Integrating molecular biogeography and community ecology to understand the evolution of habitat specialization in Amazonian forests

Abstract

I investigated the origin of western Amazonian white-sand vegetation and the evolution of plant habitat specialization to different edaphic conditions in Neotropical lowland forests. In order to address these goals I used complementary ecological as well as molecular phylogenetic approaches. Amazonian white-sand forests harbour a flora specialized to nutrient-poor sandy soils, which is distributed as habitat-islands across the Amazon and Guiana Shield regions. This flora has been suggested to have many local and regional endemics, therefore making an important contribution to overall Neotropical plant diversity. The role of habitat specialization in the origin of this flora and its relationships with other floras within the Amazon- Guiana regions is not well understood. To shed light onto these questions, this thesis studies the floristic composition of these forests as well as molecular phylogenetic patterns of selected plant lineages containing white-sand species. The floristic study focused on the white-sand forests of the western Amazon region, which contained 1180 species of vascular plants whereas the non-white-sand Amazon and Guiana Shield dataset consisted of 26,887 vascular plant species. 77% of these species occurred outside white-sand habitats, in other habitat types of the Amazon region, while 23% were white-sand specialists. This demonstrates lower endemism in western Amazonian white-sand forests than previously estimated. 88% of the total westen Amazon white-sand specialist occur within the limits of the Guiana Shield region with the remaining 12% being endemics to the white-sand forests of the western Amazon. Within the Guiana-Shield region, Caquetá Moist Forests (56%), Guayanan Highlands (55%), and Negro-Branco Moist Forests (53%) were the biogeographic regions with the highest proportions of western Amazonian whitesand specialists. Cluster analysis of province level floristic checklists across the Amazon and Guiana regions showed that western Amazonian white-sand forests are nested within floras of the western Guiana-Shield region compared to other floras in the Amazon. Molecular phylogenetic analyses were carried out for the widespread and species-rich families Sapotaceae and Chrysobalanaceae, which display an uneven number of white-sand specialists. Sapotaceae had only three white-sand specialists but Chrysobalanaceae had a larger number of white-sand specialists (14 species). Phylogenetic analysis showed that white-sand specialist species in both studied families were scattered across the phylogenies. Both families show a marked absence of edaphic niche conservatism, suggesting that evolutionary switching amongst habitat types has been frequent. Ancestral state reconstruction of habitat specialization under a maximum likelihood approach suggests that preference for poor soils may be ancestral in these clades, especially in Chrysobalanaceae, but that the evolution of species entirely restricted to white-sand soils is in general much more recent and has multiple origins. For the white-sand flora of the western Amazon in particular, there is little evidence that it comprises ancient lineages as previously hypothesized. The historical construction of the Amazonian white sand flora is more likely to be the result of a gradual accumulation of species with different degrees of edaphic specialization, both by on-going speciation driven via habitat switching from non-white-sand specialists and via regional dispersal events after these habitats became available in regions such as the western Amazon. Edaphic transitions between different habitat types were not evolutionary constrained, which may have favoured edaphic niche evolution and the accumulation of plant species diversity in Neotropical lowland forests.