Download Population Dynamics

Population
Ecology
Part Two: Population Growth
Developed by Steven Taylor Wichmanowski based in part on
Pearson Environmental Science by Jay Withgott
Remember that a
population is the
number of organisms of
a particular species that
live in one place at one
time.
As time passes, old
organisms die, new
organisms are born and
some organisms
migrate to other
locations.
All of these changes
lead to changes in
populations.
Populations change size according to the
relative rates of birth/death and
immigration/emigration:
 If the birth rate is greater than the death rate, a
population will increase.
 If the death rate is greater than the birth rate, a
population will decrease.
 If the rate of immigration is greater than the rate
of emigration, a population will increase.
 If the rate of emigration is greater than the rate of
immigration, a population will decrease.
If there are no limiting
factors to control a
population’s growth, it
should grow
exponentially.
exponential growth:
the phenomenon
where a population
grows at a fixed rate
and, therefore, the
larger a population
gets, the faster it
grows.
Here’s a graph of a population
experiencing exponential growth:
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At the early stages of an exponential
growth curve, the population grows slowly.
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As time goes by and the population gets
bigger, it begins to grow at a very fast rate.
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Populations sometimes grow
exponentially, but only until the
population reaches its carrying
capacity.
Carrying capacity: the
maximum population that
an ecosystem can support.
Remember that population is a
measure of individuals of one
species.
In any ecosystem, different
species will have different
carrying capacities.
In reality,
exponential growth
can only occur for
a limited time
because of finite
resources that a
population
depends on for
survival.
Logistic growth: the
pattern where the
growth rate slows
and stabilizes at a
carrying capacity.
An Exponential growth pattern is
sometimes called a “J curve”
a Logistic growth pattern is then called an
“S curve”
Ideally, as a population nears its
carrying capacity, the growth rate
slows until there is zero growth and the
population stabilizes.
This pattern repeats itself as the
population fluctuates above and
below the carrying capacity.
In reality, populations often overshoot their carrying
capacity and the population experiences a
“dieback” which usually results in the population
falling below the carrying capacity.
This pattern repeats itself as the population
fluctuates above and below the carrying
capacity.
Here’s a graph showing a population that
has reached its carrying capacity.
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Notice that a population that has reached
its carrying capacity still fluctuates, but
averages out at the carrying capacity.
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Populations below the carrying capacity tend to
increase.
Populations above the carrying capacity tend to
decrease.
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At times, a population will go so far past their
carrying capacity that they deplete the
resources on which they depend on for survival.
When this happens, the population crashes.
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If the population survives the crash, it may
stabilize at a new carrying capacity
much lower than the original carrying
capacity.
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Carrying capacity is usually determined by
one or more limiting factors.
A limiting factor is some kind of
environmental pressure that restrains the
exponential growth of populations.
Examples of limiting factors:
 availability of food
 availability of water
 availability of nutrients
 presence of predators
 presence of mates
 available habitat
 disease
Limiting Factors and Carrying Capacity
Some limiting
factors are related
to population
density.
Density dependent
factors are limiting
factors whose
influence depends
on population
density
(e.g., disease,
competition,
predation,
resource
availability)
Density
independent
factors are limiting
factors that do not
depend on
population density
and usually relate
to natural
disasters.
(e.g., a tsunami
would significantly
affect the
populations of a
small coastal
village as well as a
large coastal city)
One important
factor in
understanding
population growth
is a species’ biotic
potential.
Biotic potential is
the maximum
ability to produce
offspring in ideal
conditions.
(i.e., how many
babies can you
have?)
K-selection versus r-selection
Ecologists describe low
and high biotic
potentials with the
categories of K-selected
and r-selected species.
r-selected species tend
to have high birth rates
and high death rates.
They produce many
offspring, have relatively
small body size, short lifespans, and provide
relatively little paternal
care.
K-selection versus r-selection
K-selected species tend
to have lower birth rates,
but the amount of
parental care provided
tends to result in lower
death rates as well.
Compared to r-selected
species, they produce
few offspring, have large
bodies, long life spans,
and invest significant time
and energy to parental
care.
K-selection versus r-selection
Populations of K-selected
species grow slowly and
tend to fluctuate near their
carrying capacity.
Populations of r-selected
species experience rapid
bursts of exponential growth
followed by rapid decline
and thus tend to follow a
“rise and crash” population
pattern.
The difference between
between K and r strategies
can be thought of as
“Quality vs. Quantity”
This graph shows that the world human population is
currently experiencing exponential growth.
The Earth’s human carrying capacity is not known, but
is estimated to be around 10 billion.
The current human
population is
approximately 7
billion.
What limiting factors
might determine the
human carrying
capacity of the
Earth?
Historically, technology and agriculture have
helped push the Earth’s human carrying
capacity higher and higher.
It is unknown what will happen when the
human population reaches the Earth’s
ultimate carrying capacity.
We too might suffer some loss, and stabilize.
Or we may see a cataclysmic population
crash…
What do you think?