UC Santa Barbara

Rocky intertidal ecosystems in California have been significantly degraded. Of particular concern is the precipitous population decline of rockweeds attributed to increased urbanization and accompanying impacts. As foundation species, rockweeds modify the physical environment of rocky intertidal habitats and increase biodiversity by decreasing environmental stress under their canopy. No substitute macrophyte species in southern California provides a comparable function to rockweeds in the intertidal zone. Recovery of impacted rockweed populations is typically slow and unpredictable, due to their poor dispersal capacity. To determine the status of the dominant rockweed species in southern California, Silvetia compressa, I analyzed canopy cover data from 30 sites spanning 18 years. I found that the rockweed had declined markedly at sites south of the major biogeographic break, Point Conception (PC), including the California Channel Islands and southern California mainland. Analyses using climatological data revealed that substantial declines in Silvetia cover in southern California were associated with strong, hot offshore Santa Ana wind events. To assess the recovery of disturbed rocky intertidal ecosystems, I analyzed 28 years of data from a controlled disturbance experiment at a remote site with limited human visitation. I found the recolonization rates for species cleared to be highly variable ranging from years to decades, and that even relatively mild disturbances from pedestrian traffic can have long-lasting effects on rockweeds. Long-lived species such as rockweeds and mussels took more time to recover from extreme disturbances than species with shorter lifespans such as barnacles and turfweed. Since rockweeds foundational to ecosystem functioning have sustained extensive degeneration and range contractions worldwide, and recovery of impacted rockweed populations is typically slow and unpredictable due to their limited dispersal capacity, I propose that rockweeds are optimal candidates for restoration. In my third chapter, I summarize the current knowledge of rockweed ecology, highlighting studies that could inform restoration practices, and recommend ways to improve our ability to implement scalable restoration of rockweeds and accompanying ecosystem-wide benefits.