Sea urchin mass mortality rapidly restores kelp forest communities

Abstract

Giant kelp Macrocystis pyrifera is a foundational species that forms a 3-dimensional habitat and supports numerous high-value fisheries species. Constant grazing of kelp holdfasts by overabundant sea urchins causes catastrophic ecological and economic impacts on rocky reefs worldwide. Overgrazing creates urchin barrens that persist for decades in the absence of ecological forcing that would shift the ecosystem back to a kelp-dominated state. Annual surveys of kelp forest and urchin barren sites in the Southern California Bight were performed from 2011 to 2020 to assess changes in kelp forest communities as a result of restoration efforts through sea urchin culling. However, that time period also encompassed a sea urchin mass mortality event. Following drastic reductions of sea urchin densities, rocky reefs returned to a kelp-dominated state within approximately 6 mo and remained stable through the remainder of the study. Benthic cover, fish, and kelp and macroinvertebrate communities inside former urchin barrens became more similar to that of kelp forest reference sites and continued to do so for the next 5 yr. Giant kelp density increased significantly compared to existing kelp forests, while benthic indicators of urchin dominance (i.e. crustose coralline algae and bare rock cover) decreased. Kelp restoration through sea urchin culling essentially mimics sea urchin mass mortality events. If culling can produce similar declines in urchin density, it may be a viable management tool to rapidly restore persistent urchin barrens at moderate spatial scales, while a mass mortality event can drive recovery of kelp forest communities at more extensive spatial scales.