Tidal regime dictates the cascading consumptive and nonconsumptive effects of multiple predators on salt marshes

Background/Question/Methods

Prey perception of predators can dictate how prey balance the need to avoid being eaten with the need to consume resources, and this perception and consequent behavior can be strongly influenced by physical processes.  Physical factors, however, can also alter the diversity of predators that pursue prey.  Thus, it remains uncertain to what extent variable risk perception and anti-predator traits (i.e., nonconsumptive effects, NCE) versus variation in predator consumption of prey (i.e., consumptive effects, CE) underlie prey-resource dynamics in natural systems.  Here, I focused on a three-level food web in the Gulf of Mexico that resembles one in southeast Atlantic marshes where blue crabs (Calllinectes sapidus) indirectly maintain smooth cordgrass by consumptively preventing high densities of grazing snails. The following hypotheses were tested: (1) Along a 400 km tidal-inundation gradient, tidal submergence modulates how multiple predators affect the survivorship (CE), behavior (NCE), and shell morphology (NCE) of snails; (2) Tidal-induced variation in predatory CEs and NCEs on grazing snails cascades to cordgrass so that the nature and relative importance of indirect predator effects change predictably with tidal regime; (3) Shoreline variation in snail grazing and cordgrass productivity can be explained by the effects of tidal regime on snail anti-predator behavior.

Results/Conclusions

In an experimental foodweb where tide controls which predators access marsh snails (prey), multiple predators (crab and/or conch) influenced the survivorship and anti-predator behavior of snails irrespective of whether tidal inundation occurred on a diurnal or mixed semidiurnal schedule.  In contrast to these tidally consistent direct CEs and NCEs, indirect predator effects differed with tidal regime: snail grazing of marsh leaves in the presence of predators increased in the diurnal tide but decreased in the mixed semidiurnal, overwhelming the differences in snail density that resulted from direct predation.  Patterns from natural marshes spanning a tidal inundation gradient (from diurnal to mixed semidiurnal tides) across 400 km of coastline were consistent with experimental results: despite minimal spatial variation in predator density, snail density and morphology, abiotic stressors, and marsh productivity, snail grazing on marsh plants increased and plant biomass decreased on shorelines exposed to a diurnal tide.  Because both the field and experimental results can be explained by tidal-induced variation in risk perception and snail behavior rather than by changes in snail density or morphology, this study reinforces the importance of behaviorally mediated indirect effects of predators in complex natural systems and at large spatial scales.