Environmental forcing and predator consumption outweigh the nonconsumptive effects of multiple predators on oyster reefs.

Abstract

The ability to predict how predators structure ecosystems has been shown to depend on identifying both consumptive effects (CEs) and nonconsumptive effects (NCEs) of predators on prey fitness. Prey populations may also be affected by interactions between multiple predators across life stages of the prey and by environmental factors such as disturbance. However, the intersection of these multiple drivers of prey dynamics has yet to be empirically evaluated. We addressed this knowledge gap using eastern oysters (Crassostrea virginica), a species known to suffer NCEs, as the focal prey. Over 4 months, we manipulated orthogonally the life stage (none, juvenile, adult, or both) at which oysters experienced simulated predation (CE) and exposure to olfactory cues of a juvenile oyster predator (crab), adult predator (conch), sequentially the crab and then the conch, or none. We replicated this experiment at three sites along an environmental gradient in a Florida (USA) estuary. For both juvenile and adult oysters, survival was reduced solely by CEs, and variation in growth was best explained by among-site variation in water flow, with a much smaller and negative effect of predator cue. Adults exposed to conch cue exhibited reduced growth (an NCE), but this effect was outweighed by a positive CE on growth: Surviving oysters grew faster at lower densities. Finally, conch cue reduced larval settlement (another NCE), but this was swamped by among-site variation in larval supply. This research highlights how strong environmental gradients and predator CEs may outweigh the influence of NCEs, even in prey known to respond to predator cues. These findings serve as a cautionary tale for the importance of evaluating NCE processes over temporal scales and across environmental gradients relevant to prey demography.