MEPS 333:37-50 (2007)  -  doi:10.3354/meps333037

Food web interactions along seagrass–coral reef boundaries: effects of piscivore reductions on cross-habitat energy exchange

John F. Valentine1,2,*, Kenneth L. Heck Jr.1,2, Derrick Blackmon1,2, Margene E. Goecker1,2, Juliet Christian1,2, Ryan M. Kroutil1,2, Kevin D. Kirsch3, Bradley J. Peterson4, Mike Beck5, Mathew A. Vanderklift6,7

1Dauphin Island Sea Laboratory, University of South Alabama, 101 Bienville Boulevard, Dauphin Island, Alabama 36528-0369, USA
2Department of Marine Science, University of South Alabama, Mobile, Alabama 36688, USA
3NOAA Damage Assessment Center, PO Box 500368, Marathon, Florida 33050, USA
4Marine Science Division, Southampton College, Southampton, New York 11968-4198, USA
5The Nature Conservancy, 100 Shaffer Road, University of California, Santa Cruz, California 95060, USA
6CSIRO Marine Research, Underwood Avenue, Floreat, Western Australia, Australia
7Centre for Ecosystem Management, Edith Cowan University, 100 Joondalup Drive, Joondalup, Western Australia, Australia

ABSTRACT: Although early studies qualitatively documented the importance of cross-habitat energy transfers from seagrasses to coral reefs, such exchanges have yet to be quantified. Empirical evidence suggests that grazing by reef-associated herbivores along the coral reef–seagrass interface can be intense (e.g. conspicuous presence of bare-sand ‘halos’ surrounding coral reefs). This evidence must be interpreted with caution, however, as most of it comes from areas that have experienced sustained, intense overfishing. To quantify the impacts of piscivore removal on cross-habitat energy exchange at the coral reef–seagrass interface, we compared grazing intensity along fished and no-take reefs in the upper and lower Florida Keys. Using visual census techniques and direct measures of seagrass grazing, we documented the impacts of piscivore density on herbivory along the seagrass–coral reef interface. Grazing rates were greater than observed seagrass (Thalassia testudinum) production near reefs in the upper Keys, but less than 48% of production in the lower Keys. Analyses showed that these large differences were not related to regional differences in either herbivore density or species composition. Seagrass biomass was also lower near reefs in the upper Keys, where estimates of grazing were highest. Piscivores were dominated by transient predators (jacks and barracudas) whose densities varied with region and distance from reef, but not with protection from fishing. A nonsignificant negative correlation between great barracuda density and leaf losses from tethered shoots, coupled with the greater abundances of larger grazers near reefs, suggests that predation risk, rather than direct reductions in density, may limit grazers to intense feeding on seagrasses adjacent to reefs in the upper Keys. The large-scale variation in grazing intensity illustrates the need for more detailed quantifications of energy exchanges along the seagrass–coral reef boundary.

KEY WORDS: Edge effects · Herbivory · Overfishing · Parrotfish · Spatial subsidies · Trophic transfer

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