Thesis abstract: On the biogeography of vertebrate body size: ecological and evolutionary insights from assemblage-level patterns

Abstract

news and update ISSN 1948-6596 Vallecillo, S., Brotons, L. & Osborne, P. E. (under re- view) Geographical variation in ecological con- straints on species distributions along a gradient of population aggregation. Vallecillo, S., Brotons, L. & Thuiller, W. (2009) Dangers of predicting bird species distributions in re- sponse to land-cover changes. Ecological Appli- cations, 19, 538–549. Vallecillo, S., Brotons, L. & Possingham, H. (to be sub- mitted) Minimizing future threats when setting conservation targets for bird species with differ- ent response to fire disturbance. Edited by Nuria Roura-Pascual thesis abstract On the biogeography of vertebrate body size: ecological and evolutionary insights from assemblage-level patterns Miguel Angel Olalla-Tarraga PhD, Departamento de Ecologia, Universidad de Alcala, Alcala de Henares, Spain Current address: Division of Biology, Imperial College London, Silwood Park campus, Ascot, UK. E-mail: m.olalla@imperial.ac.uk; http://www.bio.ic.ac.uk/research/apurvis/miguel.htm Biogeographers, evolutionary biologists and ecolo- gists have suggested a number of ecogeographical and evolutionary “rules” to describe general trends in the variation of biological attributes across broad spatial and temporal scales. This in- cludes the latitudinal gradient in species richness, as well as Gloger’s, Bergmann’s, Allen’s, Cope’s, Jordan’s, Foster’s and Rapoport’s rules (see Lo- molino et al. 2006 or Gaston et al. 2008 for de- tails). Nonetheless, many of these “rules” were originally proposed with insufficient empirical evi- dence and have reiteratively been called into question. Recent investigations have found excep- tions to the proposed patterns and suggested that most of the “rules” are invalid or would need to be reconsidered (see e.g. Ashton 2001). Under this historical framework, the study of Bergmann’s rule is a paradigmatic example. In 1847, Karl Bergmann suggested that body size plays a major role in determining the geographic distribution of mammals and birds; large-bodied species are favoured in colder climates because of their better heat conservation (lower surface-to- volume ratio). As with the rest of ecological and evolutionary “rules”, both the original pattern and mechanism have not been exempt from criticism. Interestingly, however, after more than 160 years and despite controversies around its validity, Bergmann’s rule still attracts special attention among scientists (Blackburn et al. 1999, Ashton et al. 2000, Meiri & Dayan 2003, Blackburn & Haw- kins 2004, Rodriguez et al. 2006). This stands in contrast to some other “rules” (e.g. Gloger’s, Al- len’s or Jordan’s), whose interest largely remains historic or anecdotic. Such longstanding fascina- tion with Bergmann’s rule is undoubtedly related to the importance of organismal body size in de- termining physiological, ecological and evolution- ary processes. Along these lines, the debate around the generality of Bergmann’s rule has long been fos- tered by Ray’s (1960) and Lindsey’s (1966) pio- neering observations that some ectothermic or- ganisms also displayed intra- and interspecific body size clines as a response to environmental gradients. Apparently, these findings required al- ternative explanations to the ones offered for en- dotherms (Cushman et al. 1993). Since then, re- searchers have tried to identify ecological or evo- lutionary mechanisms accounting for geographic body size gradients in ectotherms. A critical step before searching for underlying mechanisms is indeed examining what the patterns look like in nature. Because Bergmann’s rule was originally formulated for endothermic vertebrates, numer- ous studies have reported the existence of body size gradients in mammals and birds (e.g. Black- burn et al. 1999, Ashton et al. 2000, Meiri & Dayan 2003 and references therein), whereas the geographical variation of body size for many ecto- thermic organisms remains mostly unknown (but see below). frontiers of biogeography 2.1, 2010 — © 2010 the authors; journal compilation © 2010 The International Biogeography Society