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Interspecific Competition, Island Biogeography and Null Hypotheses

P. R. Grant and I. Abbott
Evolution
Vol. 34, No. 2 (Mar., 1980), pp. 332-341
DOI: 10.2307/2407397
Stable URL: http://www.jstor.org/stable/2407397
Page Count: 10
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Interspecific Competition, Island Biogeography and Null Hypotheses
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Abstract

Some of the evidence for interspecific competition among Darwin's Finches (Abbott et al., 1977) has been challenged by Strong et al. (1979). These authors were able to predict the degree of morphological differences actually observed among the finch species from stochastic models. They concluded it is unnecessary to invoke deterministic processes such as competition. We argue that their methods of analysis are seriously flawed by a lack of realism and by biases that tend to favor acceptance of the null hypothesis, hence their conclusions are unacceptable: a) All species within a family were compared, even though some are so different ecologically that competition between them is, a priori, extremely unlikely. An analysis by genera as was done for Geospiza, or by feeding guild, is more meaningful. b) All populations of all species were assumed to have equal dispersal abilities and equal chances of reaching all islands in the archipelago; this is extremely unlikely to be even approximately true. c) A tacit assumption of no-evolution in the construction of random models leaves hanging the question of why populations of the same species differ in beak morphology among islands, and why no two congeneric, sympatric, species differ by less than 15% in at least one beak dimension. d) Expected and observed ratios of beak size were not independent. The first two objections also apply to predictions of island bird properties from a knowledge of mainland birds. Some confusions over the nature of character displacement, the degree of sympatry on islands and the relative importance of interspecific competition are dealt with. Current problems in biogeography in need of attention are (1) separating the potentially conflicting effects of different processes, such as competition and dispersal, and (2) devising tests which permit clear rejection, as opposed to nonacceptance, of an hypothesis. We conclude that stochastic models may be a useful tool in hypothesis testing in biogeography, but until they can be made realistic their usefulness will be severely limited.

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