Populations
Chapter 8
• Chapter 26 begins with the Mystery of
Easter Island. At one time, Easter Island
was forested, and supported a large
population of humans who carved huge
stone statues. Today the island has no
forests, is sparsely populated, and the
people have no memory of the culture
that created the statues. What possible
connections are there between the
disappearance of the forests and the
disappearance of the statue-carving
culture?
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Why study populations?
• In the field, populations tend to be the
unit of study. A population is a natural
grouping, so studying populations
reflects what is going on in nature.
• Even so – it’s not always easy to define a
population!
Population Science
• Studying growth rates of populations
helps us understand:
• the effects of rapid overpopulation.
• how population growth is regulated.
• We can derive important lessons for
humans from studies of populations in
nature.
General Principles
• A population consists of
members of the same
species living in the same
ecosystem at the same time.
• Total population increases or
decreases according to the
number of births, deaths,
immigration, and
emigration that occurs.
Distribution
• Individuals distribute themselves in a
population in three general patterns:
• Clumped
• Uniform
• Random
Distribution
Clumped
distribution is
typical of
organisms that
move in groups
(herds, flocks,
etc.), or that
cluster around
resources, such
as plants near a
water source.
Distribution
Uniform
distribution is
typical where
resources are
scarce.
Individuals
compete to claim
enough territory
to support them
and keep a
distance from
others.
Distribution
Random
distribution is rare.
Organisms may
distribute randomly
if resources are
abundant and the
organisms do not
form social groups.
Trees in a diverse
forest may
distribute
randomly.
Male marine iguanas are highly territorial. They also compete
for females. Male iguanas tend to be distributed uniformly
throughout their territory. Why?
1. They live in social
groups.
2. Each male has its
own distinct
breeding territory.
3. The iguana’s
resources are
localized.
90%
5%
1
5%
2
3
Which pattern of distribution do human
populations tend to show?
71%
1. Clumped
2. Uniform
3. Random
12%
1
2
17%
3
Growth Rate
• To determine the actual change in
numbers of a population in a given unit of
time, we look at the difference between
losses (deaths and emigration) and
additions (births and immigration)
• (births - deaths) + (immigrants emigrants) = change in population size.
What is the change in a population over a ten-year
period if in that time there are 9,000 births, 2,000
deaths, 800 immigrants, and 400 emigrants?
95%
1.
2.
3.
4.
1220
6600
7000
7400
0%
1
3%
2
3%
3
4
Growth Rate
• If we want to know the rate
(r) at which a population is
increasing, we need to
know:
• Birth rate (b) = number of
births in a population
during a certain time
period.
Falklands Conservation
• Example: 150 births in a
gull population of 1000 =
150/1000 = 0.15 per year.
Growth Rate
• We also need to know:
• Death rate (d) =
number of deaths in
the same time
period.
• Example: 50 deaths
in a gull population
of 1000 = 50/1000 =
0.05 per year.
Falklands Conservation
Growth Rate
minus
• Growth rate (r) = birth rate
– death rate
• r=b–d
=
• Ex: 0.15 – 0.05 = 0.1
(10% per year)
percent
increase
Growth Rate
• If we want to know the actual number of
individuals by which the population
increased, we use this formula:
• G=rxN
• G = 0.1 x 1000 = an increase of 100
individuals per year.
• Try this:
• You are studying a population of 30
ferns. This year you saw six new fern
plants become established, and 3 fern
plants died. Calculate the growth rate
of the population.
• Remember:
r=b–d
G=rxN
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What is the annual growth rate of a population of
10,000 sea turtles if there are 500 deaths and
1,500 births per year?
91%
1.
2.
3.
4.
5%
10%
15%
20%
6%
3%
1
2
3
0%
4
• Suppose your eccentric uncle says that
for your birthday, he will give you your
choice between two presents:
• Choice A: $1,000,000 on your birthday.
• Choice B: A penny on your birthday,
two pennies the next day, four the
next, and so on for 30 days.
• Which would you take? Why?
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• Choice A yields $ 1,000,000
• Choice B yields $10,737,418.23 (Why?
See:
http://mathforum.org/dr/math/faq/faq.doubling.
pennies.html)
• What happened? Why did Choice B
give you so much more money?
• What does this have to do with
population growth?
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Exponential Growth
Biotic potential = Maximum growth possible.
This assumes a maximized birth and minimized
death rate
Calculated as:
r
G
=
=
b
r
x
d
N
What happens to r and G when
b gets big and d gets little?
Exponential Growth
Exponential growth
produces a J-shaped
population graph.
Exponential Growth
Age of first reproduction
affects the rate of population
growth. Why?
Exponential Growth
Death rates and average lifespan also affects
growth rate. Why?
• Under what conditions can exponential
growth occur in nature?
• You’ve probably guessed that
exponential growth can’t go on forever.
What factors limit population growth?
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Population Limits
• Two opposing forces act on population
growth.
Environmental
resistance
Biotic
potential
Environmental Resistance
• Species introduced to a new
environment may experience exponential
growth.
• Environmental resistance will eventually
limit growth.
• Some populations experience “boom
and bust” cycles.
• Others stabilize and show logistic
growth.
Population Limits
K=
Carrying
Capacity:
# births = #
deaths
The upper limit for population growth is determined by the
carrying capacity of the environment.
Population Limits
Available
space limits
barnacle
populations.
Where there are many natural controls, populations tend to
demonstrate logistic growth.
Population Limits
If a population overshoots the carrying capacity of the
environment, the result is a population crash.
Population Limits
Where there are few natural controls, a population may
rise rapidly, exceed carrying capacity, then crash as
most of the population starves.
Which population is most likely to
experience exponential growth?
1. Algae introduced
into a small pond in
North Dakota.
