Variability in the Effects of Macroalgae on the Survival and Growth of Corals: The Consumer Connection

<div><p>Shifts in dominance from corals to macroalgae are occurring in many coral reefs worldwide. Macroalgal canopies, while competing for space with coral colonies, may also form a barrier to herbivorous and corallivorous fish, offering protection to corals. Thus, corals could either suffer from enhanced competition with canopy-forming and understorey macroalgae or benefit from predator exclusion. Here, we tested the hypothesis that the effects of the brown, canopy-forming macroalga, <i>Turbinaria ornata</i>, on the survival and growth of corals can vary according to its cover, to the presence or absence of herbivorous and corallivorous fish and to the morphological types of corals. Over a period of 66 days, two coral species differing in growth form, <i>Acropora pulchra</i> and <i>Porites rus</i>, were exposed to three different covers of <i>T. ornata</i> (absent <i>versus</i> medium <i>versus</i> high), in the presence or absence of fish. Irrespective of the cover of <i>T. ornata</i>, fish exclusion reduced mortality rates of <i>A. pulchra</i>. Following fish exclusion, a high cover of <i>T. ornata</i> depressed the growth of this branched coral, whilst it had no effect when fish species were present. <i>P. rus</i> suffered no damage from corallivorous fish, but its growth was decreased by high covers of <i>T. ornata</i>, irrespective of the presence or absence of fish. These results show that negative effects of <i>T. ornata</i> on some coral species are subordinate to those of fish predation and are, therefore, likely to manifest only on reefs severely depleted of predators. In contrast, space dominance by <i>T. ornata</i> may decrease the growth of other coral species regardless of predation intensity. In general, this study shows that susceptibility to predation may determine the severity of the effects of canopy-forming macroalgae on coral growth.</p></div>