Barrett article

Barrett article - ECOGRAPHY 27: 715 /724, 2004 Bergmann's...

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Bergmann’s rule and the mammal fauna of northern North America Tim M. Blackburn and Bradford A. Hawkins Blackburn, T. M. and Hawkins, B. A. 2004. Bergmann’s rule and the mammal fauna of northern North America. ± / Ecology 27: 715 ± / 724. The observation that ‘‘on the whole ... larger species live farther north and the smaller ones farther south’’ was first published by Carl Bergmann in 1847. However, why animal body mass might show such spatial variation, and indeed whether it is a general feature of animal assemblages, is currently unclear. We discuss reasons for this uncertainty, and use our conclusions to direct an analysis of Bergmann’s rule in the mammals in northern North America, in the communities of species occupying areas that were covered by ice at the last glacial maximum. First, we test for the existence of Bergmann’s rule in this assemblage, and investigate whether small- and large-bodied species show different spatial patterns of body size variation. We then attempt to explain the spatial variation in terms of environmental variation, and evaluate the adequacy of our analyses to account for the spatial pattern using the residuals arising from our environmental models. Finally, we use the results of these models to test predictions of different hypotheses proposed to account for Bergmann’s rule. Bergmann’s rule is strongly supported. Both small- and large-bodied species exhibit the rule. Our environmental models account for most of the spatial variation in mean, minimum and maximum body mass in this assemblage. Our results falsify predictions of hypotheses relating to migration ability and random colonisation and diversification, but support predictions of hypotheses relating to both heat conservation and starvation resistance. T. M. Blackburn (t.blackburn@bham.ac.uk), School of Biosciences, Uni v . of Birming- ham, Edgbaston, Birmingham UK B15 2TT. ± / B. A. Hawkins, Dept of Ecology and E v olutionary Biology, Uni v . of California, Ir v ine, CA 92697, USA. The observation that the body sizes of animal species vary spatially was first made by Bergmann (1847), who noted that ‘‘if we could find two species of animals which would only differ from each other with respect to size, ... the geographical distribution of the two species would have to be determined by their size ... If there are genera in which the species differ only in size, the smaller species would demand a warmer climate, to the exact extent of the size difference.’’ He concluded that ‘‘although it is not as clear as we would like, it is obvious that on the whole the larger species live farther north and the smaller ones farther south’’ (Bergmann 1847, translated in James 1970). Spatial variation in body size of this sort across species is now known as ‘‘Bergmann’s rule’’ (see Blackburn et al. 1999). Here we focus on Bergmann’s rule in the northern Nearctic region. Studies of Bergmann’s rule can take one of two
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Barrett article - ECOGRAPHY 27: 715 /724, 2004 Bergmann's...

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