Small herbivores suppress algal accumulation on Agatti atoll, Indian Ocean
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Despite large herbivorous fish being generally accepted as the main group responsible for preventing algal accumulation on coral reefs, few studies have experimentally examined the relative importance of herbivore size on algal communities. This study used exclusion cages with two different mesh sizes (1 × 1 cm and 6 × 6 cm) to investigate the impact of different-sized herbivores on algal accumulation rates on the shallow (<2 m) back-reef of Agatti atoll, Lakshadweep. The fine-mesh cages excluded all visible herbivores, which had rapid and lasting effects on the benthic communities, and, after 127 d of deployment, there was a visible and significant increase in algae (mainly macroalgae) with algal volume being 13 times greater than in adjacent open areas. The coarse-mesh cages excluded larger fishes (>8 cm body depth) while allowing smaller fishes to access the plots. In contrast to the conclusions of most previous studies, the exclusion of large herbivores had no significant effect on the accumulation of benthic algae and the amount of algae present within the coarse-mesh cages was relatively consistent throughout the experimental period (around 50 % coverage and 1–2 mm height). The difference in algal accumulation between the fine-mesh and coarse-mesh cages appears to be related to the actions of small individuals from 12 herbivorous fish species (0.17 ind. m−2 and 7.7 g m−2) that were able to enter through the coarse mesh. Although restricted to a single habitat, these results suggest that when present in sufficient densities and diversity, small herbivorous fishes can prevent the accumulation of algal biomass on coral reefs.
KeywordsEcological redundancy Diet specialization Macroalgae Phase shifts Size-dependent processes
This work was supported by the Czech Science Foundation 526/09/H025. Necessary permissions to reside and carry out research on the islands of Lakshadweep were provided by the Bombay Natural History Society and the Department of Environment and Forests, Lakshadweep. Thanks to M. K. M. Hussain, M. Noushad, Salahudheen V. K., Sakeer Hussain, P. Sahib, and K. Sahib for valuable assistance in the field.
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