populations chapter 8. chapter 26 begins with the mystery of easter island. at one time, easter...
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PopulationsChapter 8
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• 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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TOGETHER
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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!
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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.
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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.
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Distribution
• Individuals distribute themselves in a population in three general patterns:
• Clumped
• Uniform
• Random
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DistributionClumped
distribution is typical of organisms
that move in groups
(herds, flocks, etc.), or that
cluster around
resources, such as
plants near a water source.
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DistributionUniform
distribution is typical where resources are
scarce. Individuals compete to
claim enough territory to
support them and keep a
distance from others.
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DistributionRandom
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.
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Male marine iguanas are highly territorial. They also compete for females. Male iguanas tend to be
distributed uniformly throughout their territory. Why?
1 2 3
5% 5%
90%1. They live in social groups.
2. Each male has its own distinct breeding territory.
3. The iguana’s resources are localized.
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Which pattern of distribution do human populations tend to show?
1 2 3
71%
17%12%
1. Clumped2. Uniform3. Random
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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.
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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?
1 2 3 4
0%
95%
3%3%
1. 12202. 66003. 70004. 7400
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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.
• Example: 150 births in a gull population of 1000 = 150/1000 = 0.15 per year.
Falklands Conservation
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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
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Growth Rate
• Growth rate (r) = birth rate – death rate
• r = b – d
• Ex: 0.15 – 0.05 = 0.1 (10% per year)
minus
=
percent increase
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Growth Rate
• If we want to know the actual number of individuals by which the population increased, we use this formula:
• G = r x N
• G = 0.1 x 1000 = an increase of 100 individuals per year.
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• 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 – dG = r x N
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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?
1 2 3 4
3% 0%6%
91%
1. 5%2. 10%3. 15%4. 20%
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• 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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TOGETHER
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Exponential GrowthBiotic potential = Maximum growth
possible.
This assumes a maximized birth and minimized death rate
Calculated as:
r = b - d
G = r x N
What happens to r and G when b gets big and d gets
little?
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Exponential Growth
Exponential growth produces a J-shaped population graph.
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Exponential Growth
Age of first reproduction affects the rate of
population growth. Why?
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Exponential Growth
Death rates and average lifespan also affects growth rate. Why?
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• 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.
Biotic Biotic potentiapotentia
ll
EnvironmentaEnvironmental resistancel resistance
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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.
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Population Limits
The upper limit for population growth is determined by the carrying capacity of the environment.
K = Carrying Capacity
:# births
= # deaths
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Population Limits
Where there are many natural controls, populations tend to demonstrate logistic growth.
Available space limits
barnacle populations.
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If a population overshoots the carrying capacity of the environment, the result is a population crash.
Population Limits
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Population Limits
Where there are few natural controls, a population may rise rapidly, exceed carrying
capacity, then crash as most of the population starves.
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Which population is most likely to experience exponential
growth?
1 2 3
33% 33%33%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.
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Cyanobacteria
population boom
• In July growth conditions for cyanobacteria become
favorable
• 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.
• The population grows rapidly
How can this be represented graphically?
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How can the cyanobacteria example be represented
graphically?
1 2 3
33% 33%33%1. _
2. _
3. _
time
nu
mb
er
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What caused the cyanobacteria to crash was environmental
resistance that was:
1 2
50%50%
1. Dependent on the density of the population.
2. Independent of the density of the population.
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• This graph shows human population over the last 14,000 years. What kind of curve is this? What implications does this have for humans?
Date
Technical andcultural advances
Industrial andmedicaladvances
Agricultural advances
123
Billions Time to addeach billion(years)All of humanhistory
1312131433
1804
201219991987197419601927
1
234567*
*projected bubo
nic
plag
ue
billi
ons
of p
eopl
e
2012*
1830
1975
1960
1930
1987
19992006
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Population Limits
A growing population
may become self-limiting.
In fruit flies, reproduction rate drops in response to
crowded conditions.
offs
prin
g pe
r da
y lifespan
population density
days
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Population LimitsDensity-
independent factors limit populations
regardless of how large or small the
population is. Usually abiotic.
Examples: Seasonal weather
changesNatural disasters
Pollution
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Population Limits
Density-dependent factors affect a population more strongly the
larger it grows. Usually biotic.
Examples: PredationParasitesDisease
Resource competition
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Which of these is a density-dependent factor?
1 2 3 4
5% 3%
87%
5%
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.
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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:
1 2
28%
72%
1. Density-dependent factor
2. Density-independent factor
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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 density-independent
factor for these squirrels?
1 2
50%50%
1. Density-dependent factor.
2. Density-independent factor.
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• For each of these scenarios, list both density-independent and density-dependent 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
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Survivorship curves very for different species, depending on their reproductive
strategy.
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15 - 29
(a) Developed countries
0 - 14
75 and older
60 - 74
45 - 59
30 - 44
15 - 29
0 - 14
75 and older
60 - 74
45 - 59
30 - 44
age
femalemaleag
e
femalemale
(b) Developing countries
postreproductive (45–79 yr)
prereproductive (0–14 yr)
reproductive (15–44 yr)
millions of people
millions of people
2025 20502006
Different survivorship curves can have different consequences for populations, even of the same
species.
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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, statue-carving 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.
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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
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Total Deer and Wolf Populations by Year
0
500
1,000
1,500
2,000
2,500
3,000
1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
Year
Nu
mb
er o
f D
eer
0
5
10
15
20
25
30
Nu
mb
er o
f W
olv
es
Deer Wolves
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exponentialgrowth
populationcrash
Without wolves:
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bean weevils (prey)
A high predator population
reduces the prey population
The prey populationpeaks when the
predator populationis low
braconid wasp (predator)
How does this graph relate to your deer/wolf graph?
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(a) Predators often kill weakened prey
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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?
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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.