Non-consumptive effects (NCEs) of predators on their prey can be an important influence on ecosystems because predators can suppress the ecological roles of far more prey than they can consume. However, responses to predatory risk cues can differ among species with similar ecological functions. We tested for NCEs on grazing and movement behaviors of 2 species of sea urchins that have the potential to affect coral-algal interactions on Caribbean coral reefs: the small-bodied reef urchin Echinometra viridis and the larger, longer-spined sea urchin Diadema antillarum. We found that cues from a generalist predator, the Caribbean spiny lobster Panulirus argus, strongly suppressed grazing by D. antillarum but not E. viridis. Conversely, cues from simulated predation, created by crushing conspecific urchins, caused reduced grazing by E. viridis but not D. antillarum. In field tests for NCEs on movement behavior, E. viridis consistently moved away from lobsters on coral colonies of a variety of structural complexity levels, but movement rates were reduced in response to lobster cues only when on highly rugose corals. D. antillarum movement was not affected by the presence of lobsters. The contrasting responses exhibited by these 2 urchins suggest that prey respond in unexpected ways to changes in predators and habitat complexity. Different foraging strategies and the degree to which each species recognizes this predator as a potential threat appear to be the primary influences observed here. Understanding the non-consumptive effects of predators on invertebrate reef herbivores is vital because of their important roles as bioeroders and grazers on Caribbean coral reefs.
