Rhode 1996

Rhode K. 1996. Rapoport’s Rule is a local phenomenon and cannot explain latitudinal gradients in species diversity. Biodiversity Letters 3: 10-13.

This author summarizes current evidence for Rapoport’s Rule, here defined as a gradient of increasing species latitudinal ranges with increasing latitude. The studies cited fall into one of two categories: either they support the existence of Rapoport’s Rule, but only in the northern hemisphere, in terrestrial or freshwater environments north of approximately 40°N, or they did not find evidence in support of Rapoport’s Rule and studied organisms living in the tropics, the southern hemisphere, and/or marine environments. The landmasses north of 40°N are roughly coincident with the extent of the glaciation of the Pleistocene, suggesting that Rapoport’s Rule may be more a result of historical processes than current ecological conditions.

Much of the paper is a direct critique of Stevens (1989, 1996), who (in 1996) attempts to extend the generality of Rapoport’s Rule to marine teleosts and depth. According to Rhode (1996), Stevens (1989, 1996) neglects to consider two important potentially confounding effects. First, sampling bias alone (as described by Colwell & Hurtt, 1994) can lead to an apparent Rapoport’s Effect as per-species sampling effort will decrease with increasing species richness if total sampling effort is held constant, and the species in the more species rich region will appear to have smaller ranges. Second, Stevens (1989, 1996) used means of species ranges (latitudinal, depth, or altitude), but the mean is a poor measure of central tendency in this context (as pointed out by Roy et al., 1994) because of the strongly non-normal distributions of species ranges. Additionally, Rhode (1996) states that high-latitude marine fishes experience less temperature variation with depth than do tropical species, so cannot be considered to have broader temperature tolerances.

This paper clarifies some of the debate apparent in the literature extending to 2008 (e.g. Krasnov et al. 2008), but I am left with some suspicions about Rhode’s (1996) fact-checking. Besides some odd typos in the literature cited section, Rhode (1996) describes France (1992) as finding a Rapoport’s Rule pattern in North American freshwater crayfish and amphipods north of approximately 40°N. However, France (1992) includes data about crayfish and amphipod species ranges from approximately 30°N, which I do not consider equivalent to “approximately 40°N”. France (1992) also includes relevant data about other groups, molluscs, mammals, “herps”, and “fishes” derived from Stevens (1989). In short, this paper opens as many questions about the status of the debate in the biogeographical literature about Rapoport’s Rule as it answers.