Rapoport's rule, species tolerances, and the latitudinal diversity gradient: geometric considerations

Abstract

The most pervasive species-richness pattern, the latitudinal gradient of diversity, has been related to Rapoport's rule, i.e., decreasing latitudinal extent of species' ranges toward the equator. According to this theory, species can have narrower tolerances in more stable climates, leading to smaller ranges and allowing coexistence of more species. We show, using a simple geometric model, that the postulated decrease of species' potential range sizes toward the tropics would itself lead to a latitudinal gradient opposite to that observed. In contrast, an increase in extent of potential ranges toward the tropics would lead to the observed diversity gradient. Moreover, in the presence of geographic barriers constraining actual species' ranges, Rapoport's rule emerges if the latitudinal trend in extents of potential ranges (as defined by climatic tolerance) is opposite to that postulated or if variability in potential range extents decreases toward the poles. A strong implicit latitudinal diversity gradient (i.e., higher concentration of midpoints of species' potential ranges in the tropics), however, produces both observed macroecological patterns without the contribution of any latitudinal trends in species climatic tolerances or in potential range sizes. Our model underscores the necessity of discriminating theoretical processes and principles from the patterns we observe, and it is well supported by data on global distribution of species' range sizes.

Biodiversity's Big Wet Secret: The Global Distribution of Marine Biological Records Reveals Chronic Under-Exploration of the Deep Pelagic Ocean

“…The deep pelagic environment is spatially homogeneous and has been very stable over time, with little in the way of seasonal and latitudinal variability [10], and yet latitudinal gradients appear to exist in the diversity of at least some deep pelagic taxa [8]. Might this provide a means to tease apart the confounding effects of the environment, geometric constraints, and species tolerances in explaining biogeographic patterns [25], [26]? More generally, it may prove easier to unravel the multiple drivers of change in marine ecosystems, including historical human influences and future climate change, by studying those habitats that have been least affected to date – the mid-ocean, mid-water environment – before transferring this understanding back into more heavily disturbed coastal and benthic systems [27].…”

Mating system does not predict niche breath

“…In support of this argument, ecologists and evolutionary biologists have established that the distribution and range size of plant species are influenced strongly by climate (Park & Potter, ; Parker, ; Stephenson, ), and large geographical range size is thought to be associated with wider niche breadths (Brown, ; Sheth & Angert, ; Slatyer, Hirst, & Sexton, ). Species occurring at higher latitudes have also been hypothesized to have broader environmental tolerances because of larger seasonal fluctuations (Stevens, ; but see Šizling, Storch, & Keil, ). Additionally, selfing has been hypothesized to promote local adaptation and niche divergence by converting non‐additive genetic variance resulting from elements such as epistasis or dominance into additive variance for tolerance to new habitats, thus facilitating expansion into new climates (Kirkpatrick, ; Lande, ; Levin, ).…”

How to allow SAR collapse across local and continental scales: a resolution of the controversy between Storch et al. (2012) and Lazarina et al. (2013)

“…There are multiple reasons for irregular distribution of species ranges within an arena. In addition to species‐richness variation along several environmental and geographical gradients (Currie , Willig et al , Currie et al , Hillebrand , Brown , Mannion et al ), coastlines and edges of biomes will truncate ranges that otherwise would spread beyond the actual continent or biome edge (Šizling et al ). Moreover, species with small ranges (that are truncated) are more likely to become extinct than the species with large ranges (Johnson ) with their midpoints that are inevitably located closer to the centres of the continents or biomes (Colwell and Hurtt ).…”