Bioerosion by parrotfish plays a key role in coral reef ecosystems. Yet, precise bioerosion rate estimation and regional scale assessment of its distribution are lacking. In this study, the effect of fish body size on bioerosion rates was investigated through field observations. Furthermore, the distribution of bioerosion rates was quantified and predicted across New Caledonian reefs, combining available visual census data with environmental, anthropogenic and management variables. Additionally, the effectivity of MPAs was investigated and compared to both fished and pristine reefs and finally, the role of species richness as a proxy for functioning was questioned. This study showed that fish body size had a strong non-linear effect on bioerosion rates, which has implications for its vulnerability to fishing. Indeed, we found proximity to humans to be the main predictor of bioerosion, with functional extinction in highly impacted reefs. Large MPAs had a positive effect on bioerosion, while small MPAs provide little protection and both do not achieve pristine benchmark rates. Moreover, only pristine reefs sustained a size structure with a high proportion of large parrotfish. Generally, a strong mismatch between species richness and function was found, implying that conservation management should re-evaluate focus on protecting the indispensable large-sized parrotfish.
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