Download Populations

Population
Ecology
Topic 3
This trio of sea otters is
part of the population
that lives near Monterey,
California. Sea otters
often rest by wrapping
themselves in kelp to
keep from drifting away.
Kelp Forest
Ecosystem
America’s Pacific Northwest is home
to an underwater “forest” that is the
basis for a very diverse ecosystem.
• What does kelp provide?
food & habitat
• Sea Otters are an important
member of the kelp forest. Otters
eat a lot of food (sea urchins) to
stay warm in the cold water
• Sea urchins, in turn, feed on kelp
• The relationships along this food
chain set the stage for a classic
tale of population growth and
decline.
Kelp forest
Sea Otters
Sea Otters are Keystone Species
A keystone is the stone at the top of an
arch that supports the other stones and
keeps the whole arch from falling.
**A Keystone species is a species on
which the persistence of a large
number of other species in the
ecosystem depends.
Kelp Forest Continued
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A century ago, otters were nearly
eliminated by hunting.
Sea urchin populations increased
greatly, and kelp forests nearly
disappeared. Why?
Then, sea otters were declared an
endangered species and were
protected from hunting
Otter populations recovered, sea
urchin numbers dropped, and kelp
grew back
But now, some otter populations
are shrinking again…
Click to
Watch
How sea otters help save the planet
The sea otter, a
voracious urchineater and
possessor of the
densest fur in the
animal kingdom.
Photograph: Alamy
Sea otters floating in kelp.
They help kelp forests to
flourish by keeping sea urchin
numbers under control.
Photograph: Frans
Lanting/Getty Images
Sea otters use
rocks to crack
open the spiny
shells of sea
urchins.
Photograph: Noel
Hendrickson/Getty
Images
Jaguar (keystone predator)
Prairie Dog (keystone modifier)
Hummingbird (keystone mutualist)
Beaver (keystone modifier)
Saguaro Cactus (keystone host)
Snowshoe Hare (keystone prey)
…back to our topic, Populations


A group of organisms of the same
species that live in the same place
There are Four Important Characteristics
of Populations:




1. Geographic Distribution
(Range)
2. Density
3. Growth Rate
4. Age Structure
1. Geographic Distribution
(Range)


Describes the area inhabited by a
population
The range can vary in size from a few
cm3 to millions of km2
Which has a larger range?
2. Population Density

The number of individuals per unit
area

This number can vary tremendously
depending on the species and its
ecosystem
The tall saguaro
cactuses in this
Arizona desert have a
low population
density compared to
the smaller desert
plants
Density of Whitetail Deer in
the United States
Which states have the
highest density?
3. Growth Rate


How the number of individuals in a
population changes over time
Three factors can affect population
size:



Number of births
Number of deaths
Number of individuals that enter and
leave the population
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
What happens when the
Immigration: the movement of individuals birth rate is greater than
into an area
the death rate?
Emigration: the movement of individuals What happens when life
expectancy improves?
out of an area
Feedback Loop in Populations

A feedback loop is a process in which the
system responds to a disturbance either:


In the same direction (positive feedback)
In the opposite direction (negative feedback)
Example

Positive Feedback Loop =
increase in birth rate (reinforcing)
Click
to
Watch

Negative
Feedback Loop =
increase in death
rate (diminishing)
4. Age Structure
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Population growth depends, in part, on how many
individuals of different ages make up a population
Age-structure diagrams—graph of the number of
individuals in different age groups
GROWTH CURVES REPRESENT THE GROWTH OR
DECLINE OF A POPULATION GRAPHICALLY

There are two models or types of growth curves:


1. Exponential (J-shaped)—occurs when the individuals in
a population reproduce at a constant rate
2. Logistic (S-shaped)—occurs when a population’s growth
slows or stops following a period of exponential growth
EXPONENTIAL GROWTH

In the presence of unlimited resources and in the
absence of predation and disease (ideal
conditions), a population will grow exponentially
If this
growth
pattern
continued
for
several
days
without
slowing
down
bacteria
would
cover the
planet!
LOGISTIC GROWTH
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Since bacteria doesn’t cover the planet, this means
that exponential growth does not continue in natural
populations for very long
As resources become less available, the growth
of a population slows or stops
Carrying Capacity


