Rapoport's rule

Rapoport’s rule is an ecological hypothesis that states that latitudinal ranges of plants and animals are generally smaller at low than at high latitudes.

Background

Stevens (1989) [Stevens, G. C. (1989). "The latitudinal gradients in geographical range: how so many species co-exist in the tropics". American Naturalist 133, 240-256.] named the rule after Eduardo H. Rapoport, who had earlier provided evidence for the phenomenon for subspecies of mammals (Rapoport 1975 [Rapoport, E. H. (1975). "Areografía. Estrategias Geográficas de las Especies". Fondo de Cultura Económica, México] , 1982 [Rapoport, E. H. (1982). "Areography. Geographical Strategies of Species". Trad. B. Drausal, Pergamon Press, Oxford. ISBN 978-0080289144] ). Stevens used the rule to “explain” greater species diversity in the tropics in the sense that latitudinal gradients in species diversity and the rule have identical exceptional data and so must have the same underlying cause. Narrower ranges in the tropics would facilitate more species to coexist. He later extended the rule to altitudinal gradients, claiming that altitudinal ranges are greatest at greater altitudes (Stevens 1992 [Stevens, G. C. (1992). The elevational gradient in altitudinal range: an extension of Rapoport’s latitudinal rule to altitude. American Naturalist 140, 893-911.] ), and to depth gradients in the oceans (Stevens 1996 Stevens, G. C. (1996). Extending Rapoport’s rule to Pacific marine fishes. Journal of Biogeography 23:149–154.] ). The rule has been the focus of intense discussion and given much impetus to exploring distributional patterns of plants and animals. Stevens’ original paper has been cited about 330 times in the scientific literature.

Generality of the rule

Support for the generality of the rule is at best equivocal [http://dx.doi.org/10.1016/S0169-5347(97)01236-6 Gaston, K. J., Blackburn, T. M. and Spicer, J. I. (1998). Rapoport's rule: time for an epitaph? Trends in Ecology and Evolution 13, 70-74.] ] . For example, marine teleost fishes have the greatest latitudinal ranges at low latitudesRohde, K. (1992). Latitudinal gradients in species diversity: the search for the primary cause. Oikos 65, 514-527.] Rohde, K., Heap, M. and Heap, D. (1993). Rapoport’s rule does not apply to marine teleosts and cannot explain latitudinal gradients in species richness. American Naturalist, 142, 1-16.] . In contrast, freshwater fishes do show the trend, although only above a latitude of about 40 degrees North. Some subsequent papers have found support for the rule, others, probably even more numerous, have found exceptions to it [Rohde, K. (1999). Latitudinal gradients in species diversity and Rapoport’s rule revisited: a review of recent work, and what can parasites teach us about the causes of the gradients? Ecography, 22, 593-613 ] . For most groups that have been shown to follow the rule, it is restricted to or at least most distinct above latitudes of about 40-50 degrees. Rohde therefore concluded that the rule describes a local phenomenonRohde, K. (1996). Rapoport’s Rule is a local phenomenon and cannot explainlatitudinal gradients in species diversity. Biodiversity Letters, 3, 10-13.] . Computer simulations using the Chowdhury Ecosystem Model did not find support for the rule. [ [http://dx.doi.org/10.1016/j.thbio.2006.01.002 Stauffer, D., and Rohde, K., 2006. Simulation of Rapoport’s rule for latitudinal species spread. Theory in Bioscioences 125(1): 55-65.] ]

Explanations of the rule

Rohde (1996) explained the fact that the rule is restricted to very high latitudes by effects of glaciations which have wiped out species with narrow ranges, a view also expressed by Brown (1995) [Brown, J. H. (1995). Macroecology. University of Chicago Press, Chicago.] . Another explanation of Rapoport’s rule is the “climatic variability” or “seasonal variability hypothesis” [Letcher, A. J., and Harvey, P. H. (1994) Variation in geographical range size among mammals of the Palearctic. American Naturalist 144:30–42.] . According to this hypothesis, seasonal variability selects for greater climatic tolerances and therefore wider latitudinal ranges (see also Fernandez and Vrba 2005 [Fernandez, M. H. and Vrba, E. S. (2005). Rapoport effect and biomic specialization in African mammals: revisiting the climatic variability hypothesis. Journal of Biogeography 32, 903- 918.] ).

Methods used to demonstrate the rule

The methods used to demonstrate the rule have been subject to some controversy. Most commonly, authors plot means of latitudinal ranges in a particular 5^o latitudinal band against latitude, although modal or median ranges have been used by some [Roy, K., Jablonski, D. and Valentine, J. W. (1994). Eastern Pacific molluscan provinces and latitudinal diversity gradients: no evidence for Rapoport’s rule. Proceedings of the National Academy of Sciences of the USA 91, 88.71-8874.] . In the original paper by Stevens, all species occurring in each band were counted, i.e., a species with a range of 50 degrees occurs in 10 or 11 bands. However, this may lead to an artificial inflation of latitudinal ranges of species occurring at high latitudes, because even a few tropical species with wide ranges will affect the means of ranges at high latitudes, whereas the opposite effect due to high latitude species extending into the tropics is negligible: species diversity is much smaller at high than low latitudes. – As an alternative method the “midpoint method” has been proposed, which avoids this problem. It counts only those species with the midpoint of their ranges in a particular latitudinal band. An additional complication in assessing Rapoport's rule for data based on field sampling is the possibility of a spurious pattern driven by a sample-size artifact. Equal sampling effort at species-rich and species-poor localities tends to underestimate range size at the richer localities relative to the poorer, when in fact range sizes might not differ among localities Wikimedia Foundation. 2010.