Holocene bidirectional river system along the Kenya Rift and its influence on East African faunal exchange and diversity gradients

Abstract

East Africa is a global biodiversity hotspot and exhibits distinct longitudinal diversity gradients from west to east in freshwater fishes and forest mammals. The assembly of this exceptional biodiversity and the drivers behind diversity gradients remain poorly understood, with diversification often studied at local scales and less attention paid to biotic exchange between Afrotropical regions. Here, we reconstruct a river system that existed for several millennia along the now semiarid Kenya Rift Valley during the humid early Holocene and show how this river system influenced postglacial dispersal of fishes and mammals due to its dual role as a dispersal corridor and barrier. Using geomorphological, geochronological, isotopic, and fossil analyses and a synthesis of radiocarbon dates, we find that the overflow of Kenyan rift lakes between 12 and 8 ka before present formed a bidirectional river system consisting of a “Northern River” connected to the Nile Basin and a “Southern River,” a closed basin. The drainage divide between these rivers represented the only viable terrestrial dispersal corridor across the rift. The degree and duration of past hydrological connectivity between adjacent river basins determined spatial diversity gradients for East African fishes. Our reconstruction explains the isolated distribution of Nilotic fish species in modern Kenyan rift lakes, Guineo-Congolian mammal species in forests east of the Kenya Rift, and recent incipient vertebrate speciation and local endemism in this region. Climate-driven rearrangements of drainage networks unrelated to tectonic activity contributed significantly to the assembly of species diversity and modern faunas in the East African biodiversity hotspot.

Cycles of fusion and fission enabled rapid parallel adaptive radiations in African cichlids

“…Several lakes in the region could potentially have contributed colonists to the colonization of Lake Victoria after its Pleistocene desiccation and could hence have contributed to the seeding of the rapid radiation that followed. The deeper rift lakes, Kivu or Edward, likely did not completely desiccate during the Pleistocene dry spell ( 64 – 66 ) and thus may still harbor older lineages, possibly dating back to the first radiation after the original Congo-Nilotic hybridization event ( 15 ). The Kagera lakes region upstream and to the west of Lake Victoria, in the uplands between Tanzania and Rwanda, might have harbored open water bodies during the dry spell because it is located at higher elevation [1300 m above sea level ( 67 )] and received more rainfall.…”

Sympatric and allopatric Alcolapia soda lake cichlid species show similar levels of assortative mating

“…Fossil and geological data date the Magadi-Natron split to approximately 8 Ka (Dommain et al, 2022), and the lakes are currently situated approximately 25 km apart by a topographic sill (Williamson et al, 1993; Figure 1A). Using nuclear genetic data, the A. grahami (Magadi)-Natron Alcolapia species split has been estimated at 0.007-1.55 Ma (95% HPD) (Ford et al, 2019), which suggests that A. grahami populations in Magadi have been separated for at least 7 Ka and may have diverged before the lakes split (Williamson et al, 1993;Dommain et al, 2022). Therefore, our results indicate that strong mate discrimination can either evolve or persist in allopatry without recent (<7 Ka) reinforcement.…”

“…The much larger and deeper (∼55 m) paleolake Orolonga, which comprised part of rift lake network intermittently connected by rivers, contracted and split to form Natron and Magadi ∼8 Ka. Orolonga itself had more freshwater conditions, and the current highly alkaline and hypersaline conditions are thought to have developed ∼7 Ka (Roberts et al, 1993;Dommain et al, 2022). Therefore, both the adaptive evolution and the speciation of Alcolapia has been extremely rapid.…”

Magma‐Assisted Continental Rifting: The Broadly Rifted Zone in SW Ethiopia, East Africa

“…The depositional record of these basins is an important archive of the combined impacts of magmatic, tectonic, and climatic forcing on erosion and sedimentation processes, which may vary over time, as rifting is sustained over long timescales (e.g., Buiter et al., 2022; Ebinger & Scholz, 2012). Commonly, rift basins are strategic sites for hydrocarbon and geothermal‐energy exploration and they also play a pivotal role in biotic evolution, hydrological connectivity, and the dispersal of species at continental scales (deMenocal, 2004; Dommain et al., 2022; McCune et al., 1984; Prömmel et al., 2013).…”