ORCID Identifier(s)


Graduation Semester and Year




Document Type


Degree Name

Doctor of Philosophy in Quantitative Biology



First Advisor

Sophia Passy


Freshwaters are some of the most vulnerable ecosystems to global change forces, such as land use and eutrophication, implicated in the loss of biodiversity and biotic homogenization (increased similarity) of species communities. However, much less is known about the impact of these forces on the mechanisms underlying biodiversity and the patterns of species co-occurrence. In this dissertation, I use sub-continental to global stream community datasets to investigate the effects of land use and nutrients on algal, macroinvertebrate, and fish biodiversity, abundance, and complexity of co-occurrence patterns to pursue three objectives. I first investigate how agriculture contributes to biotic homogenization, and questioned if homogenization processes operate at local and regional scales (Chapter 2). Using US, French, and Canadian datasets, I determined that generally agriculture homogenizes communities, concurrent with a loss of regional biodiversity and an increase in local diversity, and that relative abundances of common species, not spatial distributions, contribute greatly to homogenization. I also observed that diatoms and insects diverged from fish in terms of biodiversity, abundance, and assembly patterns, emphasizing the importance of body size and/or dispersal capacity over trophic position. In my second investigation, I combine network theory and metacommunity approaches to explore i) whether nutrient supply and nutrient ratio constrain the complexity of sub-continental co-occurrence networks in stream algae and ii) the relative influence of climate and dispersal on co-occurrence relationships vs. metacommunity composition (Chapter 3). I found that nutrient supply and ratio are both important drivers of the size and complexity of algal co-occurrence networks. Further, climate and space (surrogate for dispersal processes) were major influences on co-occurrence relationships across networks but nutrient context determined their relative contributions. Notably, climate and space had differential effect on co-occurrence network topology compared to metacommunity composition, which indicated that processes driving individual species relationships are detached from those driving metacommunity-level patterns. In my third investigation, I examined the impact of spatial scale and body size on the shape of the node degree distribution, which is probability distribution describing how links (co-occurrences) between nodes (species) are distributed in a co-occurrence network (Chapter 4). Using diatom and fish datasets, my results are the first to show an explicit correlation between the observation scale (sub-regional to sub-continental) from which the network is constructed and the shape of the node degree distribution, which further depended upon body size. My findings advance our understanding of biodiversity and co-occurrence patterns in streams and their response to global change, scale and organismal biology. My work also demonstrates a successful integration of mathematical, metacommunity, and network approaches, which provides a solid analytical foundation to improve conservation recommendations and develop more holistic solutions in the face of the on-going biodiversity crisis.


Macroecology, Biodiversity, Species Abundance Distribution, Co-occurrence Network, Node Degree Distribution


Biology | Life Sciences


Degree granted by The University of Texas at Arlington

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