A population is a group of organisms of the same species that live in the same area. There are three characteristics of populations: geographical area, density, and growth rate. Population growth is determined by birth rate, death rate, and migration. Exponential growth occurs under ideal conditions with no limits, but logistic growth occurs when limiting factors cause the population to level off at the carrying capacity. Limiting factors can be density-dependent, affecting large populations through competition and predation, or density-independent like natural disasters.

2. Characteristics of Populations
A population is a group of organisms from the
same species that live in the same geographical
area.
There are three important characteristics of all
populations:
1.Geographical area. (Location)
2.Density (# of individuals per unit of area).
3.Growth rate.

3. Population Growth
 Some populations may stay the same size for
years.
 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

4. Exponential Growth
 What if a population
had “ideal” conditions
(no predators,
no disease, plenty of
food and space).
 Ex.) Bacterium 
“Doubling time” = 10
min

5. Exponential Growth
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?

6. Exponential Growth
 J- shaped curve
 Population grows
slowly at first and
then rapidly to
infinitely high
number.
 Does NOT
happen in natural
populations for
very long. Time
# of
Organisms

7. Exponential Growth
• Exponential growth occurs under “ideal
conditions”.
 Growth is “un-restricted.”
 Individuals will reproduce at “constant rate.”
 In case of bacteria, they constantly double every
10 minutes.
 If a population continues to reproduce at this
rate, it will reach its “biotic potential.”
 Biotic potential- highest reproduction rate
possible for a species

8. Logistic Growth
 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
populations growth.
 These factors can be abioitc or biotic.
 The environment can only support a certain
amount of individuals. This is known as the
“carrying capacity.”

9. Logistic Growth

10. 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

11. Limiting Factors
 Factors that cause a populations numbers
to level out or decrease are known as
“limiting factors.”
 Two types of limiting factors:
1.Density – Dependent Factors
2.Density – Independent Factors

12. Density – Dependent Limiting
Factors
 Factors only become limiting when the
population density reaches a certain level.
 Population size is the issue.
 These factors impact large, dense populations
the most.
1. Competition
2. Predation
3. Parasitism and Disease

13. Competition
 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
species.

14. Competitive Exclusion
Principle
 Two different
species competing
for the same
resource, one will
be better adapted
than the other and
will eventually win
that resource.

15. Density – Independent Factors
 Affect all populations
same way regardless of
population size (density).
1. Natural disasters
(storms, hurricanes,
etc.).
2. Human activity
(deforestation,
pollution, etc.).