Download Page ‹#› Human population growth

Human population growth
From here - end of course: Ecology - the
study of the distribution and abundance of
organisms
Today - population growth w/ humans as focus
Wednesday - Life histories - comparing
demographic parameters (e.g. birth rates, timing,
mortality patterns) across species
Friday- Population regulation and competition
Then 3 more lectures on interactions between
species, mutualism, parasitism, herbivory
Then, community ecology, biogeography,
conservation ecology, invasive species,
ecosystem ecology and the natural history of the
Sonoran desert
Human Population Growth
Human Population Growth
I. Concepts of population growth
A. Unlimited, exponential growth
B. Limited, logistic growth
II. Human population growth
A. History of population growth
B. Projecting into the future
I.
Unlimited population growth
A. Exponential growth
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1. Example of bacterial
growth
A. Exponential growth
Generation
0
1. Example of bacterial growth
Bacteria reproduce by binary
fission - 1 cell
2 cells
No.
1
1. Example of bacterial
growth
1. Example of bacterial
growth
Generation
0
1
Generation
0
1
2
No.
1
2
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No.
1
2
4
1. Example of bacterial
growth
1. Example of bacterial
growth
Generation
0
1
2
3
Generation
1
2
3
4
5
No.
1
2
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8
No.
1
2
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8
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1. Example of bacterial
growth
1. Example of bacterial
growth
Generation
0
1
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Generation
0
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4000
3000
2000
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No.
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1. Example of bacterial
growth
1. Example of bacterial growth
Generation
No.
Generation
0
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64
128
256
4000
3000
2000
1. Example of bacterial growth
Generation
0
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9
Generation
0
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12
2000
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Generation
10
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Number bacteria
3000
0
Number bacteria
4000
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128
256
512
1024
8
Generation
1. Example of bacterial growth
No.
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512
10
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6
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2
1000
0
Number bacteria
No.
1
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128
1. Example of bacterial growth
1. Example of bacterial growth
Generation
0
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0
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Generation
Features of simple exponential
growth
1. Example of bacterial growth
In this example,
the generation time = the
doubling time (time until
population doubles)
• A constant doubling time
Constant growth rate?
4000
3000
2000
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12
10
6
8
4
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0
1000
2
8
10
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4
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2
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No.
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1024
2048
4096
Number bacteria
4000
Number bacteria
No.
1
2
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8
16
32
64
128
256
512
1024
2048
Features of simple
exponential growth
A. Exponential growth
A constant doubling time
An accelerating growth rate
i. Example of bacterial growth
ii. The importance of
generation time
When plotted against generations,
they seem to be the same...
Imagine that 3 populations,
bacteria, mice, and elephants, all
doubled their population size at
each generation. Each bacterium
produced 2 offspring, each mouse
and elephant couple produced 4.
40
Bacteria
Mice
Elephants
20
Generation
Would their populations grow at the same rate?
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6
5
4
3
2
0
1
10
0
Number
30
When plotted against minutes,
the difference is apparent
But of course, the generation
time of these organisms is
rather different!
100000000
10000000
Minutes (log scale)
1000000
100000
0
10,512,000
10000
10
1000
Number
132,480
Generation time: the mean
length of time between birth
of parents and birth of
offspring
Elephants
20
100
Elephants (20 yrs)
30
10
Mice (3 months)
Mice
30
Generation time in minutes:
Some species of bacteria
Bacteria
40
(190
yrs!)
We need an equation that will let us
compare the population growth of
organisms with different generation times
General differential equation for unlimited
population growth:
dN/dt = rN
N is the population size
r is the instantaneous growth rate (the net
population growth at any one instant)
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The population growth rate, r, is
determined by how many individuals
are coming into the population at
any one time (the instantaneous rate
of births)
Age distribution
Projections of population growth
use instantaneous rates of birth,
rates of death, and the age
distribution
reduced by those leaving i.e. t h e
instantaneous rate of deaths
The age distribution is the way in
which a population is distributed
among different age classes.
r=b-d
b - instantaneous birth rate
d - instantaneous death rate
Examples of human a g e
distributions
1) Cambodia, 1998
Examples of human a g e
distributions
2) United States, 2000
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Which population is likely
growing more rapidly?
