Greater species diversity at the equator linked to variations in precipitation

Species diversity is higher at the equator than at the poles. In biological terms, this is referred to as the latitudinal diversity gradient (LDG), in which the number of species increases from the poles to the Equator. This ranks among the broadest and most notable biodiversity patterns on Earth. The pattern of species-rich tropics (e.g. lush rainforests teaming with ‘Life’) relative to species-poor temperate and polar areas (e.g. barren polar deserts), has been recognized for over a century, but the reasons for the LDG have remained a mystery.

A new study led by Associate Professor Erin Saupe from the Department of Earth Sciences at the University of Oxford, suggests that higher species diversity in equatorial regions may be caused by higher rates of speciation (the formation of new and distinct species in the course of evolution), driven by changes in precipitation across space and through time. In their Nature Ecology and Evolution paper, released today, Saupe and her team used simulations to test whether spatial and temporal climatic changes generated large-scale patters of biodiversity. Their simulations replicated the diversity gradients we see today for three major vertebrate groups (amphibians, birds and mammals) using models of climate change and minimal biological assumptions.

Simulated diversity patterns by Saupe and colleagues (upper left) compared to patterns observed in mammals, birds and amphibians. Provided by University of Oxford, Department of Earth Sciences


The results suggest that over Earth’s history, higher diversity in the tropics is caused by greater speciation, which is in turn largely driven by spatial and temporal variations in precipitation rather than temperature. These variations in precipitation contribute to environmental changes that can isolate populations of organisms, leading eventually to evolutionary divergence and speciation.

‘These simulations have the potential to profoundly change our understanding of the mechanisms that generate biodiversity and large-scale ecological patterns,’ explains Saupe. ‘They set an exciting future research agenda that further interrogate and test these novel ideas’.