Characteristics of Populations
A population is a group of organisms from the
same species that live in the same geographical
There are three important characteristics of all
1.Geographical area. (Location)
2.Density (# of individuals per unit of area).
Some populations may stay the same size for
Some may increase or decrease in size over time.
Three factors affect the size of a population:
1. Birth rate
2. Death rate
3. Migration (Immigration and emigration).
If BR + I < DR + E = DECREASING
If BR + I > DR + E = INCREASING
What if a population
had “ideal” conditions
no disease, plenty of
food and space).
“Doubling time” = 10
10 min 1 70 min 64 130 min 4096
20 min 2 80 min 128 140 min 8192
30 min 4 90 min 256 150 min 16,384
40 min 8 100 min 512 160 min 32,768
50 min 16 110 min 1024 170 min 65,536
60 min 32 120 min 2048 180 min 131,072
Can bacteria continue to grow like this?
J- shaped curve
slowly at first and
then rapidly to
happen in natural
very long. Time
• Exponential growth occurs under “ideal
Growth is “un-restricted.”
Individuals will reproduce at “constant rate.”
In case of bacteria, they constantly double every
If a population continues to reproduce at this
rate, it will reach its “biotic potential.”
Biotic potential- highest reproduction rate
possible for a species
Occurs when population’s growth slows or
stops following a period of exponential growth.
What could cause this to happen?
There are several factors that “limit” a
These factors can be abioitc or biotic.
The environment can only support a certain
amount of individuals. This is known as the
Logistic Growth Summarized
S- shaped curve
Lag phase – growth is stable but somewhat
slow, population is getting established
Exponential phase- population well
established and growing quickly
Equilibrium phase- population levels out
as carrying capacity reached and remains
relatively same size
Factors that cause a populations numbers
to level out or decrease are known as
Two types of limiting factors:
1.Density – Dependent Factors
2.Density – Independent Factors
Density – Dependent Limiting
Factors only become limiting when the
population density reaches a certain level.
Population size is the issue.
These factors impact large, dense populations
3. Parasitism and Disease
When populations get larger, resources are used up
more quickly. Ex: food, water, space, and other
essentials to survival.
Interspecific Between two different species.
Intraspecific Between members of the same
for the same
resource, one will
be better adapted
than the other and
will eventually win
Density – Independent Factors
Affect all populations
same way regardless of
population size (density).
1. Natural disasters
2. Human activity