The competitive exclusion
principle
Two species with similar needs for the same
limiting resources cannot coexist in the
same place for very long.
OR
No two species can occupy the same niche in
the same community indefinitely.
Gause’s Experiment
Biomass
Separately
200
150
100
50
Mixed population
Biomass
4
8
12
16
20
24
Paramecium aurelia
Separately
200
150
100
50
Mixed population
4
8
12
16
20
24
Paramecium caudatum
Days
Reduced niche overlap can
allow coexistence
• Behavioral Acclimation
Green anole lizard forced to move to
branches by introduction of Brown lizard.
• Character Displacement
Through evolution, 2 closely related
species tend to be more distinct when
they are sympatric than when they are
allopatric.
1
Examples of Character Displacement
Bill Length (mm)
Species 2
Overlap
29
26
Species 1
23
Gr
ee
Ira
ce
Ch
n
in
a
Two species of Asian nuthatches of the same genera.
Darwin’s Finches
Bill Depth
% in each class
Geospiza
fulginosa
Geospiza
fortis
40
20
same island
40
20
alone
40
20
alone
8
10
12
14
Bill Depth (mm)
Communities Can Recover
From Disturbance
Succession The sequential change in the species
composition of a community over
(ecological) time - 100s to 1000s of
years.
The change in plant species is most
often considered
2
There are 2 types of succession
• Primary (1o) - Community development in
an essentially lifeless area with no soil
remaining from a previous community.
Example - rock exposed by glaciers =>
lichens => mosses => grasses => shrubs
=> trees
• Secondary (2o) - Community development
following the disturbance of a previously
existing community. Remnant soil exists.
Example - abandoned farm => crabgrass =>
horseweed => perennial weeds => pine trees
=> hardwoods.
Primary Succession
Secondary Succession
0 years
30 years
7 years
95 years
15 years
150 years
3
Communities Don’t Always
Recover From Disturbance
Clear Cuts That Have Not Been
Successfully Reforested
4