Population Ecology

1
Population Ecology

• AP Chap 53

2

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

3
Fig. 53-1
A population is a group of individuals of a
single species living in the same general area
4
Every population has geographic boundaries.

• 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

5
Population density is often determined by
sampling techniques

• Population size can be estimated by
• Direct counting
• Random sampling based on sample plots (quadrats)
• Indexes such as tracks, nests, burrows, fecal
  droppings, etc.
• Mark and recapture method

6
Quadrat sampling
7
Mark-recapture Method
8
Mark-Recapture Formula for estimating population
size

• Estimate of Total Population   
• (total number recaptured) x (number marked)
• (total number recaptured with mark)

9

• In a mark-recapture study, an ecologist traps,
  marks and releases 25 voles in a small wooded
  area. A week later she resets her traps and
  captures 30 voles, 10 of which are marked. What
  is her estimate of the vole population in that
  area?

10
How does population density change?

• Addition birth, immigration
• Removal death, emigration

11
Fig. 53-3
Births
Deaths
Births and immigration add individuals to a
population.
Deaths and emigration remove individuals from a
population.
Immigration
Emigration
12
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

13
Fig. 53-4a
(a) Clumped
14
UNIFORM

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

15
Fig. 53-4b
(b) Uniform
16
RANDOM

• In a random dispersion, the position of each
  individual is independent of other individuals
• It occurs in the absence of strong attractions or
  repulsions

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Fig. 53-4c
(c) Random
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19
Demographics

• Demography is the study of the vital statistics
  (death and birth rates) of a population and how
  they change over time
• Death rates and birth rates are of particular
  interest to demographers
• 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, a group of individuals of the same age

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Table 53-1
The life table of Beldings ground squirrels
reveals many things about this population
21
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

22
Fig. 53-5
1,000
100
Number of survivors (log scale)
Females
10
Males
1
2
0
4
8
6
10
Age (years)
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• Survivorship curves can be classified into three
  general types
• Type I low death rates during early and middle
  life, then an increase among older age groups
• Type II the death rate is constant over the
  organisms life span
• Type III high death rates for the young, then a
  slower death rate for survivors

24
Fig. 53-6
More parental care, better health care
1,000
I
Predation, accidents, disease at all levels
100
II
Number of survivors (log scale)
10
High mortality of many offspring
III
1
0
50
100
Percentage of maximum life span
25
What type of survivorship curve?
Type 2
Type 3
Type 1
26
Reproductive tables focus on female
reproductivity.
Table 53-2
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Life history traits are products of natural
selection

• An organisms life history comprises the traits
  that affect its schedule of reproduction and
  survival
• The age at which reproduction begins
• How often the organism reproduces
• How many offspring are produced during each
  reproductive cycle

28

• Species that exhibit semelparity, or big-bang
  reproduction, reproduce once and die
• Species that exhibit iteroparity, or repeated
  reproduction, produce offspring repeatedly
• Highly variable or unpredictable environments
  likely favor big-bang reproduction, while
  dependable environments may favor repeated
  reproduction.

29
Trade-offs and Life Histories

• Organisms have finite resources, which may lead
  to trade-offs between survival and reproduction
• Examples
• brood size vs parental life span
• number of seeds and chance of gemination and
  growth

30
Fig. 53-8
RESULTS
100
Male
Female
80
60
Parents surviving the following winter ()
40
20
0
Reduced brood size
Normal brood size
Enlarged brood size
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Fig. 53-9
Some plants produce a large number of small
seeds, ensuring that at least some of them will
grow and eventually reproduce
(a) Dandelion
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) Coconut palm
32
In what way might high competition for limited
resources in a predictable environment influence
the evolution of life history traits? Semelparity
or iteroparity

• Selection would most likely favor iteroparity,
  with fewer, larger, better-provisioned or
  cared-for offspring.

33
How do we model population growth?

• By construction graphs and using mathematical
  formulas
• If immigration and emigration are ignored, a
  populations growth rate (per capita increase)
  equals birth rate minus death rate
• r b - d

34

• 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

where N population size, t time, and r per
capita rate of increase
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Exponential Growth

• Exponential population growth is population
  increase under idealized. unlimited conditions
• Under these conditions, the rate of reproduction
  is at its maximum, called the intrinsic rate of
  increase

36

• Equation of exponential population growth

Exponential population growth results in a
J-shaped curve.
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Fig. 53-10
2,000
dN
1.0N

dt
1,500
dN
0.5N

dt
Population size (N)
1,000
500
0
0
5
10
15
Number of generations
38
Fig. 53-11
The J-shaped curve of exponential growth
characterizes some rebounding populations.
8,000
6,000
Elephant population
4,000
Elephants in Kruger National Park in S. Africa
after they were protected from hunting.
2,000
0
1920
1940
1960
1980
1900
Year
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But, is this the normal state of population
growth?

