Halos of herbivory, feeding-preference, and predation risk on contemporary Belizean reefs

Abstract Biotic interactions in marine ecosystems can create visible patterns on the benthos. An archetypal example of such a pattern is the conspicuous ‘halos’ (i.e., areas largely cleared of macrophytes) that surround many tropical reefs. While early ecological studies identified the role of herbivores and the predators that feed on them in the formation of halos on Caribbean reefs, recent evidence has highlighted the context-dependent nature of these biotic interactions. Widespread ecosystem changes and associated alterations in food webs suggest potential changes in the context for Caribbean reefs since those early studies. However, the extent to which different herbivores and predators contribute to halo formation and/or maintenance on contemporary Caribbean reefs remains unclear. We quantified herbivory on five macrophyte species across adjacent reef, halo, and seagrass zones, in a manner that allowed us to partially partition herbivory between sea urchins and fishes. Furthermore, we directly tethered the urchin Diadema antillarum on reefs and in halos to quantify predation risk for this key herbivore. The removal of macrophyte assays was high on coral reefs and in halos when compared to seagrass beds, with macrophyte selection by herbivores conserved across zones. Fishes, rather than urchins, were the major herbivores in halos. Moreover, we documented higher predation on urchins in halos compared to reefs, revealing that predation may still shape diurnal urchin distribution in exposed habitats. Despite substantial ecological changes on Caribbean reefs since early studies, halo formation/maintenance by selective fish feeding activity is an enduring feature in the functioning of contemporary Belizean seascapes.