Grazer deterrence and fungal inhibition by the invasive marsh grass Phragmites australis and the native sedge Bolboschoenus robustus in a mesohaline marsh.
Tidal marsh habitats provide many important functions to coastal areas and are a valuable economic resource. Polyhaline marshes dominated by Spartina alterniflora can experience top-down control by the snail Littoraria irrorata who primarily impact Spartina through facilitating fungal growth on wounds they create. This grazing pressure may have selected for the production of chemical defenses in Spartina and other polyhaline marsh plants that deter snail feeding and fungal growth. Both Spartina and Littoraria can co-occur in mesohaline marshes that line the Chesapeake Bay, but little is known about interactions between this snail and plants in this habitat. Plant diversity, identity, and consumer abundance can differ between poly- and mesohaline marshes, and this may yield different patterns in the two marsh types. We investigated whether two abundant plants in a Chesapeake Bay mesohaline marsh of salinity ~ 13ppt deterred snail feeding and inhibited fungal growth. Through a bioassay-guided fractionation approach, we assessed palatability to snails and growth of the fungus Phaeosphaeria spartinae in response to chemical components in the invasive marsh grass Phragmites australis and the native sedge Bolboschoenus robustus. Both plants possessed chemicals that significantly deterred snail feeding compared to Spartina chemicals. In addition, both plants inhibited fungal growth, mediated by multiple metabolites. Snail density in this marsh was low (25 snails m-2), but may be enough to select for defenses in Bolboschoenus, or deterrent and inhibitory metabolites may be selected for by other consumers or factors. Chemical defenses in invasive Phragmites may contribute to its success in the Chesapeake Bay.