Cross-species and assemblage-based approaches to Bergmann's rule and the biogeography of body size in Plethodon salamanders of eastern North America

Over the past few years, there has been a resurgence of interest in the investigation of spatial patterns in body size over large-scales to explore Bergmann’s rule at different phylogenetic scales across a range of taxa (Blackburn et al. 1999, Gaston et al. 2008). In its original formulation Bergmann’s rule predicts that among closely related endotherms those living in colder regions tend to be larger than those in warmer environments as a result of their reduced surface-to-volume ratios and, hence, better heat conservation (Bergmann 1847). Remarkably, the debate around the validity of this ecogeographical ‘‘rule’’ has long been fostered by the conflation of pattern (increasing body size towards colder regions) and mechanism (heat conservation) (Blackburn et al. 1999). However, there is increasing recognition that the geographical distribution of body size is markedly idiosyncratic and Bergmann’s physiological mechanism cannot explain the observed size clines everywhere (Rodriguez et al. 2008, Diniz-Filho et al. 2009). Accordingly, a differentiation has to be made between pattern and process, so that the latter is not explicitly inherent to the former. In doing such distinction we do not only spur the scientific debate around the validity of the ‘‘rule’’, but the search of alternatives to Bergmann’s original mechanism that are able to account for the observed size clines. Part of the controversy around Bergmann’s rule stems from the seminal papers of Ray (1960) and Lindsey (1966) suggesting that some ectothermic organisms display intra and interspecific body size variation as a response to environmental gradients, which apparently requires alternative explanations to the ones offered for endotherms (Cushman et al. 1993, Ashton and Feldman 2003, OlallaTarraga and Rodriguez 2007). Since then, workers have tried to identify ecological or evolutionary mechanisms accounting for body size clines in ectotherms, but we are far from a consensus on a unifying mechanism. 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, an abundant number of studies have reported the existence of body size gradients in mammals and birds (Ashton et al. 2000, Ashton 2002a, Rodriguez et al. 2008, Diniz-Filho et al. 2009, Olson et al. 2009), whereas the geographical variation of body size for many ectothermic organisms remains mostly unknown (but see below). Several authors have documented the patterns and explored the causes of body size variation in ectotherms at different levels of biological organization (Ashton 2002b, Belk and Houston 2002, Ashton and Feldman 2003, Blanckenhorn and Demont 2004, OlallaTarraga et al. 2006, Olalla-Tarraga and Rodriguez 2007, Adams and Church 2008). Notwithstanding, the variety of methods and terminologies involved not only prevent a comparison of results from different studies but also indicate the lack of agreement on a single standard approach to investigate Bergmann’s rule. Gaston et al. (2008) recently attempted to clear up part of the confusion by identifying three kinds of approaches to studying spatial patterns in biological traits in general and Bergmann’s rule in particular: intraspecific, interspecific and assemblage-based. They stressed that the distinction between intraand interspecific approaches seems to be clear, but there is some confusion on the methodological differences between interspecific (hereafter ‘‘cross-species’’) and assemblagebased studies. Beyond semantic considerations (e.g. the assemblage-based approach has also been termed as the ‘‘community’’, ‘‘interspecific’’ or ‘‘grid-based’’ approach) (Blackburn and Hawkins 2004, Olalla-Tarraga et al. 2006), the methodological disparities between cross-species and assemblage-based approaches may not be trivial in terms of interpreting patterns and processes (Ruggiero and Hawkins 2006). In our view, this requires particular attention. Ecography 33: 362 368, 2010 doi: 10.1111/j.1600-0587.2010. 06244.x # 2010 The Authors. Journal compilation # 2010 Ecography