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.
Highlights
Coral reefs are in global decline due to chronic anthropogenic disturbances (Pandolfi et al, 2003; Bellwood et al, 2004; Hoegh-Guldberg et al, 2007; Hughes et al, 2018)
We described the state of the six biological variables: corals, juvenile corals, herbivorous fishes, sea urchins, herbivorous gastropods, and macroalgae recorded along the three Taiwanese coral reefs
The present study demonstrated that diadematid density was the primary positive driver of juvenile coral density, suggesting the important role of diadematids for coral resilience on the Taiwanese coral reefs
Summary
Coral reefs are in global decline due to chronic anthropogenic disturbances (Pandolfi et al, 2003; Bellwood et al, 2004; Hoegh-Guldberg et al, 2007; Hughes et al, 2018). This is because coral recovery is typically initiated by recruits (Hughes et al, 2007b; Gilmour et al, 2013; Graham et al, 2015), and a positive correlation between juvenile coral density and recovery of coral populations has often been observed (Gilmour et al, 2013; Dajka et al, 2019; Nozawa et al, 2020) This new approach to evaluate coral resilience based on juvenile coral density significantly shortens study time and allows scientists to explore factors influencing coral resilience more efficiently
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