The patterns and causes of elevational diversity gradients

A major focus of research in spatial ecology over the past 25 years has been to understand why the number of species varies geographically. ! e most striking, and perhaps best documented, pattern in spatial ecology is the latitudinal gradient in species diversity in which the number of species, for most taxa, declines with increasing latitude. Understanding the underlying cause(s) of the latitudinal gradient has proven challenging, perhaps because there are really only two latitudinal gradients (in the northern and southern hemispheres), and because it is often diffi cult to perform experiments at latitudinal scales. Elevational gradients in species diversity are nearly as ubiquitous as latitudinal gradients, and they off er many characteristics that make them perhaps more suitable for uncovering the underlying cause(s) of spatial variation in diversity. First, there are many replicates of elevational diversity gradients – essentially each mountain or mountain range is a replicate, so it is possible to test for the generality of the underlying cause(s). Second, it is possible to carry out manipulative experiments along elevational gradients. ! ird, fi eld data can be collected more readily along elevational gradients than along latitudinal gradients, simply because the spatial extent of elevational gradients is small relative to latitudinal gradients. Finally, many of the potential underlying causes that covary along latitudinal gradients (history, climate, time since glaciation, area) do not covary along elevational gradients (Körner 2007). Given the benefi ts of elevational gradients relative to latitudinal gradients, it seems clear that they can be useful tools to understand the underlying cause(s) of diversity gradients. And, in fact, there is a growing appreciation of the utility of elevational gradients as tools to uncover the mechanisms that shape both patterns of biodiversity and the functioning of ecosystems (Fukami and Wardle 2005, Nogues-Bravo et al. 2008). Ecography has played a major role as an outlet for many studies of elevational gradient studies, and in fact such studies are one of the strengths of the journal. Since its inception, Ecography has published more than 25 papers that have explicitly focused on elevational diversity gradients. ! e papers highlighted in this Virtual Issue indicate that Ecography has been, and will continue to be, an important outlet for papers at the cutting edge of documenting and explaining elevational gradients in diversity. Here, our goal is to highlight some elevational diversity gradient papers published in Ecography (bold-face in reference list) that we feel have made long-lasting contributions to the study of spatial ecology. ! is Virtual Issue (http://tinyurl.com/cr2lkew) is about elevational diversity gradients, though we recognize that a number of key papers have been published in Ecography on topics ranging from montane diversity at regional or continental scales (Parra et al. 2004, Ricklefs et al. 2004, Ruggiero and Kitzberger 2004, Ruggiero and Hawkins 2008), population dynamics (Ramriez et al. 2006, Gimenez-Benavides et al. 2011), interactions among species (Fuentes et al. 1992, Mazia et al. 2004), adaptation (Berner et al. 2004), and climate change (Dollery et al. 2006).