COMMUNITY SHIFTS IN MARINE ECOSYSTEMS: THE ROLE OF DISTURBANCE, DIRECTIONAL CHANGE, AND SPECIES TRAITS

Marine ecosystems face accelerating pressure from climate change, fishing, and compounding anthropogenic stressors. Conventional monitoring frameworks can be ill-equipped to detect complex, often nonlinear ecological changes. Long-term datasets paired with quantitative methods that extract meaningful information from multivariate assemblage data can improve detection and intervention response. I apply and extend a community trajectory analysis framework to test how disturbance pressures, system dynamics, and trait composition shifts shape trajectories and reversibility of community reorganization using long-term fish surveys. My first chapter examines how pulse and press disturbances structure community dynamics in the Northeast US Continental Shelf. Despite attention to marine heatwaves, no detectable reorganization was attributable to pulse events; instead, long-term warming and overfishing drove tropicalization. My second chapter evaluates fifteen demersal fish assemblages, revealing that assemblages undergoing the most consequential long-term departures did so through small, incremental steps rather than abrupt shifts. My third chapter integrates trait-based analysis with temporal beta diversity, revealing decoupled taxonomic and functional dimensions of change. Together, these studies demonstrate that interrogating temporal beta diversity across organizational levels connects assemblage dynamics to broader ecological changes and carries actionable information essential to resource stewardship.