Date of Graduation

12-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Biology (PhD)

Degree Level

Graduate

Department

Biological Sciences

Advisor/Mentor

Magoulick, Daniel D.

Committee Member

Willson, John D.

Second Committee Member

Scott, J. Thad

Third Committee Member

Evans-White, Michelle A.

Keywords

Aquatic ecology; Community ecology; Ecological model; Stream ecology; Stream systems; Stressors

Abstract

Freshwater systems experience multi-faceted degradation from a variety of ecological and environmental stressors. Three common stressors in these systems, drought, nutrient pollution, and invasive species, have wide-ranging effects on stream population- community- and ecosystem dynamics. We have a broad understanding of how each of these stressors works to influence stream systems independently. However, we still know relatively little about if, and how, these stressors might interact when they co-occur. Though drought is a natural part of many stream systems, all three of these stressors can be exacerbated or facilitated by anthropogenic actions. Accordingly, as human population and resource use continue to grow, it becomes increasingly likely that these stressors will co-occur. To address this, I undertook research that sought to better explore the effects of multiple, simultaneous stressors on stream ecosystems. Here, I performed a series of manipulative experiments and constructed mathematical models that examined the effects of varying combinations of drought, nutrients, and invasive species along several ecological scales. I found that each of these stressors can impact stream ecosystems in diverse ways. For instance, drought negatively impacted many portions of the community, while nutrients caused bottom-up trophic effects. Additionally, my models indicated that both drought and invasive species can increase terminal extinction risk for fish metapopulations. However, I also observed several interactive effects. Drought and nutrient pollution interacted both additively and antagonistically on various portions of the food web simultaneously in my experiments, and drought and invasive species interacted synergistically to increase terminal extinction risk in my metapopulation models. Across all examinations, I found that the effects of both individual and multiple stressors are context dependent. Trophic role, life history strategy, and physical habitat all shaped stressor response. This work highlights the complexity of multiple stressors in stream systems and emphasizes a growing need to undertake additional examinations across various taxonomic groups and ecological scales. Our broad knowledge of the effects of individual stressors might not translate to multi-stressor systems, and conservation and management plans that only account for individual stressors might be inadequate to protect resources in systems facing multiple stressors.

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