Transcript Th, February 5th

Populations
Possible Test Questions
1. Explain how biotic potential and/or carrying
capacity produce the J-shaped and S-shaped
population growth curves.
2. Draw the three main survivorship curves and relate
them to r selection and K selection in animals.
3. Explain how a single child born in the United States
can have a greater effect on the environment and
natural resources than a dozen or more children
born in a developing country.
Factors Affecting Population Numbers
Biotic Potential
The Biotic
Potential is the
maximum rate
at which a
population
could increase
under ideal
conditions.
Different species have different biotic potentials..
Biotic Potential of Bacteria
Biotic Potential of Flies
Biotic potential of houseflies (Musca domestica) in one year
Assuming that:
- a female lays 120 eggs per generation
- half of these eggs develop into females
- there are seven generations per year
Generation
1
2
3
4
5
6
7
Total Population: If all females in each
gene ration lay 120 eggs and then die.
120
7,200
432,000
25,920,000
1,555,200 ,000
93,312,000 ,000
5,598,720 ,000,000
Source : Data fro m E.J. Kormondy, Concepts of Ecology, 3rd ed., 1984
Harper and Row Publi shers, Inc.
Biotic Potential of Humans?
Exponential Growth
Growth = dN/dt = rN
where:
r = biotic potential (per cap.
rate of increase)
N = population size
When N is small, growth is small
When N is large, growth is large
When r is larger, the more rapid the growth
Exponential Growth
Think of exponential growth in terms of making
money. How much you increase your
earnings will depend on:
1. How much principle you have (population
size)
2. The interest rate at which you are earning
money (the biotic potential)
Density-dependent Growth Compared
to Exponential Growth
Populations will not increase forever. Ignoring all
interactions with other species (competition, predation,
parasites, herbivory), resources will still limit growth food resources and space resources.
(Remember the flies data from previous slide - In just 10
years, we would be swimming in several meters of flies if
exponential growth continued.)
Characteristics
of Densitydependent
Growth
Resources will
limit growth Food and/or
Space
Carrying capacity = the maximum number of individuals of
a species that can be sustained by an environment without
decreasing the capacity of the environment to sustain that
same amount in the future.
J-Shaped
Compared to
S-Shaped
Growth
Curves
K
r
growth = dN/dt = rN [(K-N)/K]
where
r = biotic potential
N is the population size and
K = carrying capacity
So (K-N)/K = opportunity to grow
Example of Growth Data
growth = rN [(K-N)/K]
Imagine that r = 1.0 K=100
N (t ) r
rN
(K-N)/K dN/dt
N (t+1)
N (t+1)
if no K
1
50
75
95
99
100
1
50
75
95
99
100
99/100
50/100
25/100
5/100
1/100
0/100
1.99
75
93.75
99.75
99.99
100
2
100
150
190
198
200
1
1
1
1
1
1
0.99
25
8.75
4.75
0.99
0
growth = rN [(K-N)/K]
Imagine that r = 1.0 K=100
N (t )r
rN
1
50
75
95
99
100
1
1
1
1
1
1
1
50
75
95
99
100
(K-N)/K dN/dt
N (t+1)
N (t+1)
if no K
99/100
50/100
25/100
5/100
1/100
0/100
.99
75
93.75
99.75
99.99
100
2
100
150
190
198
200
0.99
25
8.75
4.75
0.99
0
(K-N)/K = opportunity to grow
Three possible outcomes:
1. When N (population) is small, (K-N)/K ~ K/K ~ 1
so exponential growth at small pop. size.
2. Both K and r have an effect. Intermediate growth.
3. When N is large, K-N ~ 0
so the population doesn’t change in size
Population Strategies
r-Strategy Compared to K-Strategy
r-strategy: high intrinsic
growth rates - focuses on
reproduction, not on
competition with other
individuals in the
population.
K-strategy: focuses on
population at or near
carrying capacity—must
be able to compete with
other individuals in the
population.
