Host specificity and geographic range in haematophagous ectoparasites Boris R. Krasnov, Robert Poulin, Georgy I. Shenbrot, David Mouillot and Irina S. Khokhlova Krasnov, B. R., Poulin, R., Shenbrot, G. I., Mouillot, D. and Khokhlova, I. S. 2005. Host specificity and geographic range in haematophagous ectoparasites.  / Oikos 108: 449  /456. A negative interspecific correlation between the degree of habitat specialization and the size of a species’ geographic range has been documented for several free living groups of organisms, providing support for the niche breadth hypothesis. In contrast, practically nothing is known about the geographic range sizes of parasitic organisms and their determinants. In the context of the niche breadth hypothesis, parasites represent ideal study systems, because of the well documented variation in host specificity among parasite species. Here, we investigated the relationship between host specificity (a measure of niche breadth) and geographic range size among flea species parasitic on small mammals, using data from seven distinct geographical regions. Two measures of host specificity were used: the number of host species used by a flea species, and a measure of the average taxonomic distance between the host species used by a flea; the latter index provides an evolutionary perspective on host specificity. After correcting for phylogenetic influences, and using either of our two measures of host specificity, the degree of host specificity of fleas was negatively correlated with the size of their geographic range in all seven regions studied here, with only one minor exception. Overall, these results provide strong support for the niche breadth hypothesis, although other explanations cannot be ruled out. B. R. Krasnov and G. I. Shenbrot, Ramon Science Center and Mitrani Dept of Desert Ecology, Jacob Blaustein Inst. for Desert Research, Ben-Gurion Univ. of the Negev, P.O. Box 194, IL-80600 Mizpe Ramon, Israel (krasnov@bgumail.bgu.ac.il).  / R. Poulin, Dept. of Zoology, Univ.of Otago, P.O. Box 56, Dunedin, New Zealand.  / D. Mouillot, UMR CNRS-UMII 5119 Ecosystemes Lagunaires, Univ. Montpellier II, CC093, FR- 34095 Montpellier Cedex 5, France.  / I. S. Khokhlova, Desert Animal Adaptations and Husbandry, Wyler Department of Dryland Agriculture, Jacob Blaustein Institute for Desert Research, Ben-Gurion Univ. of the Negev, IL-84105 Beer Sheva, Israel. Individual species vary greatly in the size of their geographic ranges. Numerous studies have searched for the correlates of geographic range size and thus attempted to explain this variation. For example, the extent of variation in the sizes of geographic ranges appears to be specific to particular taxonomic or functional groups of organisms (Brown et al. 1996). Range sizes of closely related species are, usually, more similar to each other than to those of distantly related species (Jablonski 1987, Brown 1995), suggesting that some intrinsic characteristics of the organisms inherited from their common ancestors influence the ecological interactions that limit geographic distribution (Brown et al. 1996). Additional patterns of variation in range size as a function of characteristics of organisms include the relationships between range size and body size (Brown 1995, Gaston and Blackburn 1996a, 1996b) and between range size and abundance (Brown 1984, Gaston and Blackburn 1996c). In most cases, highly significant positive correlations between range size and both body Accepted 4 August 2004 Copyright # OIKOS 2005 ISSN 0030-1299 OIKOS 108: 449 /456, 2005 OIKOS 108:3 (2005) 449