Evolutionary and Ecological Explanations for the Elevational Flexibility of Several East African Bird Species Complexes

Jon Fjeldså,Rauri C K Bowie

doi:10.3389/fevo.2021.768062

Abstract

Africa’s montane areas are broken up into several large and small units, each isolated as forest-capped “sky islands” in a “sea” of dry lowland savanna. Many elements of their biota, including montane forest birds, are shared across several disjunct mountains, yet it has been difficult to rigorously define an Afromontane forest avifauna, or determine its evolutionary relationships with the birds of the surrounding lowland forests. In order to trace the historical relationship between lowland and highland avifaunas, we review cases of species or groups of closely related species with breeding populations at different elevations, and use phylogeographic methods to explore the historical connections between such populations within the biodiversity hotspot of East Africa. The study reveals several idiosyncratic patterns, but also a prominent number of cases of gene flow between populations in southern areas, mainly around the Malawi Rift, and mountains and coastal forests to the north, close to the equator. This may reflect more continuous past distributions through northern Mozambique and coastal Tanzania, or seasonal migrations between areas with different rainfall regimes. Over time, these distributional dynamics have resulted in a higher persistence of lineages, and an accumulation of forest-dependent lineages within the Eastern Arc Mountains of Tanzania and the northern part of the coastal forest mosaic.

Highlights

The Afromontane region comprises punctuated chains of mountains, which mostly follow the East African rift systems, and are characterized by a distinct botanical assemblage in areas above 1,500–2,000 m in elevation (White, 1981)
Based on the diversity of observed distribution patterns of birds in the Eastern Afromontane Biodiversity Hotspot, we aim to address the following questions: (1) Are disjunct elevational distributions of African bird species a rare anomaly, or can we find recurring patterns, and if so, (2) how can we explain the shifts in ecology that must have taken place? (3) Is there a specific evolutionary history that underpins the basis of joint lowland and highland residency across a species range, or are there some common ecological factors or life history traits that are not directly linked with elevation?
The results reported in this study are based on Sanger sequencing of mitochondrial markers [NADH dehydrogenase subunit 2 (ND2) and subunit 3 (ND3), Cytochrome b (Cytb), ATP Synthase membrane subunit 6 (ATP6)] and several nuclear introns [e.g., Fibrinogen beta chain intron 5 (FGB5), Glyceraldehyde 3-phosphate dehydrogenase intron 11 (GAPDH 11), Transforming growth factor beta 2 intron 5 (TGFb2)] following standard methods

Summary

Introduction

The Afromontane region comprises punctuated chains of mountains, which mostly follow the East African rift systems, and are characterized by a distinct botanical assemblage in areas above 1,500–2,000 m in elevation (White, 1981). Attempts to divide Africa into biochoria with distinct biota, have failed to identify a distinct area unit for Afromontane birds (Diamond and Hamilton, 1980; Crowe and Crowe, 1982; deKlerk et al, 2002; Linder et al, 2012; Holt et al, 2013). This is primarily because the small and patchy distribution of many Afromontane species and the high turnover across sites provide little connectivity in cluster analyses. The boundary between the lowland and highland avifaunas appears to be fuzzy, and contributes to making biogeographic subdivisions for African birds challenging (Dowsett, 1986; Bowie, 2003)

Objectives

Results

Conclusion