Rapoport's Rule: A Comparative Analysis Between South and North American Columnar Cacti

Abstract

The geographical range of species belonging to a number of taxonomic groups tends to decrease toward the Tropics (Stevens 1989). This latitudinal pattern has been well documented over the past 30 yr, and several explanations have been put forth, including environmental history (Rapoport 1975, 1982), competition (Rapoport 1975, 1982; Rosenzweig 1975; Anderson and Koopman 1981), local climate and species environmental tolerances (Stevens 1989), the mass effect (the establishment of species in sites where they cannot self-maintain; Stevens 1989, 1992), the species dispersion capabilities and their ecophysiological properties (Stevens 1989; France 1992), and sampling bias (Colwell and Hurtt 1994). The empirical biogeographical rule that states that the latitudinal extent of the geographical range of organisms is positively correlated with the latitude at which these organisms are found was named Rapoport’s rule by Stevens (1989), after the Argentine biogeographer Eduardo Rapoport, who first made reference to the correlation between range and latitude (see Rapoport 1975, 1982). The idea that geographical ranges increase from the Tropics toward the poles has received close attention over the last several years (Stevens 1989; Pagel et al. 1991; France 1992; Rohde 1992; Rohde et al. 1993; Colwell and Hurtt 1994; Roy et al. 1994; Ruggiero 1994; Smith et al. 1994; Rohde 1996; Rohde and Heap 1996). Although Rapoport’s rule has been tested on several groups in different regions, most of these studies have focused on animals (Rohde 1996; Rohde and Heap 1996), and few of them have been done for South America (Ruggiero 1994). A comparative study addressing the relationship between mean latitude and latitudinal range for one group of species in both South and North America has not been done. The purpose of this note is to examine the validity of Rapoport’s rule for species of columnar cacti along tropical-subtropical gradients in both South and North America. We chose this group of plants because they have been well col-