This item is under embargo and not available online per the author's request. For access information, please visit http://libanswers.wustl.edu/faq/5640.
Date of Award
Doctor of Philosophy (PhD)
Ecological communities across the globe are generally described in terms of their species diversity and their species abundances. The latitudinal-diversity gradient describes the systematic increase in species richness from temperate latitudes towards the equator. Complementarily, the distribution of species abundances in ecological communities is typically described by a ‘hollow curve’ where most species are rare and few are common. Recent theory in community ecology has highlighted the joint influences of speciation, dispersal, niche selection and ecological drift as drivers of species diversity and abundance. However, disentangling the relative importance of niche selection has remained challenging because different mechanisms operating at multiple scales (e.g. abiotic niches and dispersal) can result in similar patterns of species abundances, distributions, and diversity. The goal of my dissertation was to test the importance of different niche hypotheses in explaining fundamental patterns of species abundances and diversity across temperate and tropical forests. I focused on the importance of environmental variables (climate, soils, topography) and plant functional traits (leaf traits), as components of the abiotic niche, in determining changes in tree species diversity and species abundances at local, regional, and global scales. I conducted three complementary studies to test hypotheses concerning the influence of (1) abiotic niches on changes in tree species diversity across a latitudinal-diversity gradient, (2) abiotic niches on changes in tree species occupancy and mean local abundance across temperate, sub-tropical and tropical regions, and (3) abiotic niches and dispersal on species abundances and spatial distributions at local scales in a temperate-tree community. In the first study, I tested the relative importance of three prominent niche-assembly hypotheses (niche space, niche breadth, niche overlap) in explaining tree species diversity across temperate and tropical forests. Results of this study showed that species in tropical communities had larger abiotic niche breadths and higher abiotic niche overlap than species in temperate communities, patterns opposite to those predicted from resource-competition theory or “bottom-up” models of community assembly. My results suggest that tree species in tropical forests experience weaker resource competition and/or have weaker abiotic niche partitioning than tree species in temperate forests. Instead, tropical-tree communities seem to exhibit stronger “top-down” effects from biotic interactions among trees and natural enemies, and/or a stronger influence of dispersal-assembly. In the second study, I tested the relative importance of abiotic niche breadth (range of a species’ abiotic resource requirements) and niche position (location of a species’ abiotic resource requirements relative to the average resource availability of a region) in determining two patterns of species abundance (species mean local abundance and species occupancy). To explore the generality of these mechanisms, I tested these hypotheses across three regions with contrasting biogeographies and species richness (Missouri Ozarks, Argentinian Andes, and Bolivian Amazon). Results of this study showed that species mean local abundance and occupancy were initially explained by abiotic niche breadth and abiotic niche position, but after we randomized associations between the environment and species distributions within each region, only abiotic niche position explained species occupancy consistently across all three regions. My results suggest that the relative importance of abiotic niches in determining species abundances differs across spatial scales. At larger scales, niche position is more important than niche breadth in explaining why some species are widely distributed in a region (high occupancy) while others are narrowly distributed (low-occupancy). At smaller scales, niche breadth and position appear to have weak influences on mean local abundances, suggesting important roles for other drivers of species abundance within local communities (e.g. dispersal, competition, natural enemies). In the last study, I tested the importance of niche breadth, niche position and seed-dispersal limitation in determining patterns of tree species abundances and spatial distributions (intraspecific aggregation) at local scales in a temperate-tree community in the Missouri Ozarks. My results showed that species occupancy and mean local abundance is influenced by both niche breadth and niche position. Moreover, niche breadth and niche position both had a stronger influence on species occupancy than on mean local abundance. Surprisingly, dispersal did not have a strong influence on species occupancy nor mean local abundances. Species with high mean seed-dispersal distances were more spatially aggregated (clumped) at local scales than species with low mean seed-dispersal distances, a pattern opposite to the prediction that dispersal decreases intraspecific aggregation. My results suggest that abiotic niches may override the importance of dispersal in determining patterns of species abundances within low-diversity communities.
Chair and Committee
Carlos Botero, Iván Jiménez, Scott Mangan, Rachel Penczykowski,
Vela Diaz, Dilys, "Abiotic Niche-assembly as a Driver of Species Diversity and Relative Abundance in Temperate and Tropical Forests" (2019). Arts & Sciences Electronic Theses and Dissertations. 1954.
Available for download on Tuesday, August 15, 2119