Cambodia, 1998
United States, 2000
I. Unlimited population growth
A. Exponential growth
If the number of children per mother
and mortality rates were held
constant in these two countries,
would population growth be the
same ?
Cambodia, 1998
United States, 2000
In nature, we don’ t generally see
exponential growth for very long
B. Limited (logistic) growth
Thrips
(flower
feeding
insects)
on rose
bushes
1932
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1938
B. Limited (logistic) growth
B. Limited, logistic growth
I. The carrying capacity
What sorts of factors cause
population growth to slow or
decline?
We observe that most populations in
nature have some limit, called the
carrying capacity
Limited food supply
Limited habitat available
Deterioration of habitat
Predation, disease
The carrying capacity (K) is the
maximum stable population size the
environment can support.
Logistic growth
Logistic growth, the equation
K
d N/dt = rN[(K-N)/K]
logistic function
Same equation as before, with
logistic function
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W h a t’s the logistic
function?
d N/dt = rN[(K-N)/K]
What happens to the sign of the
right hand side of the equation
when N=K? When N>K? When
N< K?
A handy, but somewhat arbitrary
mathematical function that returns
the population to a line, K
What would happen if the habitat
for a particular population was
reduced?
What would happen to the
population if the habitat for a
particular population was reduced?
New K
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What would happen if the habitat
for a particular population was
reduced?
I. Unlimited population growth
II. Limited (logistic) growth
III.
Pop. would
decline
to new K
Human population growth
As of April 3, 2006 (17:19 GMT), the world population was
6,507,485,579
Every second: 4.1 births, 1.8 deaths
A net increase of 2.3 individuals
II. Human population growth
A. History of human population growth
What kind of
growth?
Year
Global population
1800
1930
1975
2000
1
2
4
6
billion
billion
billion
billion
}
W h a t’s the doubling time between
1 and 2 billion?
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Year
Global population
Year
Global population
1800
1930
1975
2000
1
2
4
6
1800
1930
1975
2000
1
2
4
6
billion
billion
billion
billion
}
W h a t’s the doubling time between
2 and 4 billion?
If a constant doubling time is a
feature of simple exponential
growth, and human populations
show a decreasing doubling time…
billion
billion
billion
billion
What can we say about the
doubling time between 1800 and
1975?
In recent years human
populations have grown faster
than simple exponential growth
t h e n… ?
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II.
II. Human population growth
A. History of human population growth
A. History of human population
growth
B. Projecting into the future
Will it continue to
grow like this
indefinitely?
What is the
capacity
planet
Earth?
What population
parameter would
we like to know?
B. Projecting into the future
i. Human carrying capacity, K
Been the subject of speculation since
van Leeuwenhoek (1679) imagined a
world like 17th century Holland (13 bill)
Estimates range from 900 mill to “a
billion billions”
Dependent on many assumptions, value
systems
Unlike other animals, humans have the
potential to move towards a sustainable
population size
Human population growth
carrying
of the
i. Human carrying capacity
What should be the standard of
living?
Hardin (1986): 1 average
American uses 300 times the
amount of energy as an average
Ethiopian. If everyone lived as
Ethiopians, the earth could
support 300 billion people…
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i. Human carrying capacity
II.
Human population growth
How does one view sustainability?
Many estimates are based on
maximum agricultural capacity and
fossil fuels, no consideration for
degradation of farm land, pollution,
global warming, or loss of
biodiversity
Pimentel et al. (1994) assumed solar
and other renewable energy at
increased rates, soil conservation
in agriculture. K=3 billion, 1-2
billion in relative prosperity
B. Projecting into the future
i. Human carrying capacity
ii. Population growth projections
United Nations world population
predictions to 2050 (2002)
ii. Population growth projections
Estimates of future populations do
not take into consideration the
environment or resources, or any
explicit estimate of K - just project
trends in birth rates and death
rates into the future
1.4E+07
1.2E+07
1.0E+07
Const. Fert. 13b
We are here
“High” 11b
“Medium” 9b
}between 7
8.0E+06
6.0E+06
4.0E+06
2.0E+06
0.0E+00
Constant
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“Low” 7b
High
Medium
Low
and 11 bill.
considered
most likely
In “low”
estimate, the
population
drops
Note the “constant fertility” projection (i.e.
fertility at current rates) is the highest and
considered unrealistic. Why?
