Date of Award
Level of Access Assigned by Author
Doctor of Philosophy (PhD)
Ecology and Environmental Sciences
Second Committee Member
Third Committee Member
Additional Committee Members
Cross-trophic-level dynamics represent biotic interactions between organisms in a food web that span two (e.g., predator-prey interactions) or more (e.g., bottom-up or top-down indirect interactions) trophic levels. These dynamics are fundamental to understanding a variety of animal attributes across ecological contexts, including life-history traits, population limitation, and resource management. A classic example illustrates how declines in sea otter (Enhydra lutris) populations had negative indirect effects on giant kelp (Macrocystis pyrifera) populations due to reduced top-down regulation of a primary kelp predator, purple sea urchin (Strongylocentrotus purpuratus). The need for management approaches that incorporate this food-web connectivity is evident in the increasing adoption of ecosystem-based management approaches in place of the traditional single-species management paradigm, particularly in aquatic ecosystems. In this dissertation, I apply a unified theme—cross-trophic-level interactions in aquatic ecosystems—across ecosystem types, vertebrate classes, and animal attributes to examine both general ecological principles and applied resource management. In Chapter 1, I examine the role of a primary prey taxon in shaping a life-history trade-off in a polymorphic amphibian population. The results of this chapter uncover sex-specific functional responses and, more broadly, highlight the importance of landscape heterogeneity in maintaining intrapopulation phenotypic diversity. In Chapter 2, I use metabarcoding of fecal DNA to describe the overwinter diet of a shorebird population that has recently experienced population declines. As the first diet study to utilize molecular approaches with this species, I provide multiple competing explanations for my novel results and illustrate the challenges of integrating initial molecular diet data into a species’ known diet history based on traditional approaches. In Chapters 3 and 4, I conduct a landscape-scale Before-After Control-Impact experiment to assess the effects of commercial harvest on a basal resource (seaweed) at a whole-bed scale (Chapter 3), and whether bottom-up effects were detectable in high trophic-level consumers (birds; Chapter 4). My results indicate that the effects of commercial rockweed harvest on two common bed characteristics (biomass and height) are lesser at the scale of the rockweed bed than at smaller spatial scales that do not capture the spatial heterogeneity of harvest impacts. My findings in Chapter 4 suggest weak bottom-up indirect interactions between commercial rockweed harvest and coastal birds, and they raise questions about the use of the focal habitat relative to other habitat types in coastal birds’ home ranges. Overall, my dissertation examines cross-trophic-level interactions across a diverse set of systems and illustrates how these food- web linkages can inform a variety of outcomes from life-history trade-offs to conservation planning.
Johnston, Elliot M., "Cross-trophic-level Dynamics in Aquatic Ecosystems and Their Application Across Ecological Contexts" (2023). Electronic Theses and Dissertations. 3764.
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