Stream habitats and the fishes they support are threatened by environmental factors operating within catchments, and efforts to contextualize species responses to those factors are essential to design and implement effective management actions. Investigating the morphological, physiological, phenological, or behavioral characteristics of species can improve mechanistic understanding of how stream fishes respond to environmental factors. Insights gained from such traits-based investigations can be used to predict changes in the structure and function of stream fish assemblages across space and through time. Despite the utility of traits-based investigations, understanding how environmental factors influence functional traits of stream fishes across broad spatial extents (e.g., continental) remains incomplete. Few traits-based investigations have been conducted at such broad extents because availability of standardized datasets representing distributions of stream fishes is often limited, as are datasets characterizing environmental factors consistently over large regions. Therefore, the goal of my dissertation is to use a functional biogeography approach to describe, explain, and predict functional responses of stream fishes to changing environmental conditions at a continental extent. In my first chapter, I use RLQ and fourth-corner analyses to describe relationships between 17 environmental variables and 16 traits for 597 stream fish species within the conterminous United States. I evaluate the generalizability of trait-environment relationships across the study region and show that while the strength and multivariate structure of trait-environment relationships vary, some relationships, including positive associations between migratory species and forested land cover, were significant in multiple ecoregions. In my second chapter, I investigate consequences of biodiversity change on the stability of stream fish metacommunities throughout the conterminous United States. To do so, I develop a structural equation model to integrate predictions of alpha, beta, and gamma diversity, functional redundancy, and compositional and functional variability at local to regional spatial scales. I show that multiple forms of biodiversity contribute to the compositional and functional variability of stream fish metacommunities and generate insights into how local and regional management efforts may help to achieve sustainable outcomes for freshwater ecosystems. In my third chapter, I develop a decision-support framework to promote the use of ecological thresholds (which represent the intensity of a human landscape stressor that leads to a severe decline in fishes) in decision-making applications. This framework integrates threshold status indices, summaries of protected area distributions, and knowledge of multiple stressor configurations within stream catchments to inform conservation and restoration efforts for nearly 1.73 million catchments located throughout the conterminous United States and Europe. The findings highlight the pervasive influences of agricultural land use on stream habitat and indicate that widespread degradation may result from increased urban development within catchments that are poorly protected. Collectively, my dissertation contributes to an improved understanding of how the functional characteristics of stream fishes vary across space and through time and demonstrates the value of using traits-based approaches to characterize vulnerability of stream fishes to human landscape stressors at broad spatial extents. The insights generated in my dissertation can be applied to support management decision-making processes to help limit further degradation of stream habitat and contribute to addressing the global freshwater biodiversity crisis.
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