Effects of grazing on benthic algal distribution in a coral reef lagoon ecosystem.

<p>A) Satellite image of algal halos within Heron Island lagoon. White lines indicate coral patch reefs; green lines indicate the approximate outer extent of bare sandy substrate (i.e., halos). B) <i>Siganus doliatus</i>, a small reef dwelling herbivore taking shelter in a tropical coral patch reef, at least partly as an anti-predator response. Although this particular species likely contributes little to halo formation beyond the reef due to its territorial ‘farming’ habits on the reef itself, the anti-predator sheltering behaviour it displays here is indicative of reef fishes in general. Predation risk is commonly cited as the ultimate mechanism leading to the spatially-constrained herbivore grazing patterns around coral patch reefs that lead to halo formation (photo: A. Harborne). C) Conceptual diagram illustrating how herbivore grazing patterns influence mean algal density and canopy height.