Asymmetric species turnover underpins the latitudinal diversity gradient in the Western Ghats biodiversity hotspot of India

Abstract

Latitudinal diversity gradients (LDGs), namely an increase in species richness from higher to lower latitudes, are one of ecology's best documented patterns, yet how the fundamental components of compositional mismatches (i.e. species gains and losses along a gradient) explain these patterns remains poorly understood. As we argue here, one of two alternative turnover scenarios must apply to a typical latitudinal richness gradient. Such gradients can be the product of either a scenario where both species gains and losses are correlated and increase with geographical distance, or of uncorrelated change where species gains increase with distance, but losses remain constant. We distinguish between these alternatives using the freshwater fish communities of the Western Ghats of India, a biodiversity hotspot with a strong latitudinal species richness gradient. We find that the uncorrelated change scenario prevails for both taxonomic and functional diversity, and on both the western and eastern slopes of the mountain range, albeit with subtle differences. Our results suggest that these uncorrelated changes result from high nestedness among communities and reflect the dominance of dispersal processes in the region. As the observed pattern arises due to an increase in range-restricted species at lower latitudes, it is consistent with Rapoport's rule. Partitioning species turnover into its gain and loss components thus helps explain the mechanisms underlying latitudinal gradients of diversity, and informs our understanding of biodiversity change.