Predator-prey relationships are not limited to consumption; prey’s fear of predators is often equally important. Predators eat one animal at a time, but can frighten many animals simultaneously. Furthermore, both the lethal and non-lethal effects of predators can influence ecosystem-level processes. In estuaries of North Carolina, our previous research suggested that predators strongly influence oyster-reef dwelling species by both consuming and altering the behavior of smaller crabs that eat oysters and clams. Meanwhile, we have also previously shown that oyster reefs can alter properties of sedimentation and nutrient cycling by promoting denitrification in the sediments surrounding reefs. Given these independent but related findings, the primary goal of our study was to answer the following questions: do the indirect effects of predators on oyster reefs cascade to influence nutrient cycling via sediment organic matter? Do the influences of predators on the ecosystem services provided by oysters vary on a biogeographic scale?

We pursued these questions from 2010-14 using field and laboratory investigations. We found that oyster reef food webs differ dramatically throughout the southeastern United States. Important differences were that predatory biomass, trophic level number, and biomass of oyster reefs all peak in the central South Atlantic Bight (SAB).  Tidal forcing in the central SAB is high, which results in larger volume of water passing over reefs each tidal cycle, greater oceanic influence inside estuaries, and deeper channels for predators to access intertidal reefs, all of which affect food web structure. Food webs were held constant in an experiment throughout the SAB to assess the influence of geography on predators.  Predators had a stronger effect on oysters and their ecosystem services in NC and FL (areas with low oyster recruitment) and a weaker effect in GA and SC (areas with high oyster recruitment).  Importantly, the effects did cascade to alter nutrient cycling.

Oysters modify their environment in many ways; altering sediment properties through filtration is one important example.  Like many response variables in our reef vs. non-reef comparisons, the pattern of oyster modification of sediment organic matter (SOM) was not what we anticipated.  The central portion of our sampling range (SC, GA) did not reveal significant enhancement of SOM on oyster reefs relative to reference sediments, likely due in part to the tidal variation identified above.  Larger tidal range results in elevated suspended material supplies and may have overwhelmed the oyster’s enhancement of SOM.  Increases in ecosystem services such as nutrient removal that are associated with oyster reefs may not be as prevalent (or as valuable) in some coastal regions.

In addition to addressing our key research objectives, we also pursued a number of unexpected and interesting patterns. For example, our observation of the food web revealed that some fish make sounds. Furthermore, these sounds are detected by crab prey and induce them to reduce their feeding on oysters. Therefore, marine invertebrate prey can “hear,” and marine predators can influence community structure through their sounds. In Florida, we also used our refined methods to address losses of oyster reef habitat and the collapse of the oyster fishery in Apalachicola, which at one point provided 10% of the nation’s oyster harvest.

Products: To date, 17 publications have resulted from this work with multiple additional manuscripts are in progress.  Data from this project are publically available on the KNB Network: http://knb.ecoinformatics.org/index.jsp. In addition, our efforts were featured in a documentary enti...

Please report errors in award information by writing to: awardsearch@nsf.gov.