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Invalidation of Principle of Competitive Exclusion Defended

Abstract

Gause and Witt1 give as necessary conditions for the coexistence of two species living in the same microcosm α<K1/K2 and β<K2/K1, where α and β are the coefficients of competition and K1 and K2 the saturation values of the two species when they are not competing with each other2. As Gause and Witt put it, these conditions mean that there is only a “slight mutual depression of species”. I showed2 that Drosophila serrata and D. pseudoobscura coexist although alpha;>K1/K2 and β>K2/K1. The two species of Drosophila coexist even though there is a “strong mutual depression”. For example, the sizes of D. pseudoobscura AR and of D. serrata are 252 and 278 flies respectively in the competing populations; in populations of single species their respective sizes are 664 and 1,251 flies. This result warrants the rejection of the principle of competitive exclusion, “two species competing for limiting resources can only coexist if they inhibit the growth of the competing species less than they inhibit their own growth”. Similarly this experimental result warrants the rejection of the competition equations from which the necessary conditions for coexistence stated above are derived.

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References

  1. Gause, G. F., and Witt, A. A., Amer. Naturalist, 69, 596 (1935).

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  2. Ayala, F. J., Nature, 224, 1076 (1969).

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AYALA, F. Invalidation of Principle of Competitive Exclusion Defended. Nature 227, 89–90 (1970). https://doi.org/10.1038/227089b0

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