Interspecific Competition 1 Ecology: Lecture 12 Interspecific Competition 1

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

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

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.

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?

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 With P. caudatum and P bursaria, Gause achieved a stable equilibrium. Why did this combination produce a stable equilibrium, whereas the other did not? www.synearth.net

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. 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?