Integrating molecular biogeography and community ecology to understand the evolution of habitat specialization in Amazonian forests
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Date
26/11/2015Author
García-Villacorta, Roosevelt
Villacorta, Roosevelt García
Metadata
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.