Top-down effects on intertidal mussel populations: assessing two predator guilds in a South African marine protected area

Mussels are important ecological engineers and occupiers of intertidal space in many parts of the world, where their populations are shaped by top-down regulation through predation on adults. In South Africa, adult predation is believed to be low, with mussel populations limited by competition or bottom-up supply of recruits. We tested the hypothesis that predation shapes these populations at the recruit rather than the adult stage by measuring predation on recruits at 2 rocky shores 100 m apart in a marine reserve, with no-take regulations. To explore the role of different guilds of predators, we used 7 treatments (fences, roofs, cages plus appropriate controls) to discriminate between the effects of benthic and pelagic predators. Factorial ANOVA showed that both types of predation had strong effects. Juvenile mussels were quickly removed in treatments with a roof (which excluded pelagic predators), a fence (which excluded benthic predators), and unprotected controls exposed to all predators. Treatments that excluded only one guild of predators were significantly different from the full cage, but not different from each other or the fully exposed control treatment. Thus, benthic and pelagic predators had similar effects that were not significantly different, suggesting that interference may occur between predator guilds. Under experimental conditions, the combined pressure of benthic and pelagic predators resulted in high mortality that may have top-down effects on mussel populations through recruit rather than adult predation.