Small herbivores suppress algal accumulation on Agatti atol
2015
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
(191 cm and 696 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.
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