The Principle of Competitive Exclusion

In nature, resources often have to be shared among a multitude of different species. Fructose, for example, is not only a favorite resource for E. coli, but for countless other microorganisms as well. In other words, E. coli faces competition for fructose with other species. The basic mechanism of resource competition is simple: Two or more species consume the same resource, and thereby max

reduce the availability of this resource both for themselves and for the other species.

What makes resource competition particularly interesting is that different species may differ considerably in their resource utilization characteristics. As an example, let us investigate competition for fructose between E. coli and a strain of the bacterium Pseudomonas. In Figure 2a, the resource-dependent growth curves of both species are shown together with their mortality rates. The maximum specific growth rate of E. coli is higher than the maximum specific growth rate of Pseudomonas. However, the mortality rate of Pseudomonas is lower than that of E. coli. Combining the growth and loss curves, we can see that the R* of Pseudomonas is lower than the R* of E. coli. Thus, Pseudomonas can grow at lower fructose concentrations than E. coli. As we have seen above, a species will continue to increase as long as the resource concentration exceeds its R*. Pseudomonas will therefore continue to grow

0 Rps Re

Fructose availability (ig l 1)

0 Rps Re

Fructose availability (ig l 1)

Time

Figure 2 Competition for fructose between the bacteria E. coli and Pseudomonas. (a) Specific growth rates (solid lines) and specific loss rates (dashed lines) of E. coli (thick lines) and Pseudomonas (thin lines) as functions of fructose availability. The R* values indicate the minimal fructose requirements of E. coli and Pseudomonas. Note that the R* of Pseudomonas is lower than the R* of E. coli. (b) Predicted time course of competition for fructose (dashed line) between E. coli (thick solid line) and Pseudomonas (thin solid line). Pseudomonas wins, because it can survive at lower fructose levels than E. coli.

Time

Figure 2 Competition for fructose between the bacteria E. coli and Pseudomonas. (a) Specific growth rates (solid lines) and specific loss rates (dashed lines) of E. coli (thick lines) and Pseudomonas (thin lines) as functions of fructose availability. The R* values indicate the minimal fructose requirements of E. coli and Pseudomonas. Note that the R* of Pseudomonas is lower than the R* of E. coli. (b) Predicted time course of competition for fructose (dashed line) between E. coli (thick solid line) and Pseudomonas (thin solid line). Pseudomonas wins, because it can survive at lower fructose levels than E. coli.

until the fructose concentration has been depleted to its R* value. E. coli, however, cannot grow at fructose concentrations as low as the R* value of Pseudomonas. E. coli will therefore decrease and die out due to depletion of fructose by Pseudomonas (Figure 2b).

The example above can be generalized to any number of species. When different species compete for a single resource in a homogeneous habitat, only the best competitor will survive. Coexistence of inferior species is not possible. This phenomenon is known as the principle of competitive exclusion. It was first demonstrated experimentally by the Russian ecologist G.F. Gause in the 1930s using three species of the protozoan Paramecium competing for bacteria or yeast cells.

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