Life history and abiotic factors mediate intra- and interspecific interactions between the perennial giant kelp and an annual invasive alga

Emily Ryznar, Peggy Fong and James O. Lloyd-Smith, Ecology and Evolutionary Biology, University of California-Los Angeles, Los Angeles, CA, Emily Ryznar, Present address: CA Sea Grant Fellow-Delta Science Program, Sacramento, CA

Background/Question/Methods
Species invasions are a leading cause of biodiversity loss worldwide, and invasions into critical marine ecosystems have accelerated over the last decade. Invasions into kelp (Macrocystis pyrifera) forests are of particular concern as kelp is a foundation species providing many ecosystem functions and services. Sargassum horneri is an annual marine macroalga that invaded Californian coastal habitats, including perennial kelp forests, in 2003 and rapidly spread throughout the region. Despite the widespread establishment of S. horneri, little is known about factors influencing its population structure or how it may be interacting with native kelp. To address these gaps, we developed stage-structured population models for kelp and S. horneri driven by light availability and seasonal temperature, and then combined these models to evaluate their interactions. To assess drivers of S. horneri population structure, we compared model outputs with assumed light and temperature relationships to population dynamics observed in the field. To evaluate the role of intra- vs. interspecific interactions where these species overlap, we compared population changes in combined model outputs with individual model outputs. Finally, to evaluate if each species could “invade” mature stands of the opposite species, we simulated recruitment at different levels of interspecific competition and assessed population trajectories.
Results/Conclusions
Modeled relationships with temperature and intraspecific competition for light for S. horneri resulted in realistic predictions of field dynamics and rapid growth during low seasonal temperatures. Additionally, the population structure of both species was strongly influenced by intraspecific competition for light, with larger stages limiting recruitment. When species initially coexisted, kelp drove S. horneri to local extinction while S. horneri influenced both timing and intensity of kelp recruitment. Further, S. horneri was unable to invade mature kelp forests during peak recruitment months, but persisted longer when invasion preceded minima in large kelp stages. In contrast, kelp readily recolonized when large S. horneri stages were sparse and competition for light was low. Our results suggest that light, temperature, and intraspecific competition structure S. horneri populations as strongly as they do for kelp. Further, interactions between kelp and S. horneri are controlled by their different life histories, with kelp dominance and reestablishment facilitated by continuous reproduction and perennial persistence, and S. horneri invasion success and resistance to kelp reestablishment limited by seasonal reproduction and annual senescence that releases resources. Taken together, our results imply that invasion of S. horneri is dependent on disturbances that remove the dominant kelp.