Evaluating the role of burrowing sea urchins as agents of coral reef bioerosion

Abstract As climate change alters reef functions, understanding factors influencing bioerosion is critical to assessing reef carbonate budgets. Census-based evaluations of reef accretion potential rely on accurate rates of carbonate production and bioerosion, but erosion rates for some locally abundant species are unavailable. Although burrowing sea urchins are abundant on many tropical reefs, little is known of what drives their distribution, abundance, and, consequently, erosion rates. Here we (i) quantify the bioerosion rates of Echinostrephus molaris , a common burrowing urchin in the Indian Ocean, and (ii) explore the factors (predation, substrate, structure, exposure, depth, urbanisation) determining sea urchin spatial distribution across 15 reefs in the Lakshadweep Islands. These bioerosion rates varied as a function of urchin size, with values ranging between 2.7g/yr for small and 4.7g/yr for large individuals, respectively. At the highest densities (> 20 urchins m − 2 ), urchin bioerosion represented as much as 100 g/m 2 /yr, compared to locations where densities were around 2 urchins m − 2 , and urchin bioerosion represented less than 10 g/m 2 /yr. In urbanised, degraded reefs with low structure, burrowing urchins could contribute to ~ 3.3% of major bioerosion. While this is a significant contributor to reef accretion budgets, it’s considerably lower than earlier estimates, based on non-burrowing urchin species. Urchins were more abundant in reefs dominated by dead, low-structure substrate, and in urbanised islands, with potentially higher nutrient input. Understanding patterns of urchin erosion and the factors affecting it helps in gauging the health of the reef, and consequently, the impact of human disturbance on reef carbonate budgets.