Tidal regime dictates the cascading effects of multiple predators
on a marsh plant in the NW Gulf of Mexico
When prey detect a predator, the prey must behaviorally balance the need to avoid being eaten with the need to consume resources. Differences in the environment, which may influence the way that prey perceive predators, may result in differences in this behavioral response. In this study, I was interested in how physical processes influenced prey perception of predators, the subsequent behavior of prey, and consequent effects of this behavior on a basal resource.
In Spartina alterniflora systems in the Southeast, snails feed on dead plant material (detritus) on the ground, as well as on fungus growing on Spartina leaves. The snails farm the fungus by slicing open the leaves, which are then colonized by a fungal infection. If this "fungal farming" becomes too intense, the plants may become stressed and die.
The snails will climb up Spartina plants at high tide in order to avoid predators such as blue crabs and crown conchs. Given that tides dictate how often and how long predators are present in the marshes, I wondered whether geographic differences in tidal regime (diurnal vs. semidiurnal) could result in differences in snail climbing behavior, the intensity of fungal farming, and plant damage.
On the panhandle of Florida, coastlines to the west experience diurnal tides (12 hours flood and 12 hours ebb each day), whereas coastlines to the east experience semidiurnal mixed tides (2 low tides and 2 high tides that are each 6 hours). In the field, I found that Spartina was less productive and had more snail-farming scars along shorelines with diurnal tides than along shorelines with semidiurnal tides. This pattern occurred despite there being equal numbers of snails and predators along both shorelines, so it was clear that densities or consumptive effects were not driving this pattern.
I next conducted a lab experiment crossing tidal regime with several predator treatments. These treatments allowed me to distunguish between the consumptive and non-consumptive effects of two different predators on snails. I found that:
(1) Predators caused snails to ascend Spartina regardless of tidal regime and predator identity.
(2) Regardless of tidal regime, blue crabs consumed more snails than crown conchs, and the highest snail consumption was seen when both predators were present. In other words, consumptive effects were consistent across tidal schedules.
(3) In the presence of predators, snail grazing on Spartina was greater under the diurnal tidal schedule than under the semidiurnal tidal schedule. In fact, the snails subject to semidurnal tides had no net effect on Spartina.
The tidal schedule therefore dictated the strength of non-consumptive predator effects, and whether predator cues indirectly benefitted or harmed Spartina through their effects on snail predator-avoidance and farming behavior. These results match the patterns observed in nature. We can see how the same assemblage of predator and prey can function differently in slightly different environments due to effects on prey behavior.