Download Ecology: Lecture 1

Ecology:
Lecture 12
Interspecific Competition 1
Outline
 Types of species interactions
 Interspecific competition
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Definition
Modeling: Lotka-Volterra equations
Laboratory Studies of Competition
Field Studies of Competition
Types of species interactions
Type of
relationship
Neutral
Mutualism
Commensalism
Parasitism
Predation
Competition
Response:
Species A
0
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+
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Response:
Species B
0
+
0
-
Interspecific competition:
Overview
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Exploitative competition: Both
species use the same limited resource.
Each makes the resource scarcer for the
other species.
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Are the two species equally affected?
Interference competition: Species
directly interact and interfere with access
to a resource that may or may not be
limiting.
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Are the two species equally affected?
Interspecific competition:
Lotka-Volterra model
 Four key assumptions
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Homogenous, stable environment
No migration
Effects of competition are instantaneous
Coexistence requires a stable equilibrium
 Are any of these assumptions realistic?
Under which circumstances?
The equations
 The pair of equations
 The meaning of each term
 Focus on competition coefficients ( and )
 Will go through on the board; be sure you
know them!
 Solving for limits
 Viewing and solving graphically
Lotka-Volterra Model: Phase plane
diagrams for individual species
 On board (see Fig. 14.1, a and b)
 Be sure you understand and can draw
the vectors as well as the isoclines
Lotka-Volterra Model: Phase plane
diagrams for both species combined
 Worksheet!
 Be sure you understand and can draw
the vectors as well as the isoclines.
Effects of change
in resource or
environment
 Scenario:
 2 species with
different seed size
preferences (top)
 3 environments
differing in seed
size distribution
(bottom)
 How will K,  and
 be affected?
Fig. 14.2: Effects of change in resource
or environment: Results
Paramecium competition (Gause)
P. aurelia vs. P. caudatum
 Set-up: fixed amount of bacteria as food
 What are the results of competition?
Paramecium competition (Gause)
P. bursaria vs. P. caudatum
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With P. caudatum and P bursaria, Gause
achieved a stable equilibrium.
 Why did this
combination
produce a stable
equilibrium,
whereas the
other did not?
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Gause’s hypothesis of
competitive exclusion
 “As a result of competition, two
similar species scarcely ever occupy
similar niches, but displace each
other in such a manner that each
takes possession of certainly
particular kinds of food and modes of
life in which it has an advantage over
its competitor.”
What evidence is required to show that
competition is responsible for a
particular pattern of distribution?
The distribution of the two species is inversely
correlated.
The two species have been demonstrated to
require the same limited resource and/or one
interferes with the other’s ability to acquire
resources
Removal of the “superior” competitor results in
movement of the “inferior” competitor into the
now unoccupied region.
It is really the presumed competitor that is
responsible for the exclusion.
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Ask: What else could be responsible?
Barnacle competition
(Connell, 1961)
Barnacle competition
(Connell, 1961)
 Is there an inverse correlation?
 Are they competing for a limited resource?
What is it?
 If the presumed “superior” competitor
(which one) is removed does the “inferior”
competitor expand its range accordingly?
 Experiments described!
 Why isn’t Chthamalus completely excluded?
 Is the “superior” competitor really
responsible for the “inferior” competitor’s
limits?
Mallards and black ducks
 Compare
distribution of the
two ducks
 What is their
interaction like in
overlap region?
 Is the evidence
presented
sufficient to show
competition?