Competition

I. A general definition of competition: an interaction between individuals brought about by a shared requirement for a resource in limited supply, leading to a reduction in survivorship, growth, and/or reproduction of the individuals concerned.

A. What sorts of interactions are we referring to? These fit into broad categories of exploitation and interference as discussed in the lab manual.

B. They can be subdivided into more specific ways that individuals may compete.

1. Exploitation--depleting resources

2. Pre-emptive--using space

3. Overgrowth--species growing over another and depriving the other of light

4. Chemical--production of toxins

5. Territorial--behavior or fighting in defense of space

6. Encounter--transient interactions directly over a specific resource

II. Intraspecific competition (Intra means within)

A. Individuals of the same species have very similar resource requirements and if these resources are limiting, they will compete for access.

B. The logistic growth equation describes growth within a species when a density dependent factor such as competition occurs, but competition can also occur between species.

III. Interspecific competition (Inter means between/among)

A. Definition of interspecific competition: individuals of one species suffer a reduction in fecundity, survivorship, or growth as a result of resource exploitation or interference by individuals of a second species.

B. How do we experimentally determine whether competition occurs between two species?

1. Removal experiments

a. Nelson Hairston, Sr.'s experiments on two species of plethodont salamanders. Plethodon glutinosus and P. jordani whose distributions sometimes overlap in the Southern Appalachians. He created experimental plots and used three treatments: remove all P. jordani, remove all P. glutinosus, and a control treatment where he left existing abundances. In the control plots and naturally P. jordani was the more abundant of the two species. In the plots where it was removed, the abundance of P. glutinosus increased, indicating that P. jordani was in some way limiting its increase. But when the reciprocal treatment was examined, P. jordani did not increase in the absence of P. glutinosus.

b. Joseph Connell's work on two species of barnacles in Scotland.

IV. Comparison and contrast between intra- and interspecific competition.

The basic mechanisms of exploitation and interference are similar.
Interspecific competition does differ in three ways.
1. Individuals of different species don't use all the same resources.
2. Individuals of different species don't use resources in exactly the same way.
3. Interspecific competition is more likely to be asymmetrical.

V. An organism's niche is a way of envisioning the environmental conditions where an organism can survive. Organisms survive, grow, and reproduce only within certain limits. For example, temperature, humidity, current flow

A. Two types of niches

Fundamental: the overall potential conditions under which a viable population can be maintained.
Realized: the precise nature of an organisms niche as dictated by competing species and predators.
B. Competitive exclusion principle: that two competing species cannot coexist in a stable environment, unless their realized niches differ.

C. How do species coexist?
1. Niche differentiation: species may specialize on part of their fundamental niche, the realized niche Ex. Cattails. Typha latifolia and T. angustifolia.

2. Character displacement: morphology differs when in sympatry to when in allopatry.
Ex 1.Change in bill morphology of House Sparrows when House Finches were introduced. Ex 2. Veromessor ants.
"Paradox of the plankton" Paradox is that numerous species of planktonic algae coexist in very simple environments with little apparent scope for niche differentiation. Hutchinson proposed that the environment was continually changing--although it was simple. Change would promote the exclusion of a certain species but before an equilibrium or total exclusion could occur, the environment would change and another species might be favored. Since all environments vary, this may be a very important factor by shifting competitive balances.

Are there general conditions under which coexistence between species is likely to occur?

Use modeling done by Lotka and Volterra to address this.Lotka and Volterra modified the logistic growth equation dN/dt = rN(K-N)/K (which is the same as dN/dt = rN[(1-(N/K)] to include interspecific competition effects.

B. How does species 2 affect species 1? Look at rate of change dN1/dt

1. If they were equal competitors, we could simply add them:

dN1/dt = rN1[K1-(N1+N2)]/K1

Species are seldom equivalent in their competitive abilities.
Add one new variable alpha (ato describe the relative effects of an individual of one species on an individual of another.

dN1/dt = rN1[K1-(N1+a12N2)]/K1

a12= the effect ON species 1 OF species 2

4. Alpha adjusts the number of N2 to being equivalent to N1. For example, if N2 is a really good competitor so that having 1 N2 around is similar to having 10 N1, alpha would be 10. If it is a lousy competitor so that 10 N2 have the same effect as 1 N1 alpha would be 0.1.

5. A similar equation applies to N2 except that the alpha has a different subscript combination to reflect that the species effects may not be reciprocal.

dN2/dt = rN2[K2-(N2+a21N1)]/K2

a21 =the effect ON species 2 OF species 1

6. When no competitor is present, the equations reduce to the logistic growth equation.

7. We want to know how population size of species 1 and species 2 change in response to each other.

8. To determine the dynamics between species 1 and species 2, we can find the line of equilibrium where dN/dt=0 (also known as zero isocline) for each species. Species 1 is at equilibrium when K1-(N1+a12N2) = 0, and Species 2 is at equilibrium when K2-(N2+a21N1) = 0. We can plot the lines of equilibrium for each species.

9. Four possible outcomes of competition based on the model.

a. Species 1 always wins.

b. Species 2 always wins.

c. Coexistence of species below each K.

d. Which species wins depends on starting conditions (Ex. Diatoms)

By the end of this lecture you should understand 1) the important components in the definition of competition; 2) similarities and differences between intraspecific and interspecific competition; 3) how studies of competition are designed and how results are interpreted; 4) the concept of niche; 5) the competitive exclusion principle; 6) how similar species are able to coexist; 7) the terms of the Lotka-Volterra competition equation; 8) the consequences for competition interactions that the model predicts; 9) how the competition equation and the logistic growth equation are similar and different.