Species richness of Madagascar is uneven, with the highest species richness and endemism found on the steep great escarpment of the eastern margin. The unevenness is further observed within the escarpment region in that phylogenic turnover shows both latitudinal and altitudinal variations. Madagascar has remained almost tectonically inactive since the last rifting with Seychelles-India such that the fundamental topographic framework has been in place since Cretaceous. The high diversity and endemism of Madagascar challenge the conventional notion of uplift-driven speciation, which argues that speciation is driven by the formation of diverse habitat types. To investigate the causal mechanisms of the diversity at the eastern escarpment, we constructed landscape evolution models, tracing the dynamics of habitable land surface patches throughout model simulations.
The landscape of a great escarpment is dynamic and the heterogenous retreat of the escarpment and the water divide makes the geographically isolated drainage basins expand landward at different rates. Within the escarpment region, habitat patches dynamically appear, disappear, fragment, or merge at a frequency that scales with the retreat rate. The models predict that escarpment retreat fosters habitat patch dynamics such that patches isolate, or reconnect with a frequency on the order of a million years, appropriate for allopatric speciation. We conclude that the spatially heterogeneous but temporally steady retreat of the Madagascar escarpment since rifting has sustained allopatric speciation over evolutionary timescales resulting in the observed high diversity and its spatial pattern of eastern Madagascar.