2. A migrating herd of
pronghorn antelope
in Eastern Oregon.
3. Chinook salmon in
the Columbia River.
33%
1
33%
2
33%
3
Cyanobacteria
• In July growth conditions for
cyanobacteria become favorable
• The population grows rapidly
population boom
• By early September the nutrient
supply has been depleted and
competition for what is left is
fierce. Most cyanobacteria can’t
get enough and die.
How can this be
represented graphically?
How can the cyanobacteria example
be represented graphically?
33%
number
1. _
33%
33%
time
2. _
3. _
1
2
3
What caused the cyanobacteria to crash
was environmental resistance that was:
50%
50%
1. Dependent on the
density of the
population.
2. Independent of the
density of the
population.
1
2
• This graph shows human population over
the last 14,000 years. What kind of curve
is this? What implications does this have
for humans?
bubonic plague
Date Billions Time to add
each billion
(years)
All of human
1804
1
history
1927
2
123
1960
3
33
1974
4
14
1987
5
13
1999
6
12
2012
7*
13
*projected
billions of people
2012*
2006
1999
1987
1975
1960
1930
1830
Technical and
cultural advances
Agricultural advances
Industrial and
medical
advances
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A growing
population may
become selflimiting.
lifespan
days
offspring per day
Population Limits
population density
In fruit flies,
reproduction
rate drops in
response to
crowded
conditions.
Population Limits
Density-independent
factors limit
populations regardless
of how large or small
the population is.
Usually abiotic.
Examples:
Seasonal weather
changes
Natural disasters
Pollution
Population Limits
Density-dependent
factors affect a
population more
strongly the larger it
grows. Usually biotic.
Examples:
Predation
Parasites
Disease
Resource competition
Which of these is a densitydependent factor?
1. Harsh, cold winters
with lots of snow
and ice.
2. A sudden tornado.
3. An outbreak of
cholera in a refugee
camp.
4. A violent
earthquake.
87%
5%
1
5%
2
3%
3
4
A population of Bluebirds is displaced when a new housing development
destroys the meadow where they nested. They move to another meadow
where other nine male bluebirds live. The males compete intensely for
nesting sites. At the end of the season, there are still only nine successful
males. Competition for nesting sites is a:
72%
1. Density-dependent
factor
2. Density-independent
factor
28%
1
2
A squirrel population is isolated on the Capitol grounds in
Salem. Heavy traffic on all sides makes it hard for squirrels to
leave the grounds. Squirrel fatalities happen as squirrels try
to cross the streets. Is traffic a density-dependent or densityindependent factor for these squirrels?
50%
50%
1. Density-dependent
factor.
2. Density-independent
factor.
1
2
• For each of these scenarios, list both
density-independent and densitydependent factors that could be involved.
• During a drought, a thick stand of young
pine trees is attacked by pine bark
beetles.
• A large herd of deer is caught by a winter
storm that buries much of their food
supply. Several of the deer, suffering
from parasites as well as lack of food, are
caught and killed by wolves.
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Survivorship
• Populations show different patterns in
survival at different ages, which in turn
can affect population growth.
• Early loss – many young die
• Constant loss – equal loss at all ages
• Late loss – high survivor of young,
most deaths in old age
Survivorship curves very for different species,
depending on their reproductive strategy.
(a) Developed countries
age
2006
75 and older
60 - 74
45 - 59
30 - 44
15 - 29
0 - 14
male
2025
2050
female
postreproductive (45–79 yr)
reproductive (15–44 yr)
prereproductive (0–14 yr)
millions of people
age
(b) Developing countries
75 and older
60 - 74
45 - 59
30 - 44
15 - 29
0 - 14
male
female
millions of people
Different survivorship curves can have different
consequences for populations, even of the same species.
Birth rates do slow down as nations become more industrialized.
However, the world population is not evenly developed, and in
developed nations, resource consumption per capita is high.
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• Based on what you have learned in
Chapter 26, how can you explain the
disappearance of the ancient, statuecarving culture on Easter Island?
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Deer and Wolves
• Using the worksheets provided, calculate
the population change in the deer
population for each year.
• Population changes = births – deaths
• In this case, deaths are due to both
starvation and predation.
Year
Wolves
Deer
Deer
offspring
Predation
Starvation
Deer
population
change
1997
10
2,000
800
400
100
300
1998
12
2,300
920
480
240
200
1999
16
2,500
1,000
640
500
-140
2000
22
2,360
944
880
180
-116
2001
28
2,224
996
1,120
26
-150
2002
24
2,094
836
960
2
-126
2003
21
1,968
788
840
0
-52
2004
18
1,916
766
720
0
46
2005
19
1,952
780
760
0
20
2006
19
1,972
790
760
0
30
3,000
30
2,500
25
2,000
20
1,500
15
1,000
10
500
5
0
0
1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
Year
Deer
Wolves
Number of Wolves
Number of Deer
Total Deer and Wolf Populations by Year
Without wolves:
exponential
growth
population
crash
How does this graph relate to your deer/wolf graph?
bean weevils (prey)
braconid wasp (predator)
A high predator
population
reduces the prey
population
The prey population
peaks when the
predator population
is low
(a) Predators often kill weakened prey
On the back of your graph:
1. Describe what happened to deer and wolf
populations between 1997 and 2006.
2. What might have happened if wolves had
NOT been introduced to the island?
3. Some people think it was cruel to introduce
wolves. Some think it would have been cruel
NOT to. Is there another management plan
that would have been as good or better?
Recap
• Population size changes through birth,
death, immigration, and emigration.
• Population size is regulated by
environmental restraints that increase
deaths or decrease births.
• Populations are distributed in various
patterns for social reasons or because of
resource availability.