Phylogenetic path analysis reveals the importance of niche‐related biological traits on geographic range size in mammals

Abstract

MIGUELA. OLALLA-T ARRAGA, ERIK JOAQU IN TORRES-ROMERO,TALITA FERREIRA AMADO andPABLO A. MARTINEZDepartment of Biology and Geology, Physics and Inorganic Chemistry, Rey Juan Carlos University, 28933-Mostoles, Madrid,SpainThe geographic range of a species is arguably the basicunit in biogeography and macroecology (Brown et al.,1996). In particular, there has been a long-standinginterest in understanding the mechanisms that shapethe immense interspecific variation in geographic rangesize, a question often framed around Rapoport’s rule(Whitton et al., 2012). As an emergent species-level trait,range sizes reflect the interplay of ecological and evolu-tionary processes and are of utmost importance for pre-dicting speciation-extinction dynamics (Jablonski,2008). Species tend to have a higher risk of extinction ifthey occupy a small geographic range (Purvis et al.,2000), which also places the investigation of patternsand processes in the variation of species’ range sizes asa central question in applied conservation science.In a recent paper, Di Marco & Santini (2015, and here-after DM&S) analysed which are the determinants ofrange size variation in extant terrestrial mammals glob-ally. They concluded that extrinsic factors (climate andhuman impacts), not intrinsic biological traits, are themost influential variables. This study brings to the tablethe importance of considering anthropogenic effects inmacroecological research. Surprisingly, even for thebest-studied taxa in macroecology such as mammalsand birds, workers have traditionally overlooked theinfluence of human pressures on the observed patterns.We ourselves have called the attention on the need toincorporate human impact metrics, such as humanfootprint or accessibility, to better understand the spa-tial distribution of extant mammal species in some bio-geographical realms (Torres-Romero & Olalla-Tarraga,2015). However, we do not agree that human effectsprevail over biological traits in determining the rangesizes of mammalian species and would like to call theattention on a few conceptual and methodologicalaspects of DM&S’s analyses and interpretation that arenot at all correct to our view.First, DM&S neglect the relevance of phylogeneticrelatedness on the geographic range sizes of species.Intrinsic autoecological features of mammals oftenreflect shared ancestry, so that closely related specieswill tend to share similar biological traits and possiblysimilar range sizes (Brown et al., 1996, Jablonski, 2008).A number of comparative methods have been designedto address potential phylogenetic correlation issues inmodel residuals (as long as there is an underlyingrobust phylogenetic hypothesis, as is the case for mam-mals). Second, DM&S also ignore the importance of therelationship niche breadth-range size as a biologicalexplanation (Slatyer et al., 2013). The concept of ecologi-cal niche was indeed formalized to describe the set ofbiotic and abiotic conditions where a species can persistand maintain stable population sizes. Its projection ontogeographic space (i.e. the duality niche-biotope) is inex-tricably linked to the spatial distribution of a speciesand is highly relevant to analyse biogeographical pat-terns (Colwell & Rangel, 2009). DM&S only incorporatea simplistic diet category variable that classifies mam-mals in terms of trophic position (i.e. carnivores, herbi-vores or omnivores), but do not consider diet breadth.Perhaps more importantly, DM&S characterized andincluded in the analyses the realized climatic niches ofeach mammal species (see also Olalla-Tarraga et al.,2011 for a similar estimation method), but erroneouslyreferred to them as extrinsic variables. These measuresof environmental tolerance breadth typically have astrong positive relationship with range size (Slatyeret al., 2013). Third, DM&S use random forest regressionmodels that allow estimating direct effects betweeneach predictor and the response variable, but cannotcalculate indirect effects via other dependent variables.We have used a recently developed phylogeneticconfirmatory path analysis (von Hardenberg & Gonz-alez-Voyer, 2013) and included previously untestedvariables to provide a reassessment on which are themain determinants of range size in mammals globally(for methodological details see supplementary infor-mation). We find that the range sizes of both nonvo-lant mammals and chiropterans strongly depend ontheir thermal and hydric niches, an intrinsic biologicalproperty, followed by a secondary extrinsic effect of