1.4E+07
1.2E+07
1.0E+07
8.0E+06
6.0E+06
4.0E+06
2.0E+06
0.0E+00
Const. Fert. 13b
We are here
Constant
Note the “constant fertility” projection (i.e.
fertility at current rates) is the highest and
considered unrealistic. Why?
Fertility rates are dropping
worldwide
Part of a robust pattern without
one clear explanation - “ t h e
demographic transition”
“High” 11b
“Medium” 9b
“Low” 7b
High
Medium
Low
C. Projecting into the future
i. The demographic transition
C. Projecting into the future
i. The demographic transition
Countries go from having small, slow
growing populations to having large, slow
growing populations in 4 steps
In the past 200 years, all “developed”
countries have undergone this process
Most “developing” countries are
somewhere in the process - fertility rates
are dropping but populations are still
growing
1) High death rates, high birth rates fi no growth,
small population
2) Death rate falls, birth rates stay same fi rapid
growth, larger population
3) Birth rate falls, death rates stay the same fi
population increases (because of age
distribution), large population
4) Birth rate and death rates stay the same fi no
growth, large population
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Children per mother
Fertility rates 1950 - 2050 (U.N. report
2002)
We are here
8
7
6
5
4
3
2
1
0
World
More
developed
Less
developed
Least
developed
If fertility rates are dropping why is the
world population still growing?
2) Age distribution - if most individuals in
a population are young, population will
still grow after fertility drops below
replacement
Most world population growth is coming
from developing countries
If fertility rates are dropping why is the
world population still growing?
1) Fertility is still above replacement
8
7
6
5
4
3
2 per mother 2
- replacement 1
0
Children per mother
C. Projecting into the future
i. The demographic transition
We are here
World
More
developed
Less
developed
Least
developed
C. Projecting into the future
i. The demographic transition
Why the demographic transition?
What causes the dropping mortality
rates?
Better public health
In developing countries the “public health
revolution” has been from 1945- present
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C. Projecting into the future
i. The demographic transition
C. Projecting into the future
i. The demographic transition
Life expectancy from 1950 - 2050 (U.N.
2002)
Why is life expectancy not currently rising
in the least developed countries?
we are here
90
80
70
60
50
40
30
20
10
0
90
80
70
60
50
40
30
20
10
0
World
More
developed
Less
developed
Least
developed
World
?
More
developed
Less
developed
Least
developed
Why is life expectancy not currently
rising in the least developed countries?
C. Projecting into the future
i. The demographic transition
HIV/AIDs : huge economic and social costs in
some of the least developed countries in Africa
Some tragic statistics:
-In Botswana, 1 in 3 adults is currently infected
-Life expectancy has dropped from 63 years to
44 years
-Population is expected to be in decline by 2015
Outside of Africa, no. of countries where HIV/AIDS
is a problem is growing, especially in Asia, Latin
America, Caribbean
Peak prevalence has passed in many countries,
but prevalence expected to stay relatively high
until 2050
Why the demographic transition?
What causes the dropping fertility rates?
No one really knows.
Some likely factors in recent decline in Africa:
Drop in the “ideal family size” in many countries
coupled with
Education for girls
Access to contraception
In some cases, costs of children borne increasingly
by families
No one factor appears universal to fertility decline
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C. Projecting into the future
C. Projecting into the future
iv. What else can we see ahead?
Where do we fall on this
curve?
Or possibly up here
What is K?
We may be somewhere in here
1965-1970 max. world pop. growth
For some time, an ageing population
A much larger population, but one
that seems to be slowing its
growth, and may even start a slow
decline within 100 yrs.
With the prospect of a world
population that is not growing
exponentially in the future, we can
begin to think about what
population size the earth can
sustain
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