Influence of past and present environment on the ecology and biogeography of tree species in the Western Ghats biodiversity hotspot

Multiple and entangled ecological and evolutionary processes determine the architecture of biodiversity in space and time. Deciphering these influences is key to understanding fundamental questions relating to the current distribution and assemblage of species, the susceptibility of species to environmental change, and the influence of the biogeographic context on diversification. In the face of ongoing and large-scale habitat alterations of climatic and anthropogenic origin in species-rich tropics, this understanding is all the more important to inform conservation strategies. This is particularly the case in the Western Ghats of India which, along with Sri Lanka, is one of the eight hottest global biodiversity hotspots and one of the most densely populated. Little is known about the ecological and biogeographical drivers of Western Ghats biodiversity, and the basic objective of the thesis is to characterize the role of past and present environmental conditions on the distribution and diversification of Western Ghats tree species. We did this by integrating species distribution models and the analysis of phylogenetic and reproductive trait variation across broad-scale gradients in this region spanning 8 degrees of latitude. First, we examined how past environmental fluctuations could have influenced endemic species distributions over time. We modelled the potential distribution of tree species over the last glacial cycle, and identified basic scenarios of species stability/expansion, contraction, and migration (article 1). Second, we investigated the phylogenetic structure of tree communities to characterize the legacy of past adaptations and the signatures of current environmental filters along gradients (article 2). We found lower phylogenetic diversity under hydric stress or historical instability, suggesting filtering of lineages with specific adaptations. Overdispersion occurs either in least seasonal and more stable forests or in high elevation ecosystems; in the former case, this could be ascribed to ecological diversification over the long term and conservation of old lineages in refugia, and in the latter, to the assemblage of species pools of distinct biogeographic and evolutionary backgrounds (temperate and tropical). Lastly, we addressed in what conditions separation of sexes (dioecy) has been selected for in the course of evolution and in extant ecosystems. We found reverse spatial patterns of dioecious species frequency for endemics and non-endemics, pointing to different mechanisms selecting for dioecy: conservation of non-endemic dioecious lineages in least seasonal southern WG forests, vs. diversification of dioecious endemics in more seasonal and unstable environments northward (article 3). The results concur to highlight the key role of environmental gradients and biological adaptations in the biogeography and ecology of Western Ghats trees. They stress the importance of long-term evolutionary processes and climatic variations in shaping current species pools and patterns of tree species diversity in a highly diverse yet little studied region.