Long-term monitoring reveals the foundational roles of three rockweed species on understory community structure and stability

Erica Nielsen^1*Walter N Heady¹Thomas Lamy²Peter T Raimondi³Mark Reynolds¹

• ¹The Nature Conservancy, California, Sacramento, CA, United States
• ²UMR MARBEC, University of Montpellier, CNRS, Ifremer, IRD, France, Montpellier, France
• ³Ecology and Evolution, University of California Santa Cruz, Santa Cruz, United States

A group of brown algae, known as rockweeds, influence rocky shore ecosystems as ‘foundation species’ by increasing habitat complexity and ameliorating environmental stressors for other species. Rocky shore habitats shaped by foundational canopy-forming rockweed species are threatened by human disturbance and climate change impacts. Thus, it is of pressing conservation concern to understand how changes in rockweed densities lead to shifts in associated community structure. Here, we tested the hypothesis that higher cover of rockweed canopy correlates with higher cover, richness, and stability of understory communities, using the unique study system of California’s coast, where several species of rockweed co-occur within the mid-to-high intertidal and are experiencing severe declines. With restoration approaches being tested across taxa, we compared the role of three dominant Californian rockweed species (Silvetia compressa, Fucus distichus, Pelvetiopsis limitata) on understory composition and stability. We analyzed eleven-year time-series data across 37 sites throughout California, with plots consistently sampling both benthic and canopy communities. We found a positive impact of rockweed cover on total benthic community cover for all three rockweed taxa. There was also a positive impact of rockweed cover on benthic richness, diversity, and stability, although this relationship was only significant for certain rockweed species. All three rockweed taxa showed a positive association with non-coralline crusts, otherwise there were species-specific positive associations with benthic taxa such as limpets, barnacles, and red turf algae. Our findings provide evidence of Californian rockweed species playing a foundational role in the rocky intertidal as their canopy changes benthic community structure. The positive impact of rockweed cover was most pronounced in S. compressa and P. limitata, which suggests that these species offer greater protection against physical stresses as they exist at higher tidal elevations and lower latitudes where desiccation stress may be stronger. As restoration efforts continue to aid declining Californian rockweed populations, these findings can inform conservation management to target sites and species to have the largest benefits to ecosystems and coastal resilience.