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Santa Rosalia Reconsidered: Size Ratios and Competition

Daniel Simberloff and William Boecklen
Evolution
Vol. 35, No. 6 (Nov., 1981), pp. 1206-1228
DOI: 10.2307/2408133
Stable URL: http://www.jstor.org/stable/2408133
Page Count: 23
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Santa Rosalia Reconsidered: Size Ratios and Competition
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Abstract

Hutchinson's (1959) claim that there is a minimum size ratio between adjacent species in a size-ranking and the related contention that size ratios tend to be constant between adjacent species pairs in a suite of three or more species have often been repeated and interpreted as resulting from interspecific competition, but never subjected to statistical analysis. Many data sets for which these claims have been made cannot be tested since they are incomplete, but we have attempted to examine the other sets by tests based on those of Barton and David (1956). Our null hypothesis was generally that the logarithms of species sizes did not differ from an independent set of points uniform-randomly distributed on a line segment between the logs of the largest and smallest observed sizes, and the alternative was constant ratios or unusually high minimum ratios. At the .05 level few of the data sets falsified the null hypothesis, though at the .30 level one would reject the null in favor of the alternative hypothesis about half the time (though rarely without ambiguity). Related claims about size ratios are a mixed bag; some are consistent statistically with the data on which they are based, others are not. In no sense is the "1.3 rule" of size ratios a rule of nature; even published results tend to support it weakly at best, and negative results are probably much less frequently reported. Its continued popularity is likely due to its inclusion in a larger tacit hypothesis that sizes differ and this is caused by competition. Since one can usually find other differences between similarly sized species, failures of the rule or its variants in specific instances are not generally taken to falsify the larger hypothesis. We do not claim that sizes are not partly determined by competition, nor that our hypothesis of independent, random sizes is correct. But we do feel that the evidence presented to date that sizes are competitively determined is weak, and that in particular the "1.3 rule" was probably always a red herring and has certainly outlived its usefulness to evolutionary ecologists.

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