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Marine Ecology Progress Series

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MEPS 507:197-206 (2014)  -  DOI:

Omnivory dampens trophic cascades in estuarine communities

Keith D. Johnson1,*, Jonathan H. Grabowski2, Delbert L. Smee3

1Department of Biological Sciences, Stevenson University, 1525 Greenspring Valley Road, Stevenson, MD 21153, USA
2Marine Science Center, Northeastern University, 430 Nahant Road, Nahant, MA 01908, USA
3Department of Life Sciences, Texas A&M University - Corpus Christi, 6300 Ocean Dr., Corpus Christi, TX 78412, USA
*Corresponding author:

ABSTRACT: Predators often have large effects on prey populations and entire communities, but the nature of top-down control in speciose food webs is often difficult to assess due to consumer interactions such as complementarity, predator interference, and omnivory. To assess how top and intermediate predators affect community structure, we used predator exclusion experiments designed to isolate predator effects at different trophic levels in a species-rich community. Using oyster Crassostrea virginica reefs as a model system, predators were excluded using mesh-covered cages with openings of 1.0 cm2 (small mesh) to exclude all potential oyster predators, 5.0 cm2 (large mesh) to allow intermediate consumers but not large predators to access oyster reefs, and controls with complete predator access. Natural oyster settlement and survival were greatest in the small mesh cage when oysters were completely protected from consumers. Meanwhile, abundances of adult Panopeus herbstii, the Atlantic mud crab, an intermediate consumer and known oyster predator, were an order of magnitude greater in the large-mesh cage than in the other treatments. Oyster survival did not differ between the large-mesh cage and controls, suggesting that top predators such as blue crabs Callinectes sapidus and sheepshead Archosargus probatocephalus are likely consuming mud crabs (or inducing mud crabs to avoid controls) and consuming juvenile oysters on these oyster reefs. Top-down processes are known to be important on oyster reefs, but in this system, omnivory can overwhelm density-mediated cascading effects of top predators. We also found mud crabs to induce oysters to grow heavier shells and less soft tissue. Efforts to model the direct and indirect effects of predators on ecological communities will benefit from a more thorough understanding of the relative strength of omnivory on cascading effects of top predators.

KEY WORDS: Crassostrea virginica · Indirect effects · Lethal and non-lethal effects · Mud crabs · Predator–prey interactions · Food web · Top-down

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Cite this article as: Johnson KD, Grabowski JH, Smee DL (2014) Omnivory dampens trophic cascades in estuarine communities. Mar Ecol Prog Ser 507:197-206.

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