Regional influences on community structure across the tropical-temperate divide

Alexander E White,Dhananjai Mohan,Trevor D Price,Kushal K Dey,Matthew Stephens

doi:10.1038/s41467-019-10253-6

Abstract

Many models to explain the differences in the flora and fauna of tropical and temperate regions assume that whole clades are restricted to the tropics. We develop methods to assess the extent to which biotas are geographically discrete, and find that transition zones between regions occupied by tropical-associated or temperate-associated biotas are often narrow, suggesting a role for freezing temperatures in partitioning global biotas. Across the steepest tropical-temperate gradient in the world, that of the Himalaya, bird communities below and above the freezing line are largely populated by different tropical and temperate biotas with links to India and Southeast Asia, or to China respectively. The importance of the freezing line is retained when clades rather than species are considered, reflecting confinement of different clades to one or another climate zone. The reality of the sharp tropical-temperate boundary adds credence to the argument that exceptional species richness in the tropics reflects species accumulation over time, with limited transgressions of species and clades into the temperate.

Highlights

Many models to explain the differences in the flora and fauna of tropical and temperate regions assume that whole clades are restricted to the tropics
To evaluate consequences at the local level, we show that bird communities in the Himalaya below and above the freezing line are largely populated by different tropical and temperate biotas with affinities to tropical South Asia (India and Southeast Asia) or to temperate Asia, respectively
Even when K = 11, subSaharan Africa falls into two motifs, bounded approximately by the tropics, whereas Holt et al.[2] considered this entire region to be 1 of the 20 zoogeographic regions, perhaps because species that span both areas contribute to their metric

Summary

Introduction

Many models to explain the differences in the flora and fauna of tropical and temperate regions assume that whole clades are restricted to the tropics. A long independent history of species accumulation in tropical and temperate climatic regimes (here defined as locations separated by the line of regular freezing) is implied if turnover between these realms is sharp, in much the same way that biotas on different continents have evolved independently. Such an independent history is integral to many models to explain differences in species composition and richness between communities in the tropics and the temperate[8,9,10], yet evidence has been elusive.

Methods

Results

Conclusion