WLF 448: Fish & Wildlife Population Ecology

Fall 2008

Interspecific Interactions

I.  Competition

Definitions of Competition

• Competition occurs when animals utilize common resources that are in short supply; or if resources are not in short supply; competition occurs when the animals seeking those resources nevertheless harm one another in the process. -Birch 1957

• Interspecific competition occurs when two or more species experience depressed growth rate or equilibrium population level attributed to their mutual presence in an area. -Emlem 1973

            How could we measure "depressed growth rate"?

• neutralism:  neither species is adversely affected by the other's presence

• amensalism:  individuals of only one species are adversely affected by the other's presence

• competition:  both species are adversely affected by the other's presence

• allopatric:  occurring in separate, non-overlapping geographic areas

• sympatric:  occupying the same or overlapping geographic areas

               parapatric:  within a sympatric geographic distribution areas of occurrence are disjunct 

Ecological theory of competition:

• Competitive exclusion:  Gause (1934) in his book The Struggle for Existence stated: "Complete competitors cannot coesist."

• Niche

• Character displacement

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

• Look for depressed growth rate

            Time-series of abundance analysis (approach based on Lotka-Voltera competition model) 

• Look for partitioning of resources (e.g., food, temporal, habitat/space)

            Example:  Ocelots and bobcats

• Investigate reasons for parapatric distributions

            Example:  Stream fish in Oklahoma

References

Ayala, F. J., M. E. Gilpin, and J. G. Ehrenfeld. 1973. Competition between species: theoretical models and experimental tests. Theor. Pop. Biol. 4:331-356.

Beuchamp, R. S. and P. Ullyett. 1932. Competitive relationships between certain species of freshwater triclads. J. Ecol. 20:200-208.

Cody, M. L. 1974. Competition and the structure of bird communities. Princeton Univ. Press, Princeton, N.J. 318pp.

Connell, J. H. 1961. The influence of interspecific competition and other factors on the distribution of the barnacle Chthamalus stellatus. Ecology 42:710-723.

Elton, C. S. 1927. Animal ecology. Sidgwich and Jackson, London.

Gause, G. F. and A. A. Witt. 1935. Behavior of mixed populations and the problem of natural selection. Amer. Natur. 69:596-604.

Grinnell, J. 1917. The niche relationships of the California thrasher. Auk 21:364-382.

Heller, H. C. 1971. Altitudinal zonation of chipmunks (Eutamias): Interspecific aggression. Ecology 52:312-319.

Horn, H. S. and R. M. May. 1977. Limits to similarity among coexisting competitors. Nature 270:660-661.

Hutchinson, G. E. 1957. Concluding remarks. Cold Spring Harbor Symp. Quant. Biol. 22:415-427.

Hutchinson, G. E. 1959. Homage to Santa Rosalia, or why are there so many kinds of animals? Am. Nat. 93:145-159.

Miller, R. S. 1964. Ecology and distribution of pocket gopher (Geomyidae) in Colorado. Ecology 45:256-272.

Miller, R. S. 1967. Pattern and process in competition. Adv. Ecol. Res. 4:1-74.

Pulliam, H. R. 1975. Coexistence of sparrows: a test of community theory. Science 189:474-476.

Tilman, David. 1982. Resource Competition and Community Structure. Monographs in Population Biology No. 17, Princeton Univ. Press, Princeton, NJ.