David J. Currie and Jeremy T. Kerr. 2008. Tests of the mid-domain hypothesis: A review of the evidence. Ecological Monographs 78:3–18.

Appendix B. Continental variation in richness and mid-domain effects (MDEs).

Romdal et al. (2005) argue that the very striking latitudinal mid-domain peak of bird richness in the Americas “support[s] the hypothesis that the distribution of species ranges may be influenced by geometric constraints” (see also Willig and Lyons 1998). As Fig. 2 shows, the pattern is sufficiently strong to be evident without statistical analysis. MDE models make the additional prediction of maximal richness in mid-continents, declining toward the east and west coasts as well as north-south (e.g., Bokma et al. 2001: Fig. 2, Jetz and Rahbek 2002: Fig. 2, Diniz-Filho et al. 2002: their Fig. 2; Kerr et al. 2006: Fig. 1). Based on the reasoning in Romdal et al. (2005), one would expect similarly strong gradients of richness in many taxa, on all continents, in both latitudinal and longitudinal directions. Is this the case? Maps of continental variation in richness (Table B1) can be used to test these predictions.

Consider North America (see references listed in Table B1). Richness of essentially all groups (birds, mammals, fish, amphibians, reptiles, insects, plants) shows strong latitudinal gradients. Longitudinally, the continental mid-domain (the American short-grass prairies) shows a diversity minimum for all groups of organisms studied. Amphibian and plant richness are both maximal near the southeastern corner of the continent (in the southern Appalachians), and reptiles and many insects have richness maxima in the deserts of the Southwest. Breeding-bird richness is maximal in two strips through the Rockies and the coastal Sierras, and eastward around the Great Lakes. We are unaware of any taxon in which peripheral California or South Carolina are species-poor relative to central Kansas and Nebraska. Longitudinal variation in richness is inconsistent with a two-dimensional mid-domain effect.

In Africa (again, see references in Table B1), a maximum of plant richness is found in the fynbos in the southwestern corner of the continent, with a curious local minimum in the central Congo basin (exacerbated or possibly created by poor sampling). Birds vary similarly. The gross pattern in Africa is the opposite of a mid-domain effect.

Maxima of bird and plant richness in South America are found along the eastern slopes of the Andes (close to the west coast), in the mountains near the North and Northeast coasts, and in the Atlantic forest on the East Coast. Again, the gross pattern is the opposite of a mid-domain effect.

In Eurasia, plant richness is highest along the southeast coast, with local maxima in the Himalayas and in (peripheral) southern India. Across the northern half of the continent, richness varies strongly along climatic isoclines (north–south). Thus, the latitudinal gradient shows the typical strong increase in richness toward the tropics, with no mid-domain peak, since the Eurasian landmass does not cross the equator. A longitudinal transect through Eurasia along the 45th parallel stretches from the Atlantic near Bordeaux to the Sea of Japan near Vladivostok. It is nearly twice the length of the latitudinal gradient from the tip of India to the Arctic Ocean. There is no mid-domain peak; on the contrary, there is a mid-domain minimum of richness.

In Australia, bird richness is maximal along the eastern coast, with a mid-domain minimum in the central deserts. Plant richness is maximal on the north-east coast, with a secondary peak in the southwest: again, the opposite of a mid-domain effect. Lizard richness, in contrast, is maximal in the center, consistent with a mid-domain effect (but also consistent with a positive monotonic relationship between richness and temperature, e.g., Currie 1991:Fig. 6).

In sum, with the exception of Australian reptiles, we see no evidence of approximately radially symmetric gradients of richness centered in continental masses. Only the latitudinal pattern of richness in the Americas is at all consistent with a MDE; all other latitudinal patterns all longitudinal patterns including the Americas, are inconsistent with the mid-domain hypothesis.

TABLE B1. Studies presenting maps of continental to global variation in richness, by reference number (below). References in bold show a mid-domain peak in richness, decreasing radially. References in italics show a mid-domain local minimum of richness with maxima near the opposite edges. The evidence is overwhelming inconsistent with the mid-domain hypothesis.

1 = Barthlott and Lauer (1996); 2 = Cook (1969); 3 = Currie (1991); 4 = Currie and Paquin (1987); 5 = Francis and Currie (2003); 6 = (H.-Acevedo and Currie 2003); 7 = Hawkins et al. (2003); 8 = Orme et al. (2005); 9 = (Schall and Pianka 1978); 10 = (Simpson 1964).

LITERATURE CITED for Table B1:

Barthlott, W., W. Lauer, and A. Placke. 1996. Global diversity in species diversity in vascular plants: toward a world map of phytodiversity. Erdkunde 50:317–327.

Bokma, F., J. Bokma, and M. Mönkkönen. 2001. Random processes and geographic species richness patterns: why so few species in the North? Ecography 24:43–49.

Cook, R. E. 1969. Variation in species density of North American birds. Systematic Zoology 18:63–84.

Currie, D. J. 1991. Energy and large scale patterns of animal and plant species richness. American Naturalist 137:27–49.

Currie, D. J., and V. Paquin. 1987. Large-scale biogeographical patterns of species richness in trees. Nature 329:326–327.

Diniz-Filho, J. A. F., E. R. De Sant'Ana, M. C. De Souza, and T. F .L. V. B. Rangel. 2002. Null models and spatial patterns of species richness in South American birds of prey . Ecology Letters 5:47–55.

Francis, A. P., and D. J. Currie. 2003. A globally-consistent richness-climate relationship for angiosperms. American Naturalist 161:523–536.

H.-Acevedo, D., and D. J. Currie. 2003. Does climate determine broad-scale patterns of species richness? A test by natural experiment. Global Ecology and Biogeography 12:461–473.

Hawkins, B. A., E. E. Porter, and J. A. F. Diniz-Filho. 2003. Productivity and history as predictors of the latitudinal diversity gradient of terrestrial birds. Ecology 84:1608–1623.

Jetz, W., and C. Rahbek. 2002. Geographic range size and determinants of avian species richness. Science 297:1548–1551.

Kerr, J. T., M. Perring, and D. J. Currie. 2006. The missing Madagascan mid-domain effect. Ecology Letters 9:149–159.

Orme, C. D. L., et al. 2005. Global hotspots of species richness are not congruent with endemism or threat. Nature 436:1016–1019.

Romdal, T. S., R. K. Colwell, and C. Rahbek. 2005. The infulence of band sum area, domain extent, and range sizes on the latitudinal mid-domain effect. Ecology 86:235–244.

Schall, J. J., and E. R. Pianka. 1978. Geographical trends in the numbers of species. Science 201:679–686.

Simpson, G. G. 1964. Species densities of North American mammals. Systematic Zoology 13:361–389.

Willig, M. R., and S. K. Lyons. 1998. An analytical model of latitudinal gradients of species richness with an empirical test for marsupials and bats in the New World. Oikos 73:579–582.