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Test of Community-Wide Character Displacement Against Null Hypotheses

Donald R. Strong, Jr., Lee Ann Szyska and Daniel S. Simberloff
Evolution
Vol. 33, No. 3 (Sep., 1979), pp. 897-913
DOI: 10.2307/2407653
Stable URL: http://www.jstor.org/stable/2407653
Page Count: 17
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Test of Community-Wide Character Displacement Against Null Hypotheses
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Abstract

Community-wide character displacement, a general tendency for sympatric species to be unusually distinct morphologically, has been inferred by several authors in the recent ecological literature. We test the notion by comparing contiguous morphological ratios between actual and null communities of birds, of three avifaunas. Contiguous ratios are those between adjacent species in rankings by size. Null communities are formed by random assembly, by computer, of populations from source faunas. The hypothesis of community-wide character displacement would be supported by actual ratios that were larger than ratios from null communities. We find that contiguous bill and wing ratios in actual communities are not statistically distinct from those in null communities, for birds of the Tres Marias and California Channel Islands. An analysis of the Galapagos finches gives similar results. Actual communities on individual Galapagos islands show no tendencies of character displacement, compared to null communities that have been drawn from the archipelago as a whole. The actual finch communities of some of the islands depart from expected, but in the direction of character convergence. In sum, we cannot detect community-wide character displacement in any of these three avifaunas. Our results are opposite from those in the literature, probably because our null hypothesis is opposite from that normally used; we assume no a priori structure for communities, for the characteristics in question. Our null hypothesis is that actual communities cannot be differentiated from similar, but randomly assembled, sets of populations from the source pool, and we find no evidence to reject the null hypothesis. Our approach emphasizes apparent randomness in actual communities, and that species often persist together independently of their morphological characteristics. We suggest that apparent randomness would account for a substantial proportion of variation in many real ecological communities, were null hypotheses employed that assumed no structure at the outset. Null hypotheses of this sort have not been customary, and for this reason we suspect that more structure and pattern have been deduced by ecologists than actually exists.

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