Ecological consequences of ontogenetic shifts in predator diet: Seasonal constraint of a behaviorally mediated indirect interaction

Predators play an important role in structuring assemblages through direct and cascading indirect effects. While there has been recent interest in how the strength and direction of trophic cascades vary spatially, seasonal variability in trophic links is seldom considered. In North Carolina, recruitment-failure of bay scallops typically occurs following the spring but not the fall spawning despite the presence in each of these seasons of predatory blue crabs. One explanation for this pattern is that in the fall, seasonally abundant predators of blue crabs reduce the foraging efficiency of crabs on scallops and thus the overall magnitude of top-down effects. Quantification of bay scallop consumption by blue crabs in closed mesocosms with or without pinfish supported the hypothesis that seasonally abundant adult pinfish indirectly increase survivorship of bay scallop recruits in fall by reducing predation by blue crabs. Despite voracious consumption of bay scallops during both the day and night in mesocosms to which only small blue crabs were added, blue crabs in mesocosms with visually-foraging adult pinfish consumed bay scallops only by night. Juvenile pinfish that dominate estuarine populations in spring did not impede consumption of bay scallops by blue crabs. In mesocosms from which animals could not emigrate, the addition of neither adult nor juvenile pinfish increased the mortality of blue crabs, indicating a behaviorally mediated interaction. Blue crabs restricted by adult pinfish to nocturnal feeding did not compensate for lost feeding time by increasing their night-time consumption of bay scallops. These results strongly suggest that greater survivorship of bay scallops in fall than spring is due to adult pinfish, potential predators of small blue crabs, restricting blue crab foraging to hours of dark. In spring, when pinfish are small and incapable of consuming blue crabs, blue crabs consume bay scallops by day and by night. Such seasonal variation in the number of trophic links in a system may have important evolutionary implications. By timing reproduction to occur in fall when the pinfish-crab-scallop cascade is in operation, bay scallops maximize recruitment.