Herbivory is an important structuring factor in coral reefs, influencing seaweed abundance, competitive interactions between seaweeds and corals, and coral reef resilience. Despite reports of a drastic increase in the cover of benthic algae and turf dominancy in the coral reefs of Eilat, Red Sea, very little is known about the factors responsible for this phenomenon or the possible effects of herbivory on turf algae and coral recruits. Here, we examine the effects of herbivory by experimentally exposing turf algae and coral recruits to grazing activities of herbivorous fish and sea urchins. Using remote video cameras to document removal of algae and coral spats, we show that the main grazing impact is due to daily grazing by fishes, whereas the significant impact of sea urchins is mainly expressed in their adverse effect on the survival of coral recruits, with a relatively low effect on algal biomass. These findings contribute to our understanding of the factors influencing turf algae establishment and proliferation, and the survival of coral recruits on the coral reefs of Eilat. The clear differences between the impact of herbivorous fish and that of sea urchins, on the Eilat reefs, have critical implications for reef resilience and restoration measures. 1. Introduction The escalating environmental threats and the concern for the future existence of the world’s coral reefs have led to increased studies and speculations on coral reef resilience. Resilience has been defined, among others, as the ability of the reefs to absorb recurrent disturbances and subsequently rebuild coral-dominated systems . Resilience is critically dependent on maintaining a balance among the different reef dwellers or functional groups (e.g., algae, corals ). Changes in this balance can lead to a phase shift, that is, an alternative assemblage, typically characterized by algal takeover and dominancy . The takeover of coral reefs by algal turf is a process that has significant ecological implications . Algal communities, dominated by highly productive, small filamentous algal turf, can hinder coral settlement and overgrow coral recruits, thus contributing to the demise of a coral population . The factors determining the relative abundance of either corals or algae on coral reefs are most often an outcome of the complex interactions between environmental factors (bottom-up controls such as nutrient levels) and biological factors (top-down controls such as grazing [5–7]). Herbivory, the removal of plant biomass, is one of the most important structuring factors of
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