Navigating Coexistence: Ecological Drivers and Social Implications of Predator-induced Regime Shifts in the Northeast Pacific

Societies are greatly challenged by regime shifts, when ecosystems undergo fundamental changes that are rapid, unexpected, and difficult to reverse. In order to better navigate these transitions, we need information on the drivers, species interactions, and feedbacks that influence ecosystem dynamics, and an understanding of how human communities are adapting to the profound shifts in ecosystem resources. My thesis applies this social-ecological system lens to an iconic regime shift – the recovery of sea otters (Enhydra lutris) in the Northeast Pacific that is triggering a trophic cascade which causes sea urchin and shellfish-dominated rocky reefs to become productive macroalgae-dominated forests. To examine how predation and herbivory interactions affect the structure, function, and resilience of reef communities on the central coast of British Columbia (BC), I conducted four years of subtidal surveys and experiments. These data confirm the critical role of sea otter predation in suppressing urchin populations, but also demonstrate for the first time, that complementary predation by mesopredators (i.e. sunflower sea star Pycnopodia helianthoides) further enhance the resilience of kelp forests by consuming smaller-sized urchins that are otherwise unconsumed by otters. I also experimentally quantified how numerical and behavioural factors collectively influence herbivory rates that maintain alternative reef states. Kelp consumption rates showed a positive but non-linear relationship with urchin biomass, whereas food subsidies and predator-avoidance behaviour reduced urchin grazing rates. Next, to understand how sea otter recovery influences coastal Indigenous communities, I worked in a collaborative Indigenous partnership to host workshops and conduct survey interviews in a comparative case study. We identified 22 social-ecological conditions that can influence Indigenous peoples’ ability to adapt to otters, and revealed how perceptions and adaptive capacity differed between a BC First Nations community and an Alaska Sugpiaq Tribe. These quantitative and qualitative data suggest that coexistence with sea otters could be improved through strengthening Indigenous agency and authority and enabling collaborative adaptive otter management grounded in traditional knowledge and western science. As a whole, this thesis highlights the complexities, surprises, and contextual nuances that characterize sea otter recovery in tightly coupled social-ecological systems, and provides the foundations for a road map to improve future human-otter coexistence.