In logistic growth, why does the growth slow or
stop?
The ecosystem reaches the carrying capacity,
or the largest number of individuals of a
population that a given environment can support
Note that even when the
population growth is said
to stop, it still rises and
falls, but averages out to
a certain population size.
Limiting Factors
Concept Map
Let’s Review…
Population
Growth
can be
Logistic
growth
Exponential
growth
characterized by
No limits on
growth
Unlimited
resources
represented by
Constant
growth rate
J-shaped
curve
characterized by
Limits on
growth
which cause a
Falling
growth rate
represented by
S-shaped
curve
Many factors may cause a decline in a
population over time.
Remember…predators such as killer whales are
now feeding on populations of sea otters 
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
Another Example: primary productivity of an ecosystem can be
reduced when there is an insufficient supply of a particular
nutrient – limiting nutrient
This is an example of a larger concept – a limiting factor.
Limiting Factors

Cause population growth to decrease

Ex. Competition, Predation, Parasitism,
Disease, Drought, Human disturbances
Think of it like a dam holding water back.
Density-Dependent Limiting Factors

Depends on population size—only becomes
limiting when the density (number of
organisms per unit area) reaches a certain
level

This includes factors such as competition,
predation, parasitism, & disease

Ex. Where is your chance of getting the flu the highest?
Predator-prey relationships

The regulation
of a population
by predation is
one of the
best-known
mechanisms of
population
control
How many moose does a wolf
need to eat to survive a year?
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In order to survive a 50 kg
wolf needs to eat about
2700 kg of moose/yr
The average mass of a
moose is 385 kg

How many “average”
moose does a wolf need to
eat each year?
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~7
If there are 8 wolves in a
pack?

2700 x 8 =21,600/385 = ~56
Density-Independent Limiting Factors

Affect all populations in similar ways,
regardless of the population size

This includes factors such as weather, natural
disasters, seasonal cycles, & human activity

In response to such factors, many species show a
characteristic crash in population size
What could
be the
potential
cause for
this crash?
Human Population Growth
Like other organisms, the size of the human pop. tends to
increase over time.
Industrial
Revolution
begins
Agriculture
begins
Plowing
and
irrigation
Bubonic
plague
• To begin: Life was often difficult…many limiting factors
• Until fairly recently only ½ of children survived to
adulthood
• When the food supply became more reliable, technology,
sanitation, medicine, & healthcare reduced death rate.
Demography – the scientific study of
human populations
 Thomas Malthus predicted that
human populations would not grow
indefinitely
 due to war, famine, disease
providing limits
How large is the human population?
http://www.census.gov/population/international/
NPR population video
Birthrates, death rates, and the age
structure of a population help predict why
some countries have high growth rates
while other countries grow more slowly.
The Demographic
Transition
C.
A.
A. All countries begin here
B. As countries modernize,
they have better nutrition,
sanitation, and medicine
which lowers the death
rate
C. As modernization
continues, families have
fewer children which
lowers the birth rate
D. The transition is complete
when population growth
stops
D.
B.
 Know how
changes to birth &
death rates affect
the population.
Age-Structure Diagrams: used for predicting
future growth
Section 5-3
U.S. Population
Males
Females
Rwandan Population
Males
Females
Click to
Watch
 Population growth depends on how many people of
different ages make up a population.
Animal
Societies
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Some of the most extreme examples
of relatedness, and the most complex
animal societies (apart from humans)
are found among social insects!
An animal society is a group of animals of the same
species that interact closely and often cooperate
Members of a society are often related to one another
An Ant Society
Click to Watch
Figure 1: Social insects have well
protected or defended nests, including
termites (a), wasps (b), and bees (c).
Click to Watch
Animal Societies
Figure 1: Social groups which have formed to improve the probability
of survival and reproduction of individual members: pack of wolves
(a) and school of fish (b)
Group Behavior
Figure 1: Group-living is widespread in the animal kingdom.
Although many species form temporary associations, such as (a)
flamingo (Phoenicopterus minor) colonies and (b) zebra (Equus
quagga) herds, some species such as (c) African elephants
(Loxodonta africana), (d) snapping shrimps (Synalpheus brooksi),
and (e) superb starlings (Lamprotornis superbus) form more
permanent social groups and live together year-round.
Great Migrations
Click
to
Watch
Tens of millions of bison once rumbled across the Great
Plains on a quest for grazing. By the late 1800s nearly all
had been slaughtered. Today most of the half million
remaining bison are in captivity, like these on the Triple U
ranch in South Dakota.
The Genius of Swarms
Click
to
Watch
A single ant or bee isn't smart, but their colonies are. The
study of swarm intelligence is providing insights that can
help humans manage complex systems, from truck routing
to military robots.