1. Interspecific Competition 1
Ecology: Lecture 12
Interspecific Competition 1

2. Outline Types of species interactions Interspecific competition
Definition
Modeling: Lotka-Volterra equations
Laboratory Studies of Competition
Field Studies of Competition

3. Types of species interactions
Type of relationship
Response:
Species A
Response:Species B
Neutral
Mutualism +
Commensalism
Parasitism -
Predation
Competition

4. Interspecific competition: Overview
Exploitative competition: Both species use the same limited resource. Each makes the resource scarcer for the other species.
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.

5. Interspecific competition: Lotka-Volterra model
Four key assumptions
Homogenous, stable environment
No migration
Effects of competition are instantaneous
Coexistence requires a stable equilibrium
Are any of these assumptions realistic? Under which circumstances?

6. 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

7. 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

8. 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.

9. 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?

10. Fig. 14.2: Effects of change in resource or environment: Results

11. Paramecium competition (Gause) P. aurelia vs. P. caudatum
Set-up: fixed amount of bacteria as food
What are the results of competition?

12. Paramecium competition (Gause) P. bursaria vs. P. caudatum
With P. caudatum and P bursaria, Gause achieved a stable equilibrium.
Why did this combination produce a stable equilibrium, whereas the other did not?

13. 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.”

14. 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.
Ask: What else could be responsible?

15. Barnacle competition (Connell, 1961)

16. 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?

17. 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?