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Latitudinal Patterns of Range Size: Methodological Concerns and Empirical Evaluations for New World Bats and Marsupials
S. Kathleen Lyons and Michael R. Willig
Vol. 79, No. 3 (Sep., 1997), pp. 568-580
Stable URL: http://www.jstor.org/stable/3546901
Page Count: 13
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Controversy surrounds the existence and causes of latitudinal gradients in range size, as well as the methodologies for detecting them. We show that results based on traditional methods used to evaluate Rapoport's Rule (i.e., a positive correlation between range size and latitude) for New World bats and marsupials are conflicting and subject to problems associated with statistical independence and mathematical bias. To avoid these shortcomings, we used simulation models to assess the degree to which latitudinal patterns are a product of stochastic or deterministic processes. Two different kinds of simulations were used to generate range sizes. The simulations differed in the kinds of spatial constraints that were incorporated into random algorithms. The first model randomly produced upper and lower latitudinal limits, without any spatial constraint except that species ranges were entirely within the continental New World. To reflect aspects of empirical latitudinal gradients of diversity, the second model incorporated the constraint that the set of randomly generated ranges had a distribution of mid-latitudes or most-distal points that corresponded exactly to the distribution of mid-latitudes or most-distal points in each taxon. The correlation between latitudinal range size and latitude was calculated separately for each taxon. Random distributions of correlation coefficients were generated from 1000 simulations for each taxon. When mid-latitude was used as a descriptor, New World bats and marsupials had ranges that are smaller in the tropics and larger in the temperate zone than would be expected by chance alone. In contrast, when most-distal point was used as a descriptor, relationships were consistently indistinguishable from those produced by stochastic processes.
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