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Chance, Habitat and Dispersal in the Distribution of Birds in the West Indies

John Terborgh
Evolution
Vol. 27, No. 2 (Jun., 1973), pp. 338-349
DOI: 10.2307/2406973
Stable URL: http://www.jstor.org/stable/2406973
Page Count: 12
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Chance, Habitat and Dispersal in the Distribution of Birds in the West Indies
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Abstract

The first part of this paper investigates the degree to which the species composition of insular avifaunas in the West Indies can be predicted from simple biogeographical data. The problem was reduced to a manageable form by limiting the comparisons to ecologically homogeneous sets of islands in three size categories (small 62-160 mi2, medium 120-583 mi2, large 3,400-44,000 mi2). Position effects were eliminated by regressing an index of faunal similarity for each possible pair of islands within each size class against interisland distance and extrapolating to zero distance. For the small and medium-sized islands, the zero distance similarity (endemics deleted) was 0.89 in both instances. The small remainder of 0.11 includes differences due to random colonizations and extinctions as well as uncontrolled variation in species number and habitat quality. It is argued that random processes contribute more to faunal differences within the groups of ecologically similar small and medium-sized islands than does interisland variation in habitat. In contrast, habitat is shown to be comparatively important in accounting for faunal differences between large islands. The second part of the paper examines the degree of sympatry within insular faunas as a function of island size. Speciesarea regressions for the important families fall into four categories that reflect differing family trends in dispersal ability, population density and habitat requirements. As faunal size shrinks with island area, the proportions of most major families hold approximately constant, suggesting that competitive sorting works to preserve an adaptive balance. The analysis supports the notion that insular faunas oscillate about an equilibrium state in which both the number of species and the level of sympatry within competing groups are held within close bounds by the opposition of contrary tendencies.

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