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Marine Ecology Progress Series

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MEPS 411:149-159 (2010)  -  DOI:

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

Jeremiah G. Plass-Johnson*, Christopher D. McQuaid, Francesca Porri

Coastal Research Group, Zoology and Entomology Department, Rhodes University, P. O. Box 94, 6140 Grahamstown, South Africa

ABSTRACT: 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.

KEY WORDS: Intertidal · Mussels · Predator exclusion experiment · South Africa · Marine protected areas · MPA · Top-down regulation

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Cite this article as: Plass-Johnson JG, McQuaid CD, Porri F (2010) Top-down effects on intertidal mussel populations: assessing two predator guilds in a South African marine protected area. Mar Ecol Prog Ser 411:149-159.

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