Can Rapoports Rule Be Rescued?
Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, California 93106 USA Abstract. The latitudinal gradient in species richness, wherein species richness peaks near the equator and declines toward the poles, is a widely recognized phenomenon that holds true for many taxa in all habitat types. Understanding the causative mechanism or mechanisms that generate the latitudinal gradient in species richness (LGSR) has been a major challenge, and the gradient remains unexplained. A different latitudinal trend (named “Rapoport’s rule”), in which the mean size of species geographical ranges tends to decline toward the equator, has been hypothesized by G. C. Stevens to play a key role in generating the LGSR when coupled with a version of the “rescue effect,” in which local populations toward the fringes of geographical ranges are sustained by immigration. The Stevens hypothesis is now commonly cited as a potential explanation for the LGSR and has provoke
