WLF 448: Fish & Wildlife Population Ecology

Fall 2004

Population Dynamics

III.  Determining the Importance of Competition

A.  2 Opposing views on the importance of competition in population dynamics

1. Competition is the dominant ecological interaction Diamond (1978)

2. Variable environments are primary determinants of population dynamics (Andrewartha and Birch 1954, Weins 1977)

B.  Six propositions of interspecific competition:

• Species too similar in their resource use cannot coexist for long without competitive exclusion
• Species that coexist do so by sufficient differences in their realized niche
• Competition is a powerful evolutionary force selecting for adaptations that allow differential resource use
• Distributions of species are often determined by competitive pressures.  The result is that similar species will have disjunct ranges (i.e., parapatric).
• Species may compete by interference or exploitation (i.e., depletion of resources)
• Experiments performed on species with substantial overlap in resource use should detect interspecific competition (i.e., depressed population growth or fitness when together and the reverse when one is removed)

C.  Approaches to determine importance of competition

1. Experimental: Perturb the system away from equilibrium by adding or removing individuals

• Niche expansion/retraction
• Change in equilibrium density

2. Observational: Attempts to implicate competition indirectly by predicting the result of competition and looking for this result in the natural system

• Partitioning of resources (e.g., food, temporal, habitat/space)
• Example:  Ocelots and bobcats; Stream fish in Oklahoma