Rift escarpment retreat sustains dynamic landscape for Malagasy flora speciation

A high-relief escarpment characterizes the modern topography of the rifted margin of eastern Madagascar. Although it remained tectonically inactive since the formation from rifting with Seychelles-India in the late Cretaceous, the escarpment landscape has been evolving actively, in that the escarpment retreats laterally due to differential erosion between the escarpment and the high plateau since rifting. The topographically active escarpment of Madagascar corresponds to the island's high endemic species richness, although the island itself is a well-known biodiversity hotspot globally.

To investigate the role of tectonic-geomorphic processes in shaping the plant diversity of Madagascar, we constructed a model of elevation change over the entire island. Four geological processes were analyzed for the elevation change: the dynamic uplift from mantle upwelling, the escarpment retreat, the faulting in the Ankay-Alotra Graben, and the sporadic volcanism on the island. We then related the model elevation changes to the phylogenetic patterns and mapped species richness of seed plants. Correlation analysis showed consistence between the observed species richness pattern and the elevation change, in particular, elevation change from the escarpment retreat showed the best correlation with the high species richness.

We hypothesized that escarpment retreat leads to vicariant speciation and accumulates the species lineages along the escarpment region over geological time. To demonstrate how the surface processes on an escarpment are linked to speciation, we constructed a landscape evolution model simulating fluvial erosion of an escarpment on the edge of a pre-existing, topographic highland. The model shows that the escarpment retreats laterally, and the drainage basins become longer and broader. However, the basin growth rates are heterogeneous, and the main divide develops sinuosity as individual drainage basins grow at different rates. The differential and transient erosion rates between catchments lead to increased segregation of elevation bands between watersheds, creating isolated highlands detached from the escarpment slope, and providing a highly fragmented habitat in both space and time. The habitat connectivity constantly evolves during the retreat, which we believe, links closely to speciation through the various observed morphological processes.  

Retreat of the Madagascar escarpment indirectly influences the orographic precipitation and climatic conditions in that the tropical climatic conditions are sustained, and the tropical habitat expands area during the retreat, both are known favorable conditions of speciation. Overall, we conclude that the escarpment retreat sustains a dynamically evolving landscape, which consequently fosters the flora species hotspot of Madagascar, likely through vicariant speciation.