Sea Urchins Play an Increasingly Important Role for Coral Resilience Across Reefs in Taiwan

Abstract

Herbivores are an important functional group that control algae, create new space, and promote recruitment for coral recovery. However, on many coral reefs, overfishing has greatly decreased the density of herbivores, especially fishes and gastropods, impairing coral resilience. On such overfished reefs, remnant herbivores that are not target species of local fisheries, e.g., sea urchins, are expected to play an increasingly important role, yet few studies, except for those in the Caribbean and Kenya have examined non-fish herbivores in relation to coral resilience. Here, we conducted field surveys at 30 sites along three coral reefs in Taiwan between 2016 and 2017, to examine the relative importance of six key factors for coral resilience: herbivore abundance (fishes, gastropods, sea urchins), coral cover, macroalgal cover, habitat complexity, water depth, and wave exposure. The density of juvenile coral was used as a proxy of coral resilience. Diadematid sea urchins (Echinothrix spp. and Diadema spp.) dominated most sites (19 of 30 sites) and multivariable regression models showed sea urchin density as the best positive predictor of coral juvenile density. The results elucidated the increasing role diadematid sea urchins play as remnant herbivores on overfished coral reefs in Taiwan. Given that overfishing is a widespread issue, this phenomenon may be occurring globally. More studies are needed to examine the role of remnant, but often ignored, sea urchin herbivory on coral resilience. Reef managers should consider monitoring locally remnant herbivores and incorporating them into management strategies.

Coral reef benthic community changes in the Anthropocene: Biogeographic heterogeneity, overlooked configurations, and methodology

“…Whilst these initiatives are very important and may enable protecting key functional species (e.g. species that might limit algal proliferation; Bellwood et al, 2006b; Burkepile & Hay, 2011; Dang et al, 2020; Kuempel & Altieri, 2017), there is still an extremely poor understanding on the mechanisms that drive and reinforce non‐algal alternative communities (González‐Rivero et al, 2012). Therefore, a better understanding of the drivers, reinforcing mechanisms and the persistence of non‐algal alternative communities is urgently required to design appropriate conservation approaches.…”

Post-bleaching alterations in coral reef communities

“…In moderate numbers, sea urchins also play a key functional role in maintaining levels of algal growth and can substitute for fish when these are reduced or absent (Bellwood et al, 2004;Graham et al, 2013). Their effective grazing has particularly been highlighted to facilitate coral recovery following disturbances, by promoting coral recruitment and reef resilience (Carpenter and Edmunds, 2006;Dang et al, 2020a;Dang et al, 2020b;Nozawa et al, 2020). Contrary to expectation, the majority of the investigated reefs, particularly those with the greatest coral mortality, showed a decline in sea urchin densities.…”

Pioneer assessment on megabenthic community suggest the recent ecological condition of coral reef in Senggigi Beach, Western Lombok Island, Indonesia

“…A population of sea urchin in an area could potentially reduce algae cover by 30 to 60%, facilitating the attachment of coral juveniles by opens up a living space for coral recruitments (Dang et al 2020a;Nozawa et al 2020). Although several studies reported that the dominance of sea urchins in waters has the potential to reduce the space for coral attachment through a bioeroder mechanism (Peyrot-Clausade et al 2000), the density of sea urchins in Senggigi Beach cannot yet be categorized as potentially damaging the settlement space for coral recruitment as the threatening aggregation of sea urchin supposed to be more than 16 ind•m -2 (Dang et al 2020b).…”