Predation refuge values of marsh and mangrove vegetation for the marsh periwinkle Littoraria irrorata

Abstract

Black mangroves Avicennia germinans are becoming increasingly common in coastal wetlands in the Gulf of Mexico (USA). As mangroves displace salt marsh vegetation, there may be consequences for associated wetland fauna. In a series of field studies, we compared prey refuge values between marsh and mangrove vegetation for a vertically migrating gastropod, the marsh periwinkle Littoraria irrorata. Littoraria were tethered to marsh grasses (Spartina alterniflora) or the aerial roots (pneumatophores) of Avicennia in arrays that fully crossed vegetation type (Spartina vs. Avicennia), tether height (base vs. canopy), and wetland location (edge vs. interior marsh platform). After 1 d, acute predation rates were twice as high on Littoraria tethered to the base of Spartina stems than on those tethered to pneumatophores, suggesting that mangroves provided superior refuge from benthic predators like blue crabs. In the canopy, Spartina reduced acute predation rates by 75%, indicating that marsh grasses may provide superior refuge from aerial predators (possibly wetland birds). After 7 d, the effect of vegetation type diminished, but Littoraria mortality was 2 times higher on the benthos than in the canopy. Links between vegetation type and predation intensity on this important basal consumer may have broader consequences for trophic dynamics in coastal wetlands that are experiencing mangrove encroachment.