Species invasions are a leading cause of biodiversity loss worldwide, and invasions into critical marine ecosystems have accelerated over the last decade. Marine invasions into kelp forests are of particular concern as kelp is a foundation species providing many ecosystem functions and services. Sargassum horneri is a marine macroalga that invaded Californian coastal habitats, including kelp (Macrocystis pyrifera) forests, in 2003 and rapidly spread throughout the region. Despite the widespread establishment of S. horneri, especially during El Niño disturbance conditions that are unfavorable for M. pyrifera, little is known about mechanisms facilitating invasion success nor its impact on native species. We evaluated if herbivory, species interactions (S. horneri vs. M. pyrifera) and disturbance influence growth and survivorship of both species on subtidal reefs. To assess herbivory as an invasion mechanism, small (<10cm) stages of both species were caged or open to herbivory in the field. To evaluate if species interactions and disturbance enhance invasion success or recolonization by the native, we transplanted 3 sizes of S. horneri and 1 size (<10cm) M. pyrifera into 3 sites: 1) S. horneri dominated, 2) M. pyrifera dominated, and 3) lacking algal canopy (i.e. “recently disturbed”). We monitored light and growth at each site.
Results/Conclusions
Results of the herbivory experiment indicated that while both S. horneri and M. pyrifera were consumed, less of the invasive was eaten. Further, S. horneri grew faster than M. pyrifera when caged. All S. horneri sizes grew fastest in the bare, “recently disturbed” site, and slower in algal-dominated sites. Further, out of the 3 S. horneri size classes, small S. horneri grew slowest. In contrast, all small M. pyrifera lost biomass, regardless of whether the site was algal-dominated or lacking canopy, suggesting reestablishment is limited at the smallest stage of M. pyrifera. Further, light levels were low in algal-dominated sites and high in the bare, “disturbed” site. Taken together, our results suggest colonization into established beds of S. horneri or M. pyrifera by the opposite species is unlikely due to competition for light. Further, they imply that declines in M. pyrifera due to disturbance can facilitate the spread of S. horneri via competitive release. Finally, competition with S. horneri and herbivore preference for M. pyrifera may inhibit recovery of the native foundation species following disturbance events such as El Niño, with severe implications for the multitude of ecosystem services and nearshore species dependent on the foundation species, M. pyrifera.