• Exponential growth cannot be sustained for long
  in any population
• A more realistic population model limits growth
  by incorporating carrying capacity
• Carrying capacity (K) is the maximum population
  size the environment can support

40
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
  approaches K.

41
Table 53-3
As N approaches K, rate nears 0.
42

• The logistic model of population growth produces
  a sigmoid (S-shaped) curve

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Fig. 53-12
Exponential growth
2,000
K
dN
1.0N

dt
1,500
K 1,500
Population size (N)
Logistic growth
1,000
1,500 N
dN
1.0N

1,500
dt
500
0
0
5
10
15
Number of generations
44
Fig. 53-13a
Some populations overshoot K before settling down
to a relatively stable density
1,000
800
These organisms are grown in a constant
environment lacking predators and competitors
Number of Paramecium/mL
600
400
200
0
0
5
10
15
Time (days)
(a) A Paramecium population in the lab
45
Fig. 53-13b
Some populations fluctuate greatly and make it
difficult to define K.
180
150
120
Number of Daphnia/50 mL
90
60
30
0
0
20
40
60
80
100
120
140
160
Time (days)
(b) A Daphnia population in the lab
46

• Some populations show an Allee effect, in which
  individuals have a more difficult time surviving
  or reproducing if the population size is too
  small

47
The Logistic Model and Life Histories

• Natural selection shapes the final life history
  of individual species.
• Some members of populations are subject to
  r-selection and some to k-selection.
• When population size is low relative to K,
• r-selection favors r-strategies
• high fecundity (ability to reproduce),
• small body size,
• early maturity onset,
• short generation time, and
• the ability to disperse offspring widely.

48
Characteristics of r - Selected Opportunists

• Very high intrinsic rate of increase.
• Opportunistic
• Populations can expand rapidly to take advantage
  of temporarily favorable conditions
• Ex Bacteria, some fungi, many insects, rodents,
  weeds, and annual plants.

49

• In environments that are relatively stable and
  populations tend to be near K, with minimal
  fluctuations in population size, K-selection
  favors K strategies large body size, long life
  expectancy, and the production of fewer offspring
  that require extensive parental care until they
  mature.
• These populations are strong competitors.
• They are specialists rather than colonists and
  may become extinct if their normal way of life is
  destroyed.

50
Characteristics of K - Selected Species

• Population responds slowly, usually with negative
  feedback control so that constancy is the rule.
• Their numbers are controlled by the availability
  of resources. In other words, they are a density
  dependent species

Most birds
Most predators
Elephants
Whales
Oaks
Chestnuts
Apple
Coconut
51
r or k-selected?

• Nature is more complex though and most
  populations lie somewhere in between these two
  extremes.
• Ex- Gymnosperms and angiosperms are typically
  classified as K-strategists but they release many
  seeds.
• Cod fish are large fish but
• release large numbers of
• gametes into the sea with
• no parental investment.
• So, cod are considered
• r-strategists.

52
K or r-selected ?

• When a farmer abandons a field, it is quickly
  colonized by fast-growing weeds. Are these
  species more likely to be K-selected or
  r-selected species?

r
53
What about the bluegill fish?

• Bluegill exhibit one of the most social and
  complex mating systems in nature. Parental males
  delay maturation and compete to construct nests
  in colonies, court females, and provide sole
  parental care for the young within their nest.

54
Many factors that regulate population growth are
density dependent

• There are two general questions about regulation
  of population growth
• What environmental factors stop a population from
  growing indefinitely?
• Why do some populations show radical fluctuations
  in size over time, while others remain stable?

55
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

56
Fig. 53-15
Density-dependent birth rate
Density-dependent birth rate
Density- independent death rate
Density- dependent death rate
Birth or death rate per capita
Equilibrium density
Equilibrium density
Population density
Population density
(a) Both birth rate and death rate vary.
(b) Birth rate varies death rate is constant.
Density-dependent death rate
Density- independent birth rate
Birth or death rate per capita
Equilibrium density
Population density
(c) Death rate varies birth rate is constant.
57
So, to determine if the environmental factor is
density dependent or independent.

• Density-independent factors may affect all
  individuals in a population equally
• rainfall, temperature, humidity, acidity,
  salinity, catastrophic events

58

• Density-dependent factors have a greater affect
  when the population density is higher.

