Mangroves alter predator-prey interactions by enhancing prey refuge value in a mangrove-marsh ecotone

Black mangroves (Avicennia germinans) are becoming more common along the edges of their range, potentially displacing salt marsh vegetation in transition zones such as the Texas coast in the Gulf of Mexico (USA). Consequences of this shift on trophic interactions within wetland ecosystems are largely unknown. We investigated how increasing mangrove cover might alter predator-prey interactions involving an important consumer in coastal wetlands, blue crabs (Callinectes sapidus), and one of their key prey items, penaeid shrimp (Family Penaeidae). We conducted a series of mesocosm experiments to assess predation rates and prey refuge values in artificial vegetation (AV) matrices simulating flexible marsh grasses (e.g., Spartina patens, Distichlis spicata) and rigid mangrove pneumatophores (aerial root structures). In predation experiments within each of these AV matrices, blue crabs captured 50% fewer penaeid shrimp in the rigid mangrove treatment compared to the flexible marsh treatment, and time between captures was 83% longer among the simulated pneumatophores. In a prey refuge experiment that allowed both predator and prey to move freely among the AV matrices, penaeid shrimp occupied the rigid pneumatophore matrix 30% more often than the grass matrix when blue crabs were present but had no preference when the predator was absent. This study suggests that where A. germinans and marsh grasses co-occur, the mangroves may provide superior refuge for prey items such as penaeid shrimp. Pneumatophores will likely restrict the mobility of predators such as blue crabs, resulting in lower predation efficiency. These results suggest that as mangrove stands continue to increase in cover within mangrove-marsh ecotones, there are likely to be consequences for upper trophic level dynamics.