Estimating sea urchin bioerosion: global synthesis and a case study inside and outside of no-take MPAs around O‘ahu, Hawai‘i

Estimating sea urchins’ contribution to reef erosion—and how it is shaped by local interventions—is central to understanding the balance of accretion and erosion that supports coral reef persistence in a warming, acidifying ocean. Here, we paired a literature review with a field study on O‘ahu, Hawai‘i to (1) summarize the spatiotemporal distribution of studies on urchin erosion compared to herbivory, (2) examine the role of no-take marine protected areas (MPAs) relative to environmental drivers of urchin community composition around O‘ahu, (3) assess how this then affects urchin bioerosion, gross bioerosion, and net accretion on O‘ahu. Our literature review revealed that urchins have been widely studied but the annual number of studies examining bioerosion has remained relatively flat over the past 35 years, especially compared to the increasing number of studies on herbivory. O‘ahu urchin community composition was affected by wave energy, depth, and coral cover, but not MPAs or the biomass of urchin competitors and predators. These differences drove patterns in gross bioerosion, and despite MPAs not affecting urchin community composition, bioerosion was higher outside of three of four O‘ahu MPAs, driven largely by urchin contributions to gross bioerosion. Estimated net accretion was low around O‘ahu, and was only consistently positive in Kāne‘ohe Bay, where urchins were scarce. Our results suggest that urchin community composition can affect carbonate budgets on O‘ahu and highlight the need for more empirical study on the effects of urchin bioerosion on reef structure in the face of continued environmental change.