Emergent effects of two rocky intertidal predators on prey: Interaction of crabs and sea-stars may reduce mussel mortality

Abstract

Two predator – one prey systems are widespread in nature, and have been important to both ecological theory and practice. A primary question for these systems is whether the two predators produce “emergent effects”: greater or less prey mortality than expected based on the two predators acting alone. We conducted an in-lab experiment on a two predator (the crab Cancer productus and the sea-star Evasterias troschelii), one prey (the bivalve Mytilus trossulus) system common to the marine rocky intertidal of the northeastern Pacific. In 156 mesocosm experimental trials, mortality rates of M. trossulus were compared between Crabs only, Sea-stars only, and a Mixed treatment with both predators present. The experiment used a substitutive design, i.e., predator density was held constant. Based on a Linear Mixed Effects Model, the interactive effects of the number of predator species and water temperature had the greatest effect on mussel mortality. For the observed temperature range, the two predators in the Mixed treatment killed fewer mussels than expected based on the average mussel mortality of the one-predator treatments. As the two predators were never observed to interfere with each other directly, the reduced prey mortality is possibly a trait-mediated indirect interaction, involving behavioral modification (mutual avoidance) on the part of the predators. Temperature may mediate these results; crabs consumed more mussels at higher temperatures while sea-stars did so at lower temperatures. As these experiments were conducted over a large part of the known temperature range for this region, these results suggest that mussels may experience seasonal shifts in both predation rates and the identity of the predator, which should be investigated in future field studies. Increasingly warmer temperatures anticipated in the coming century could increase mussel mortality due to predation by cancrid crabs, which appear to have temperature responsive feeding rates. Understanding this two-predator one-prey system better is particularly important given that mussels are important foundational species, and recent climate change has affected abundances of the taxa involved.