A NULL MODEL FOR DETECTING NONRANDOM PATTERNS OF SPECIES RICHNESS ALONG SPATIAL GRADIENTS

I present a null model that can be used for detecting nonrandom patterns of species richness along spatial gradients, such as latitude and elevation. Because estimates of species richness along a single spatial gradient are nonindependent, tests of statistical inference should not be applied. The null model described here circumvents this problem of nonindependence by randomly placing the actual range width of each species along the gradient. The simulated richness curve generated in this way can be compared to an “average” random curve composed of many such simulations. The comparison involves measuring the mean displacement (D) of the simulated curve from the average random curve. By repeating these steps a specified number of times, one can obtain a distribution of D values. The displacement of the actual species richness curve from the average random curve can also be determined and compared to the distribution of D values of the simulated random curves. This comparison allows one to determine whether...

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