Food supply, disease, parasites, competition,
predation
59
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,
  disease, predation, toxic wastes, and intrinsic
  factors

60
Fig. 53-17a
In many vertebrates and some invertebrates,
competition for territory may limit
density Cheetahs are highly territorial, using
chemical communication to warn other cheetahs of
their boundaries
(a) Cheetah marking its territory
61
Fig. 53-17b

• Oceanic birds exhibit territoriality in nesting
  behavior

(b) Gannets
62
Disease

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

63
Predation

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

64
Toxic Wastes

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

65
Intrinsic Factors

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

66
Population Dynamics

• The study of population dynamics focuses on the
  complex interactions between biotic and abiotic
  factors that cause variation in population size
• Long-term population studies have challenged the
  hypothesis that populations of large mammals are
  relatively stable over time
• Weather can affect population size over time

67
Fig. 53-18
2,100
1,900
1,700
1,500
1,300
Number of sheep
1,100
900
700
500
0
1955
1965
1975
1985
1995
2005
Year
68
Fig. 53-19
2,500
50
Wolves
Moose
2,000
40
1,500
30
Number of moose
Number of wolves
1,000
20
500
10
0
0
1955
1965
1975
1985
1995
2005
Year
Changes in predation pressure can drive
population fluctuations
69
Population Cycles Scientific Inquiry

• Some populations undergo regular boom-and-bust
  cycles
• Lynx populations follow the 10 year boom-and-bust
  cycle of hare populations
• Three hypotheses have been proposed to explain
  the hares 10-year interval
• - winter food supply
• - predators
• - sunspot activity (quality of food)
• affected cycles

70
Fig. 53-20
Snowshoe hare
160
120
9
Lynx
Number of lynx (thousands)
Number of hares (thousands)
80
6
40
3
0
0
1850
1875
1900
1925
Year
71
The human population is no longer growing
exponentially but is still increasing rapidly

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

72
Fig. 53-22
7
6
The human population increased relatively slowly
until about 1650 and then began to grow
exponentially
5
4
Human population (billions)
3
2
The Plague
1
0
8000 B.C.E.
4000 B.C.E.
3000 B.C.E.
2000 B.C.E.
1000 B.C.E.
0
1000 C.E.
2000 C.E.
73
Fig. 53-23
2.2
2.0
1.8
1.6
1.4
2005
1.2
Annual percent increase
Projected data
1.0
0.8
Though the global population is still growing,
the rate of growth began to slow during the 1960s
0.6
0.4
0.2
0
1950
1975
2000
2025
2050
Year
74
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

75
Fig. 53-24
The demographic transition in Sweden took about
150 years, from 1810 to 1960. It will take about
the same length of time for Mexico.
50
40
30
Birth or death rate per 1,000 people
20
10
Sweden
Mexico
Birth rate
Birth rate
Death rate
Death rate
0
1750
1800
1900
1950
2000
2050
1850
Year
76

• The demographic transition is associated with an
  increase in the quality of health care and
  improved access to education, especially for
  women
• Most of the current global population growth is
  concentrated in developing countries

77
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
• Age structure diagrams can predict a populations
  growth trends
• They can illuminate social conditions and help us
  plan for the future

78
Fig. 53-25
Rapid growth
Slow growth
No growth
Afghanistan
United States
Italy
Male
Female
Age
Age
Male
Female
Male
Female
85
85
8084
8084
7579
7579
7074
7074
6569
6569
6064
6064
5559
5559
5054
5054
4549
4549
4044
4044
3539
3539
3034
3034
2529
2529
2024
2024
1519
1519
1014
1014
59
59
04
04
10 
10 
8
8
6
6
4
4
2
2
0
6
6
4
4
2
2
0
8
8
6
6
4
4
2
2
0
8
8
Percent of population
Percent of population
Percent of population
79
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

80
Fig. 53-26
60
80
50
60
40
Life expectancy (years)
Infant mortality (deaths per 1,000 births)
30
40
20
20
10
0
0
Less indus- trialized countries
Indus- trialized countries
Less indus- trialized countries
Indus- trialized countries
81
Global Carrying Capacity

• How many humans can the biosphere support?
• The carrying capacity of Earth for humans is
  uncertain
• The average estimate is 1015 billion

82
Limits on Human Population Size

• 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

83
Fig. 53-27
Log (g carbon/year)
13.4
9.8
5.8
Not analyzed
84

• Our carrying capacity could potentially be
  limited by food, space, nonrenewable resources,
  or buildup of wastes