UC Santa Barbara

Coastal habitats contain some of the most biodiverse and productive ecosystems on the planet. Many of these habitats are formed by foundation species (e.g., coral, macroalgae), organisms that shape the structure and function of the ecosystems they support. Foundation species are often highly susceptible to disturbance and anthropogenic stressors, yet there is still much we don’t understand about the drivers of their resilience (or lack thereof)– particularly the role that community-level interactions play in these dynamics. In this dissertation, I explored these questions in coral reefs and kelp forests by integrating calculus-based mathematical models with observational data from the Long-Term Ecological Research network’s Santa Barbara Coastal (SBC) and Moorea Coral Reef (MCR) sites.In Chapter 1, I used a dynamic energy budget (DEB) model to investigate how nitrogen excretion by coral-dwelling damselfish influences the bleaching susceptibility of host corals. When parameterized for reefs in Moorea, French Polynesia, the DEB model predicted that fish-derived nitrogen can promote coral growth, but could potentially exacerbate bleaching due to complex effects of nutrients on the coral-Symbiodiniaceae symbiosis.In Chapter 2, I transitioned from individual organisms to the metapopulation scale and explored the regional post-disturbance recovery dynamics of the foundation species giant kelp (Macrocystis pyrifera). I used a combination of mathematical and statistical models to investigate how these dynamics are mediated by meta-ecosystem processes (spatial connectivity of kelp detritus) and trophic interactions (grazing by sea urchins). I found that kelp patch dynamics were best explained by connectivity of both kelp detritus and spores, with detritus playing a dominant role when urchins were abundant.In Chapter 3, I continued to investigate how spatial processes and species interactions influence the resilience of foundation species, this time focusing on stony corals and their spatial competition with macroalgae. I used partial differential equations to explore how herbivore behavior interacts with fishing pressure to influence the spatial dynamics of coral and macroalgae. I found that herbivore attraction to coral produced self-organized spatial patterns that eroded coral resilience, allowing localized macroalgal patches to persist at fishing pressures where nonspatial models predicted uniform coral dominance.In Chapter 4, I returned to the coral-damselfish system and used it as the basis for a matrix model of a stage-structured mutualism in which host quality increases with host size. I used adaptive dynamics to explore the evolution of partner choice (strength of preference for adult hosts) in this system and compared evolutionarily stable partner behaviors to the behaviors that maximized population-level partner biomass. I found that the evolutionarily stable and ecologically optimal behaviors could be quite different, and that evolved behaviors lead to greater host and partner sensitivity to intensifying disturbance regimes compared to ecologically optimal behaviors.Overall, this research provides novel insights into how species interactions and spatial processes mediate the responses of two important foundation species, stony corals and giant kelp, to a range of stressors. As coastal marine ecosystems become increasingly threatened by climate change and direct anthropogenic activities, successfully conserving and managing these ecosystems will require a strong understanding of how environmental changes impact foundation species. The combination of mathematical models and observational data used in this thesis represents an important tool for studying these impacts, allowing one to explore questions about community-level interactions not easily addressed by empirical work.