Population
Trends
Why do populations grow
and shrink?
Populations
A
population is the sum of one species
living in a set area at the same time.
 For example, if we measured the number
of one specific species of bass in a lake in
Iowa, we would say that is a population of
bass.
 Note that all adults in a single population
form a reproductive group – they can
breed within their own population.
Populations
 All
populations have several traits that
can be used to describe them:



Size, the total number of organisms in the
population
Density, the number of organisms divided
by the total area or volume
Dispersion, the placement of organisms
within the region
Population Growth
A
population’s growth can basically be
calculated by subtracting the number of
deaths from the number of births. This
gives you the net birth rate.
 If the growth rate is:



Positive: population is increasing
Zero: population is static
Negative: population is decreasing
Population Growth
 Population
growth can also be described
in terms of the rate of growth.
 Biotic potential is the fastest rate that any
given population can grow. The upper
limit for this is the reproductive potential,
the maximum number of offspring that
each individual organism can produce.
Population Growth
 Reproductive
potential differs from
species to species.


Some bacteria have a reproductive
potential of as many as 19 million offspring!
However, a pair of bowhead whales might
take centuries to produce that many
descendants!
Population Growth

Reproductive
potential is
determined by:



Number of offspring
produced at one
time
Rate of reproduction
Age of viable
reproduction
Population Growth
 For
instance, small organisms like bacteria
have very short generation times and
produce offspring early in life. As such,
they have a very high reproductive
potential.
 By contrast, larger organisms have long
generation times and only produce
offspring if sexually mature. They have a
much lower reproductive potential.
Exit Ticket
1.
2.
3.
How do we determine a population’s
growth rate?
Assume a population’s growth rate is
virtually zero. What would happen if new
organisms migrated into the region?
Would pandas have a high or low
reproductive potential?
Exponential Growth
 When
a population’s growth rate is
approaching the reproductive potential,
we say the population is achieving
exponential growth.
 Exponential growth requires sufficient
access to resources to support all
offspring. It also requires the absence of
predators or competitors in the local
ecosystem.
Exponential Growth
Exponential Growth
 As
a population grows, it begins to face
increasing pressures on population
growth. These pressures will eventually
suppress future growth.
 The carrying capacity is the theoretical
limit on the number of organisms that can
be supported in one area. Birth and
death rates are virtually equal.
Exponential Growth
Limiting Factors
 Limiting
factors are those that oppose
future population growth. There are two
types of limiting factors:


Density-dependent factors, which cause
more deaths as a population increases in
density
Density-independent factors, which affect
all populations equally, regardless of density
Limiting Factors
Density-dependent
 Scarcity
of
resources
 Disease
 Predation
Density-independent
 Natural
disasters
 Adverse weather
 Climate change
Bell Ringer
1.
2.
3.
Estimate the carrying
capacity of deer.
List ONE reason that
the deer population
declined from month
five to eight.
Predict what would
happen if the number
of deer exceeded
the carrying capacity
for a significant time.
Importance of Population
Studies
 Population
studies are important for
monitoring the health of various
populations.
 Through population studies, we can
anticipate whether a given population is
likely to grow or decline in the future. In
doing so, we can monitor an ecosystem
for new threats, such as disease or
invasive species.
Importance of Population
Studies
 Population
studies are of particular value
in looking at the present and future
growth of the human population.
 For instance, we use age-structure
diagrams to identify the potential for
future local population growth.
Importance of Population
Studies
 The
bottom of the
graph shows the
proportion of
young people, and
the top shows the
proportion of older
people.
Importance of Population
Studies
 Future
population
growth is predicted
by a large number
of young people.
 On the other hand,
a small base
means that there
will be fewer births.
Importance of Population
Studies
 Seeing
a large
percentage of old
people indicates
that people live a
relatively long time.
 In contrast, if few
people live to see
old age, then
death rates are
likely high.
Importance of Population
Studies
Importance of Population
Studies
Exit Ticket
 Looking
1.
2.
3.
at this graph:
Are birth rates high or low?
Are death rates high or low?
Predict whether future growth will be positive
or negative.
Bell Ringer
A
1.
2.
3.
Match each description to the correct
age structure.
B
C
D
The U.S. experiences slow, but steady population
growth.
Nigeria is a developing country experiencing
rapid population growth.
Germany is a developed country experiencing
negative population growth.
Importance of Population
Studies
A
survivorship
curve shows the
percentage of
offspring that
remain after a
given time frame.
 It is very useful for
looking at
mortality.
Importance of Population
Studies
 The
mortality rate
shows the rate of
death over time.
 Often, this is not a
static value – note
the parabolas to
the right!
Importance of Population
Studies
Importance of Population
Studies
 Type
I survivorship
shows most
individuals live to
long age, and
shows a low
mortality rate.
 Death is only
common at
advanced ages.
Importance of Population
Studies
 Type
II survivorship
shows a constant
mortality rate.
 As such, death
occurs at the same
rate for all ages.
Importance of Population
Studies
 Type
III survivorship
relatively few
individuals live to
adulthood.
 The mortality rate is
high starting from
birth. To offset this,
a large number of
offspring are born.
Importance of Population
Studies
Exit Ticket
1.
2.
Which curve do
you believe best
describes the
U.S.? Why?
Name TWO
countries that
would show a
Type III curve.
Fertility Rates
 Officially,
the fertility rate is the number of
babies born each year per 1000 women.
 The total fertility rate is the average
number of babies a woman will give birth
to in her lifetime.
 The total fertility rate is compared to the
replacement level, the average number
of children each parent must have to
maintain the population.
Demographic Transition
Stage 1
Stage 2
Stage 3
Stage 4
Birth rate
Natural
increase
Death rate
Time
Note: Natural increase is produced from the excess of births over deaths.
Lesson Plan: The Demographic Transition, Activity One
Demographic Transition
Births/Deaths per 1,000
60
50
40
Sweden
Birth Rate
Mexico
Birth Rate
30
Death Rate
20
Death Rate
10
0
1950 1775 1800 1825 1850 1875 1900 1925 1950 1975 2000
Sources: B.R. Mitchell, European Historical Statistics 1750-1970 (1976): table B6; Council of Europe, Recent
Demographic Developments in Europe 2001 (2001): tables T3.1 and T4.1; CELADE, Boletin demografico 69 (2002):
tables 4 and 7; Francisco Alba-Hernandez, La poblacion de Mexico (1976): 14; and UN Population Division, World
Population Prospects: The 2002 Revision (2003): 326.
Lesson Plan: The Demographic Transition, Activity One