Constraining null models with environmental gradients: a new method for evaluating the effects of environmental factors and geometric constraints on geographic diversity patterns

Abstract

The relative effects of climate and geometric constraints on geographic diversity patterns have long been controversial. We developed a new method to assess the role of geometric constraints in shaping altitudinal richness patterns. We showed how plant species richness on four mountains in southwest China are shaped by geometric constraints and environmental gradients together. Contrary to previous studies, our results suggested that: 1) small-and large-ranged species richness were largely controlled by the same environmental gradients, and differed mainly in the effect of geometric constraints.2) The contribution of geometric constraints (in addition to environmental gradients) to explaining species richness was greater when species richness peaked at low altitudes than at mid-altitudes, suggesting that geometric constraints may be very important when richness peaks are far away from mid-domains. 3) Relating species richness directly to environmental factors (the most widely used method in biodiversity studies) may be misleading when geometric constraints may be affecting the richness pattern, because this method may overestimate the effect of environmental factors by failing to distinguish the confounding effect of geometric constraints. Instead, the effect of environmental factors can be evaluated with an underlying gradient derived from small-ranged species. 4) The geometric constraints effect cannot be fully evaluated by pure geometric constraints models, and is better evaluated with range-based models constrained with environmental gradients. 5) If the generality of our findings is supported for other taxa on other gradients, then many previous studies on the effects of climate and of geometric constraints on geographic diversity patterns may need to be re-visited.

Climate stability is more important than water–energy variables in shaping the elevational variation in species richness

“…Our results supported the finding that boundary constraint has a relatively significant effect on elevational variation in species richness, which was similar to that found by Colwell () and Wang and Fang (). When we reduced the effect of boundary constraint, plants showed a more significant gradient along the elevation (Figures and ).…”

“…One of the main hypotheses used to explain species richness gradients is the water-energy hypothesis (Francis & Currie, 2003;Kaspari, O'Donnell, & Kercher, 2000;Wang & Fang, 2012). It predicts that regions with higher water and energy availability can hold more species, although the strength of this relationship is dependent on the spatial scale (Evans, Newson, Storch, Greenwood, & Gaston, 2008;Gaston, 2000;O'Brien, 1993).…”

Midpoint attractors and species richness: Modelling the interaction between environmental drivers and geometric constraints

“…We take a novel approach to integrating environment with geometric constraints over elevational gradients. Inspired by Wang & Fang's (2012) evidence that large-and small-ranged species respond similarly to environmental drivers and by Rangel & Diniz-Filho's (2005) mechanistic model, we postulate the presence of an underlying unimodal 'favourability' gradient, specific to each elevational transect and to each taxon or functional group.…”

Explaining the species richness of birds along a subtropical elevational gradient in the Hengduan Mountains

“…Mechanistic models (Rangel & Diniz-Filho, 2005;Colwell & Rangel, 2009) and new statistical approaches (Wang & Fang, 2012) hold promise in explaining the prevalence of low-elevation richness peaks, by integrating climatic drivers of species richness with the geometric constraints posed by domain edges. Although the pooling of taxa adapted to conditions at different elevations, together with climatic factors, productivity and geometric constraints may explain the prevalence of low-elevation richness peaks, additional factors may contribute to this pattern.…”