Hierarchies of Biodiversity: Integrating Traits, Genes, and Landscapes to Understand Freshwater Vulnerability Across Scales

Abstract

Freshwater ecosystems occupy less than one percent of the Earth's surface yet support a disproportionately large share of global biodiversity, including nearly 10% of all known species and over 40% of all fishes. These systems are highly vulnerable to the dual pressures of climate change and habitat fragmentation, which threaten biodiversity from genes to species and communities. A key challenge in freshwater conservation is understanding how spatial scale, environmental variability, and species traits interact to shape patterns of biodiversity and vulnerability. In Chapter 1 of my dissertation, I introduce this challenge in the context of three integrated studies, each addressing different dimensions of freshwater biodiversity, ranging from genetic structure in stream macroinvertebrates to climate sensitivity in freshwater fishes, within the physical and hierarchical context of river networks. In Chapter 2, I assess whether geographic extent influences species-level climate sensitivity rankings for 137 native freshwater fishes of the United States. Using occurrence data and trait-based indices, I find that while national-scale rankings are broadly conserved across regions, regional analyses reveal context-specific vulnerabilities not apparent at broader scales, underscoring the value of multi-scale conservation assessments. Chapter 3 investigates the relationship between species traits and genetic structure in freshwater macroinvertebrates. Across 21 case studies encompassing diverse species and spatial extents, I demonstrate that dispersal ability is a fundamental driver of population genetic differentiation, while life history traits show weaker and less consistent associations. These findings clarify the relative importance of species traits in structuring genetic variation and inform trait-based predictions of genetic vulnerability. Chapter 4 focuses on population genetic diversity and connectivity in Baetidae mayflies across five U.S. basins spanning a gradient of climatic aridity. Using landscape genomic approaches, I compare models of isolation by distance and isolation by resistance. Results indicate that while basin-scale aridity is not a consistent predictor of genetic structure, incorporating flow intermittency improves the ability to explain spatial patterns of differentiation, highlighting the role of fine-scale hydrologic variability in shaping connectivity. Chapter 5 brings together these studies to illustrate how the hierarchical structure of river networks, combined with species-specific traits and spatial scale, governs patterns of biodiversity and vulnerability in freshwater systems. By integrating trait-based, range-based, and genetic perspectives, this work contributes to a broader understanding of freshwater resilience and offers insight for informing stream conservation in a changing world.