K-strategy
r-strategy
Reproductive Strategies
Charac teristics of contra sting reproductive stra tegies
Extrinsically Contro lled Grow th
1.Shor t li fe
2.Rapid g rowth
3.Early maturit y
4.Many small offspring
5.Little parental care or protection
6.Little inves tment in individua l offspring
r-strategy
Instrinsically Controlled Grow th
1.Long li fe
2.Slower growth
3.Late maturit y
4.Fewer la rge offspring
5.High parental care and p rotection
6.High inv estment in individu al offspring
K-strategy
Demography
Demography = study of population change
Natality = production of new individuals
Mortality = death rate
(+ migration contributes to local numbers)
Factors affecting natality and mortality:
(1) Age composition of population
(2) Environment
Life Expectancies Change With Age
Type I: humans and other large mammals; high
mortality when reach old age
Type II: birds (seagulls); probability of death is
unrelated to age.
Type III: aquatic organisms that release fertilized
eggs. High mortality of very young individuals.
Which is r-strategy? Which is K-strategy?
Charac teristics of contra sting reproductive stra tegies
Extrinsically Contro lled Grow th
1.Shor t li fe
2.Rapid g rowth
3.Early maturit y
4.Many small offspring
5.Little parental care or protection
6.Little inves tment in individua l offspring
r-strategy
Survivorship curves:
Type I related to K,
Type III related to r.
Instrinsically Controlled Grow th
1.Long li fe
2.Slower growth
3.Late maturit y
4.Fewer la rge offspring
5.High parental care and p rotection
6.High inv estment in individu al offspring
K-strategy
Reproduction changes with age - proportion of individuals in
each reproductive class can have a large effect on population growth.
Expanding: Population momentum: when young make large
proportion of the population, potential for rapid increase in
natality when young reach reproductive age. ‘Bottom heavy’
Stable: stationary phase. Mortality such that each class goes to the
next class at the size the next class was at. The population is at
replacement numbers for births.
Diminishing: natality has fallen below replacement numbers. ‘Top
heavy’.
HUMAN POPULATION
2000 years ago
300 million people
200 years ago
under a billion people
40 years ago
3 billion people
1999
6 billion people
Now, the population is growing by almost 78 million
more people each year.
Two possible causes:
Life expectancy (age at mortality)
Fertility
Life expectancy at birth, selected countries, aro und 1910 and in 1998
Country
Around 1910
1998
Males
Females
Males
Females
Australia
56
60
75
81
Chile
29
33
72
78
Italy
46
47
75
81
Japan
43
43
77
83
New Zealand
60
63
74
80
Norway
56
59
75
81
United States
49
53
73
80
_______________________________________________________________________
Source : Wo rld Health Orga nization, Wor ld Health Report 1999 pg. 2.
Mortality and death rates: The primary cause of
population growth has been declining mortality.
In the last 100 years, average life expectancy has risen by
about 25 years - due to modern medicine, better food, and
better sanitation (environment changed).
Declining Mortality
Population USA Year 2000
Population USA Year 2025
Population USA Year 2050
Comparison Population USA 2000
with 2050
Post-repro
Reproductive
Pre-repro
2000
2050
Columbia River Salmon - Carrying Capacity
Estimates as Pounds Caught.
Conclusion
By changing the salmon’s environment (DAMS), we have
changed the carrying capacity for salmon.
Are we doing
the same
thing with the
earth for
human
beings?
Economics and Life Expectancy
Although life expectancy is predicted well by annual
per capita income, the correlation is good only up to
about $4000.
Birth Rates and Fertility Rates
Crude birth rates are the number of births per 1000
people. It is crude because it is not adjusted for the
number of women of reproductive age.
Total fertility rate is the number of children born to
an average woman in a population during her
reproductive life.
Obviously only women give birth, so women
must be considered specifically.
Why are fertility rates so high in sub-Saharan
Africa?
Infant mortality rates strongly correlated with
high fertility rates.
Other Correlations with Infant
Mortality Rates?
Can increased female literacy decrease infant
mortality, and so decrease fertility rates?
EMPOWER WOMEN? What would
this mean for some societies?
Fertility Rates and
Female Literacy
Fertility Rates and Birth
control (linked to literacy?)
Religious
Beliefs
Religious
Beliefs
Cultural Norms
Cultural
Norms