Faster Speciation and Reduced Extinction in the Tropics Contribute to the Mammalian Latitudinal Diversity Gradient

Frederic Jiguet,Jonathan Rolland,Craig Moritz,Fabien L Condamine,Hélène Morlon

doi:10.1371/journal.pbio.1001775

Abstract

Author SummaryWhy are there more species in the tropics? This question has fascinated ecologists and evolutionary biologists for decades, generating hundreds of hypotheses, yet basic questions remain: Are rates of speciation higher in the tropics? Are rates of extinction higher in temperate regions? Do the tropics act as a source of diversity for temperate regions? We estimated rates of speciation, extinction, and range expansion associated with mammals living in tropical and temperate regions, using an almost complete mammalian phylogeny. Contrary to what has been suggested before for this class of vertebrates, we found that diversification rates are strikingly consistent with diversity patterns, with latitudinal peaks in species richness being associated with high speciation rates, low extinction rates, or both, depending on the mammalian order (rodents, bats, primates, etc.). We also found evidence for an asymmetry in range expansion, with more expansion “out of” than “into” the tropics. Taken together, these results suggest that tropical regions are not only a reservoir of biodiversity, but also the main place where biodiversity is generated.

Highlights

The global increase of species richness toward the equator has been the subject of wonder, debates, and speculations since Darwin’s times [1,2]
Why are there more species in the tropics? This question has fascinated ecologists and evolutionary biologists for decades, generating hundreds of hypotheses, yet basic questions remain: Are rates of speciation higher in the tropics? Are rates of extinction higher in temperate regions? Do the tropics act as a source of diversity for temperate regions? We estimated rates of speciation, extinction, and range expansion associated with mammals living in tropical and temperate regions, using an almost complete mammalian phylogeny
Contrary to what has been suggested before for this class of vertebrates, we found that diversification rates are strikingly consistent with diversity patterns, with latitudinal peaks in species richness being associated with high speciation rates, low extinction rates, or both, depending on the mammalian order

Summary

Introduction

The global increase of species richness toward the equator has been the subject of wonder, debates, and speculations since Darwin’s times [1,2]. Known as the ‘‘out of the tropics’’ hypothesis, lineages originate in the tropics, where they massively diversify, and disperse from the tropics to the temperate regions Under this hypothesis, dispersal is higher out of than into the tropics, acting ‘‘against’’ the latitudinal diversity gradient. Known as the ‘‘tropical niche conservatism’’ hypothesis, lineages originate in the tropics and have difficulties to disperse and adapt into temperate regions, accumulating in tropical regions [14,15]. Under both hypotheses, the origin of diversity is tropical, such that intense dispersal from temperate to tropical regions is not considered a plausible explanation for high tropical species richness

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