Commentary: One step closer towards disentangling competing hypotheses in macroecology

Abstract

news and update ISSN 1948‐6596 commentary One step closer towards disentangling competing hypotheses in macroecology Macroecological studies of continental and global gradients are constrained to correlative ap‐ proaches — including studies of so‐called natural experiments — because of their large scale. Even when studies are carefully performed, strong in‐ ference may be difficult, since variation and co‐ variation of potential driving factors are not easily controlled. Often such macroecological data sets are characterized by strong intercorrelations among the predictor variables, so‐called multicol‐ linearity. Such problems may cripple the ability to differentiate among competing hypotheses. Nev‐ ertheless, new approaches provide greater ability for tackling these problems and progressing our understanding of what determines species diver‐ sity. These approaches include analyses that as‐ sess spatial variation in relationships, differentiate the components of species diversity, and integrate phylogenetic information into ecological studies. The recent study by Hortal et al. (2011) exempli‐ fies an approach that combines these analytical advances and provides important progress in our understanding of species diversity patterns. Hortal et al. (2011) are concerned with a complex and controversial, but core, question in ecology and biogeography, namely to what extent current diversity patterns are driven by current environmental conditions, notably current cli‐ mate, relative to deep‐time events and processes, so‐called historical factors. Europe was strongly impacted by the Pleistocene glaciations, with large parts of Northern Europe covered by glaciers as late as the Last Glacial Maximum (LGM) just 21 thousand years ago. A number of recent studies have attempted to assess whether these massive past climate changes are still shaping current di‐ versity patterns, e.g. for amphibians and reptiles (Araujo et al., 2007) and mammals (Flojgaard et al. 2011). However, predictors describing historical factors and current climate are often strongly spa‐ tially correlated, making it difficult to tease their respective effects apart. With their innovative, multifaceted ap‐ proach, Hortal et al. (2011) have taken an impor‐ tant step towards overcoming such difficulties and present novel insights into the potential drivers of dung beetle diversity patterns across Europe. They used geographically weighted regression (GWR) to show that there is a remarkable geographic vari‐ ability (non‐stationarity) in the association of spe‐ cies diversity to climate variability since the LGM, indicating a stronger influence of past climate on the current diversity pattern of dung beetles in southern Europe than northern Europe. This find‐ ing for dung beetles is consistent with what Araujo et al. (2007) previously observed for other ectotherms, namely that the location of the LGM 0°C isotherm of mean annual temperature de‐ notes a change in species composition, from small ‐ranged amphibians and reptiles south of the iso‐ therm to widespread species north of the iso‐ therm (see also Svenning et al. 2009 for a related GWR‐based finding for European plants). GWR is generally useful for investigating macroecological processes where associations between drivers and patterns are likely to change in space, for exam‐ ple, such as in regions where the impact of past glaciations varies strongly across space (e.g. Europe). Given that traits often show some degree of phylogenetic conservation, phylogenetic analyses can further help us understand the physiological mechanisms involved in shaping macroecological patterns. Hortal et al. (2011) applied phylogenetic analyses to show that there was a remarkable phylogenetic difference between the southern and northern assemblages, with northern species being phylogenetically clustered, corresponding to particular cold‐tolerant clades of species within the dung‐beetle family studied. The deconstruc‐ tion of beta diversity into its components, turn‐ over (change in species composition) and nested‐ ness (species richness differences) (see Baselga 2010), furthermore supported this finding as © 2011 the authors; journal compilation © 2011 The International Biogeography Society — frontiers of biogeography 3.2, 2011