Reply from R.K. Colwell and D.C. Lees

We agree with Bokma and Monkkonen1xThe mid-domain effect and the longitudinal dimension of continents. Bokma, F. and Monkkonen, M. Trends Ecol. Evol. 2000; 15: 288–289Abstract | Full Text | Full Text PDF | PubMedSee all References1 in viewing departure from the predictions of appropriate geometric null models as evidence for climatic, historical and other influences on geographical patterns of species richness. We would be astonished if distributional patterns on the continental scale were not importantly affected by such influences. However, their argument and verbal model regarding latitudinal gradients in the Americas fail to provide such evidence.In their letter, Bokma and Monkkonen1xThe mid-domain effect and the longitudinal dimension of continents. Bokma, F. and Monkkonen, M. Trends Ecol. Evol. 2000; 15: 288–289Abstract | Full Text | Full Text PDF | PubMedSee all References1 envision ranges as randomly placed circles on a map of the Americas as a two-dimensional model for the mid-domain effect (MDE)2xThe mid-domain effect: geometric constraints on the geography of species richness. Colwell, R.K. and Lees, D.C. Trends Ecol. Evol. 2000; 15: 70–76Abstract | Full Text | Full Text PDF | PubMed | Scopus (501)See all References2. We agree that such a circle model would produce a bimodal-richness pattern. But, in moving from a line segment (which can represent a path through a range of any shape) to a circle, new and unrealistic constraints are introduced.Using circles placed within an hourglass-shaped domain (such as the Americas) precludes all trans-isthmian distributions and falsely limits occurrence in the isthmus to narrow-ranged species (small circles), assuming that the entire range circle (or some minimum percentage of it) must lie on land. In reality, ranges of most species that occur in Central America are quite ‘laterally compressed’, or even, themselves, funnel or hourglass-shaped3xEstimating global biodiversity: tropical beetles and wasps send different signals. Bartlett, R. et al. Ecol. Entomol. 1999; 24: 118–121Crossref | Scopus (29)See all References, 4xEstimating terrestrial biodiversity through extrapolation. Colwell, R.K. and Coddington, J.A. Philos. Trans. R. Soc London Ser. B. 1994; 345: 101–118Crossref | PubMedSee all References. Because the MDE is disproportionately caused by wide-ranging species2xThe mid-domain effect: geometric constraints on the geography of species richness. Colwell, R.K. and Lees, D.C. Trends Ecol. Evol. 2000; 15: 70–76Abstract | Full Text | Full Text PDF | PubMed | Scopus (501)See all References2, it is not surprising that eliminating all such species from the isthmus and its funnels (by the design of the model) subordinates or eliminates a mid-tropical MDE (which might or might not fall on the isthmus itself).If we are to use real continental shapes (and elevations) as limiting domains in geographic null models, then range shapes and range size distributions must be modeled realistically. Unfortunately, as this worthwhile exchange illustrates, this is no simple matter for two or three dimensions. Meanwhile, one-dimensional ‘transect’ models, using orthogonal dimensions5xA null model for species richness gradients: bounded range overlap of butterflies and other rainforest endemics in Madagascar. Lees, D.C. et al. Biol. J. Linn. Soc. 1999; 67: 529–584CrossrefSee all References5, allow incorporation of empirical range-size distributions explicitly and range shape implicitly, and stand on their own merits for interpreting one-dimensional patterns, such as the latitudinal gradient of local richness.