MEPS 456:139-148 (2012)  -  DOI:

Density-dependent indirect effects: apparent mutualism and apparent competition coexist in a two-prey system

W. Christopher Long1,2,*, Emily F. Gamelin1,3, Eric G. Johnson1, Anson H. Hines1

1Smithsonian Environmental Research Center, 617 Contees Wharf Road, Edgewater, Maryland 21037, USA
2Present address: Kodiak Laboratory, Resource Assessment and Conservation Engineering Division, Alaska Fisheries Science Center, National Marine Fisheries Service, NOAA, 301 Research Ct., Kodiak, Alaska 99615, USA
3Present address: National Oceanic and Atmospheric Administration, Office of Oceanic and Atmospheric Research (OAR),
1315 East-West Highway, Silver Spring, Maryland 20910, USA

ABSTRACT: A predator consuming multiple prey species usually causes indirect effects. Apparent mutualism results when multiple prey species reduce predation risk for each other by altering a predator’s functional response. Short-term apparent competition occurs when multiple prey species increase predation risks for each other through the numerical response, i.e. increasing the predator’s birth rate, or aggregative response, i.e. attracting a higher density of predators. Our objectives in this study were to determine the aggregative response and 2-prey functional response of a predator and to examine indirect effects over a range of prey densities. We used the clam Macoma balthica and juvenile blue crabs Callinectes sapidus as prey for adult blue crabs. In laboratory experiments, we determined the single-prey functional responses of the crabs to each prey species and the 2-prey functional response. We combined the 2-prey functional response with the known blue crab aggregative response to clams to estimate field predation rates. Our model predicts that at low prey densities, clams and juvenile blue crabs exhibit apparent mutualism, whereas at high clam densities, this relation switches to short-term apparent competition. These unexpected results highlight the need to incorporate multiple aspects of predation at multiple scales when considering indirect effects.

KEY WORDS: Predator–prey interactions · Functional response · Indirect effects · Blue crab · Macoma balthica

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Cite this article as: Long WC, Gamelin EF, Johnson EG, Hines AH (2012) Density-dependent indirect effects: apparent mutualism and apparent competition coexist in a two-prey system. Mar Ecol Prog Ser 456:139-148.

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