Population Growth
ECOLOGY UNIT
***ALL NOTES MUST BE HAND WRITTEN!
The Human Population

From 1900 to 2003, the
population tripled in size to
reach 6.3 billion people

Recently, the 7 billionth
person was born and we
are projected to reach 9
billion within the next 50
years.

While our population has
increased, others have
dramatically decreased.
Properties of
Populations





Remember, populations are a group of
organisms that belong to the same
species and live in a particular place at
the same time.
Geographic
Range of the
Luna Moth
Geographic Range

Definition: the area inhabited by a population.

Varies enormously in size depending on the species.
Population Density

Definition: the number of individuals per unit area.

Populations of different species often have different
densities, even in the same environment.
Types of Population Distribution
Distribution

Definition: how individuals in a population are spaced
out across the range of the population.

Random, uniform, or clumped.
Growth Rate: determines whether the size of the
population stays the same, increases, or decreases
over time.
Age Structure: the age and number of all
Age Structure
individuals with male or female reproductive
Represented
organs that a population contains.
Graphically
Population Growth Rate
Definition: the amount by which a
population’s size changes in a given
time.
 Whether a population grows, shrinks, or
remains the same size depends on:





Immigration and birth add to a
population.
 Emigration and death subtract from a
population.

Calculate growth rate using the following
equation. Note that immigration and
emigration are ignored.
Birth rate – Death Rate = Growth Rate
Birth
Death
Immigration: movement of individuals
into a population.
Emigration: movement of individuals out
of the population.
The Exponential Model
In exponential growth, the larger the
population gets, the faster it grows.
 Under ideal conditions with unlimited
resources, a population will grow
exponentially.
 A graph of the population size over time for
exponential growth makes a J-shaped
curve.



Population
growth
increases as
individuals join
population.
If nothing interferes with this kind of growth,
the population will become larger and
larger, faster and faster, until it approaches
an infinitely large size.
Example: sometimes when an organism is
moved to a new environment, its
population grows exponentially for a time.
Population growth
starts slow.
The Logistic Model
Builds on the exponential
model but accounts for the
influence of limiting factors.
 Logistic growth occurs when a
population’s growth slows and
then stops, following a period
of exponential growth.
 The graph of this model looks
like a stretched-out letter S.
 Carrying capacity (K) is the
number of individuals the
environment can support over
a long period of time.

Population growth stops. Birthrate
= death rate. Immigration =
emigration. Population remains at
carrying capacity.
Population grows
rapidly. Resources
unlimited.
Population growth slows.
Birthrate may decrease.
Death rate may increase.
Immigration may decrease.