Population Growth
Graphs & Histograms
When conditions are ideal for growth and
reproduction, a population will experience a
rapid increase in size.
Initially the population grows slowly, but
the larger the population gets, the faster it
grows. As more offspring survive and
reproduce, even more offspring are born.
If things were perfect for a population and all the
individuals survived and reproduced at the maximum
rate, that growth rate is called the biotic potential.
For example, human females could theoretically
produce 1 child every 9 months between the ages of
approximately 12 and 45.
Organisms do not usually reproduce according to their
biotic potential.
The graph below is called an exponential
population growth curve, or a J-shaped curve.
Can a population continue to grow at this
rate forever? The answer, of course, is no.
The environment becomes limiting.
Resources such as food and water become
scarcer and the rate of population increase
begins to slow. The graph below illustrates
a population growth curve of this nature.
The graph below is called a logistic population
growth curve, or a S-shaped curve
Compare Population
Growth Curve 1 and
Population Growth Curve 2
and note the similarities
between the two graphs.
Assignment
The population growth on the second graph begins to
slow. Perhaps resources such as food and water are
becoming scarcer. Perhaps some of the population
migrates to a new area to obtain resources.
Eventually the population growth on this graph
reaches zero, and the size of the population remains
fairly stable. This is because the number of deaths in
the population equals the number of births.
Remember…
The formula for determining population
growth is as follows:
Population growth = (births + immigrants) - (deaths + emigrants)
If population growth is less than zero, there
are more deaths and emigrants in the
population than there are births and
immigrants. The size of the population begins
to decline.
The largest population of a species that a
particular environment can support is known
as the
carrying capacity
The carrying capacity of the environment is different
at different times.
At some times, when resources such as food and
water are more abundant, a population can increase
in size. At other times, resources may be scarcer,
causing the population to decline.
These population fluctuations occur over time, but the
carrying capacity represents an average of a series of
ups and downs.
On a graph
If you draw a line through the middle of
the population fluctuations, that line
represents the carrying capacity of that
environment for that species.
Population Growth Curve: Carrying Capacity
Carrying
Capacity
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Assignment:
Carrying Capacity
 Predator – Prey interactions
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Population Growth - Histograms

As well as showing how population
changes with time, (which is the purpose
of a growth curve), it is also useful to
consider how many organisms there are of
a certain age within a population.
Histograms: Age Structure

Age structure is usually shown as a graph
called a population histogram.

The histogram can be used to predict
future changes in population size and
provide governments with information on
the types of changes that might be
required

for example, if there are more children, more
schools will be needed.
Histogram

These histograms only represent a particular
moment or snapshot in time.

The shapes of histograms can often tell you
whether a population is increasing,
stabilizing or decreasing.
Stable
Increasing
Decreasing
About the Graph

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The population is along the horizontal axis
The age is along the vertical axis
The graph is drawn with horizontal bars.
Each bar represents the number of organisms of a
certain age group. There are as many bars as age
groups. There is one bar for males and one bar for
females.
Increasing Populations


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In this histogram many offspring are being produced.
As the population ages, the young replace the current parents and
have offspring of their own.
The histogram is shaped like a pyramid with a wide base. The wide
base means there are lots of young organisms and few older ones.
The population increases since there are fewer older organisms dying
and many more organisms being born.
Steady/Stable Populations

In this histogram organisms of reproductive age are having just
enough offspring to replace themselves.

As the population ages, there will be enough young organisms to
replace the current parents and have offspring of their own.
Since there are as many organisms being born as are dying, the
population remains steady.

Decreasing Populations

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In this histogram there are many organisms of reproductive age but
they are having few offspring.
As the population ages, there will be fewer young organisms to
replace the current parents.
This histogram has the widest part in the middle section because
there are more parents than young.
The population decreases since there will be fewer organisms to
reproduce and replace the young.
Summary

This lesson has dealt with two types of population graphs.

The population growth curves show how population changes
with time.
 These graphs show that populations grow differently
depending on whether the conditions for growth are ideal (Jshaped curve) or if there are restrictions (S-shaped curve).
 Populations in real life fluctuate around the ideal number of
organisms which the ecosystem can support called the
carrying capacity.

An age-population histogram shows a snapshot of the
organisms, both male and female, that fall into categories of
age.
 While the data represented is for only a particular instant in
time, the shape of the distribution allows us to predict if the
population is increasing, decreasing or steady.
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Assignment:
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Population pyramids
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QUIZ NEXT CLASS!