Testing For Indirect Effects of Predation in an Intertidal Soft‐Bottom Community

The killifish Fundulus heteroclitus and the grass shrimp Palaemonetes pugio are abundant predators in salt marshes along most of the United States Atlantic coast. Both species feed on benthic invertebrates, but large (>70 mm total length) killifish also prey on grass shrimp. As a test of the hypothesis that killifish have a positive indirect effect on some benthic invertebrates by controlling the abundance of grass shrimp, I maintained both predators together and separately in 5—m2 cages on the intertidal marsh surface for 10 wk. Predation by killifish significantly reduced the survival of grass shrimp in cages to which both predators were added, and the numerical responses of the dwarf mud anemone Nematostella vectensis provided unequivocal evidence for a positive indirect effect of killifish in this system. However, most benthic prey taxa were not significantly affected by the predator treatment. Either predation is generally unimportant in this community or another intermediate predator has a greater effect than P. pugio. Larval killifish and post—larval grass shrimp, which invaded the cages and became abundant in predator—exclusion treatments, may have confounded the predator effects.