Abstract
Parrotfish provide important ecological functions on coral reefs, including the provision of new settlement space through grazing and the generation of sediment through bioerosion of reef substrate. Estimating these functions at an ecosystem level depends on accurately quantifying the functional impact of individuals, yet parrotfish feeding metrics are only available for a limited range of sites, species and size classes. We quantified bite rates, proportion of bites leaving scars and scar sizes in situ for the dominant excavator (Cetoscarus ocellatus, Chlorurus strongylocephalus, Ch. sordidus) and scraper species (Scarus rubroviolaceus, S. frenatus, S. niger, S. tricolor, S. scaber, S. psittacus) in the central Indian Ocean. This includes the first record of scar frequencies and sizes for the latter three species. Bite rates varied with species and life phase and decreased with body size. The proportion of bites leaving scars and scar sizes differed among species and increased with body size. Species-level allometric relationships between body size and each of these feeding metrics were used to parameterize annual individual grazing and bioerosion rates which increase non-linearly with body size. Large individuals of C. ocellatus, Ch. strongylocephalus and S. rubroviolaceus can graze 200–400 m2 and erode >500 kg of reef substrate annually. Smaller species graze 1–100 m2 yr−1 and erode 0.2–30 kg yr−1. We used these individual functional rates to quantify community grazing and bioerosion levels at 15 sites across the Maldives and the Chagos Archipelago. Although parrotfish density was 2.6 times higher on Maldivian reefs, average grazing (3.9 ± 1.4 m2 m−2 reef yr−1) and bioerosion levels (3.1 ± 1.2 kg m−2 reef yr−1) were about 15% lower than in the Chagos Archipelago (4.5 ± 2.3 and 3.7 ± 3.0, respectively), due to the dominance of small species and individuals in the Maldives (90% <30 cm length). This demonstrates that large-bodied species and individuals contribute disproportionally to both grazing and bioerosion. Across all sites, grazing increased by 66 ± 5 m2 ha−1 and bioerosion by 109 ± 9 kg ha−1 for every kg increase in parrotfish biomass. However, for a given level of parrotfish biomass, grazing and bioerosion levels were higher on Maldivian reefs than in the Chagos Archipelago. This suggests that small-bodied fish assemblages can maintain ecosystem functions, but only if key species are present in sufficiently high numbers.
Highlights
IntroductionParrotfishes (family Labridae, subfamily Scarinae) are an abundant, diverse and ecologically important group of coral reef fishes
Parrotfishes are an abundant, diverse and ecologically important group of coral reef fishes
The proportion of bites leaving scars (%scars) differed significantly between functional groups (F1,314 = 38.96, p < 0.001, Figure 2D) and among species (Figure 2C, Table S1), the latter mainly driven by differences between S. scaber and S. psittacus compared to large-bodied parrotfish
Summary
Parrotfishes (family Labridae, subfamily Scarinae) are an abundant, diverse and ecologically important group of coral reef fishes. Commonly characterized as herbivores, they feed on cyanobacteria and other autotrophic microorganisms on (epilithic) or within (endolithic) calcareous substratum by cropping, scraping or biting the reef surface with their beak-like jaws [1,2]. Through this feeding strategy, parrotfish provide critical ecosystem functions on coral reefs, including the provision of settlement space for corals and other benthic organisms by clearing benthic substrate, and acting as primary agents for bioerosion and sediment production (reviewed by [3]). Hoey and Bellwood [9] quantified cross-shelf variation in parrotfish densities and ecosystem functions on the Great Barrier Reef (GBR) and found that the differences in scraping, bioerosion and coral predation were driven by only a few species
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