LINKING CLIMATE, GEOLOGY AND LANDSCAPE EVOLUTION TO UNDERSTAND HABITAT DISTRIBUTION AND MICRO-ENDEMISM IN MADAGASCAR

Habitat connectivity and fragmentation are crucial parameters in driving gene flow, speciation and terrestrial ecosystem evolution. A number of abiotic factors, such as climate and topography, are commonly used to map the spatial distribution of habitats and explain observed biodiversity patterns. We used numerical modeling experiments to explore how landscape evolution and climate evolve through geological time to define habitat distribution. We predicted orographic precipitation patterns on a series of landscape evolution models under varying geologic and climatic conditions, and habitats were defined using a combination of precipitation rate and elevation. Model results were validated against the biogeography and geology of Madagascar, a biodiversity hotspot due to its species richness and microendemism.

We find that drainage basin geometry is highly influenced by the wavelength of flexural isostasy along the island margins in response to erosion of steep escarpments. This control on topography thus has an important effect on altitudinal temperature and orographic precipitation gradients. In cases where the effective elastic thickness and the wavelength of isostatic rebound is large relative to island width, a more linear drainage divide develops, leading to orographic precipitation patterns generally occurring parallel to the divide. This results in connected habitats on both the windward and leeward sides of the island that remain relatively intact through time. Where isostatic rebound occurs at small wavelengths, rivers are diverted closer to the island margin, resulting in a more complex drainage evolution history that leads to the formation of large high elevation or “perched’’ catchments. River capture and divide migration events on the central and leeward side of the landscape lead to fragmented habitat distribution through both space and time. We observe these patterns of divide and catchment geometries associated with a short wavelength isostatic rebound on Madagascar. Furthermore, vegetation, precipitation and species range patterns show higher connectivity on the windward side and fragmented distribution on the leeward side of the island, suggesting a potentially strong link between landscape evolution and climate in long-term ecosystem evolution.