.2
1
0.8
0.6
0.4
0.2
0
2000
2050
2025
1975
1950
Year
33
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

34
LE 52-24
50
40
30
Birth or death rate per 1,000 people
20
10
Sweden
Mexico
Birth rate
Birth rate
Death rate
Death rate
0
2050
1950
1850
1750
2000
1900
1800
Year
35

• The demographic transition is associated with
  various factors in developed and developing
  countries
• Family planning
• Volunteer contraception
• Delayed marriage and reproduction
• Education

36
Age Structure

• One important demographic factor in present and
  future growth trends is a countrys age structure
• Age structure is the relative number of
  individuals at each age
• It is commonly represented in pyramids
• Age structure diagrams can predict a populations
  growth trends
• They can illuminate social conditions and help us
  plan for the future

37
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

38
Estimates of Carrying Capacity

• The carrying capacity of Earth for humans is
  uncertain

• At more than 6 billion people, the world is
  already in ecological deficit

39
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

40
LE 52-27
16
Ecological deficit
14
12
New Zealand
10
USA
Germany
Australia
Ecological footprint (ha per person)
8
Netherlands
Japan
Canada
Norway
6
Sweden
UK
4
Spain
World
2
China
India
0
6
16
0
2
4
10
12
14
8
Available ecological capacity (ha per person)