Rapoport effect and speciation/extinction rates in the tropics

Abstract

In their comprehensive TREE review, Chown and Gaston 1xAreas, cradles and museums: the latitudinal gradient in species richness. Chown, S.L. and Gaston, K.J. Trends Ecol. Evol. 2000; 15: 311–315Abstract | Full Text | Full Text PDF | PubMed | Scopus (140)See all References1 reviewed evidence for and against Rosenzweig's hypothesis that the geographic area of the tropics is responsible for the latitudinal gradient of species diversity. The hypothesis is based on the fact that the tropics, as the largest climatic zone, can host species with larger geographic range sizes, resulting in higher speciation rates and lower extinction rates 2xRosenzweig, M.L. CrossrefSee all References2. The authors claim, in accordance with Rosenzweig, that the existence of a Rapoport effect (the lower geographic extent of species in lower latitudes) does not falsify the area model. It might be true if we consider Rapoport's rule only as a decline in the mean or median latitudinal extent of geographic ranges towards lower latitudes; but, it does not seem reasonable if the range sizes of the species in the tropics are actually smaller, which is advocated by some authors 3xRapoport's rule: time for an epitaph?. Gaston, K.J. et al. Trends Ecol. Evol. 1998; 13: 70–74Abstract | Full Text | Full Text PDF | PubMed | Scopus (204)See all References3. Rosenzweig 2xRosenzweig, M.L. CrossrefSee all References2 claims that his model cannot predict real differences in range sizes at unequal tropical and temperate diversities because only in similar diversities can one predict that tropical range sizes will be larger. Indeed, the theory predicts that, in equilibrial conditions (and the theory actually is equilibrial), the tropical range sizes should be either larger or the same as temperate ones, but not smaller. According to the theory, smaller ranges should lead to a relative lowering of speciation rates or elevation of extinction rates. Therefore, if range sizes only are responsible for speciation and extinction rates, then the relatively small ranges of species in the tropics should lead progressively to lowering of tropical species richness, and thus the latitudinal gradient is not in equilibrium. In other words, the theory does not explain the persistence of the smaller species ranges in the tropics.Therefore, the existence of smaller species’ ranges in lower latitudes falsifies the mechanism of generating latitudinal gradient in species diversity proposed by Rosenzweig. However, it does not mean that the geographic area is not a primary determinant of the gradient through some other mechanism. One promising mechanism is that proposed by Chown and Gaston. Because of the large, climatically similar surface of the tropics, tropical species can be adapted to relatively narrow ranges of climatic conditions, even if they can potentially have relatively large range sizes. For these species, any barriers posed by climatic conditions and promoting allopatric speciation might be more important. This hypothesis, contrary to Rosenzweig's hypothesis, actually does not predict any equilibrial range sizes.