Ecological processes driving the structure and function of marine algal communities: from species traits to species invasions

Marine ecosystems are complex and diverse, with multiple, often simultaneous processes influencing community structure and functioning. While algae play pivotal roles in supporting biodiversity and ecosystem function, dramatic population increases due to human-induced factors are causing shifts to undesirable alternate states that may not be reversable, motivating research into understanding the processes shaping algal communities in the Anthropocene. Functional groups cluster species assumed to have similar responses to environmental drivers and effects on ecosystem functioning, but these assumptions are rarely tested for marine algae. In Chapter 1, I evaluated the Functional Group Model (FGM), which groups species based on morphological complexity, and makes predictions for traits assumed to correspond with ecological function. We tested these predictions by measuring growth, toughness, and tensile strength across tropical and temperate algae. Only toughness aligned with FGM predictions. Further, there was significant within-group variability among species for all traits, implying the FGM does not predict community responses to ecological drivers and/or contributions to ecosystem function. Marine algal invasions pose severe threats to marine communities, but mechanisms driving invasion success are poorly understood. In Chapter 2, I evaluate how community diversity, herbivory, and interactions with the foundational kelp, Macrocystis pyrifera, influence success of the invasive alga, Sargassum horneri, in southern California. I found neither herbivory nor diversity provide strong explanations for invasive success, suggesting S. horneri requires disturbance to invade. In Chapter 3, I use stage-structured population models to assess whether interactions between kelp and S. horneri influence invasion success and kelp persistence. Modeled relationships between temperature and intraspecific competition for light resulted in accurate predictions of S. horneri population structure. Life history differences mediated S. horneri invasion success and kelp reestablishment, demonstrating S. horneri invasion is dependent on disturbances that remove kelp. My research suggests that new approaches are needed to understand the link between algal community change and ecological drivers, particularly as algal communities are shifting in the Anthropocene. Further, as disturbances are predicted to increase in frequency and intensity with global change, my research implies that invasive species such as S. horneri will persist if the native community is disproportionately impacted