Mountains exhibit a stronger latitudinal diversity gradient than lowland regions

Abstract

Aim: The latitudinal diversity gradient (LDG) is a well-explored biogeographical pattern, yet regional differences across geographical and geomorphological areas have been poorly examined. Here, we compare the magnitude of the LDG between mountain and lowland regions and hypothesize that the LDG is much stronger in mountain ranges due to their high species packing and turnover. Location: Global. Taxon: Birds, mammals and amphibians. Methods: Mountain regions were defined following two criteria: one based on elevation and another based on ruggedness. Species richness maps were constructed using geographical information of ~22,000 species from Birdlife and International Union for Conservation of Nature over a grid of 10 km2. We quantified species packing as the number of species per unit area that belong to mountains or lowlands globally, and per latitudinal bands of ~3°. We related the number of species to latitude and extracted a distribution of this relationship using a Monte Carlo approach. Results: We showed that the number of species per unit area increased rapidly towards low latitudes, being faster in mountains and reaching values of up to four times higher than that observed in the lowlands. After accounting for area size, species richness increased faster towards the equator and was better predicted by latitude in mountains than in the lowlands. Main Conclusions: These results support our hypothesis, where the high beta-diversity characterizing mountains has an additive effect on the inherent increase of diversity towards the equator. More broadly, our findings may question the predictive power of well-supported hypotheses, which state that regions with greater area, time and energy accumulate more species richness, because in comparison to the lowlands, mountains are geologically younger and exhibit less energy in less area. Hence, mountains represent a conundrum that invites to re-evaluate hypotheses considering the regional dependency of macroecological and evolutionary processes driving species diversity gradients.