Oysters Crassostrea virginica on red mangrove Rhizophora mangle prop roots: facilitation of one foundation species by another

Red mangroves Rhizophora mangle and oysters Crassostrea virginica are foundation species that co-occur in subtropical Florida, USA. In the Indian River Lagoon (IRL), oysters living on prop roots and in isolated clumps on the ground (ground oysters) near mangroves comprise most of the natural population. Overall mean and median densities on prop roots were 750 and 177.8 live oysters m(-2) but only similar to 20 live oysters m(-2) in ground oysters. We collected data on C. virginica abundance and size, mangrove habitat, and physical and geographical variables ( fetch, sediment firmness, distances from inlet and nearest freshwater discharge source) and obtained water quality data (salinity, chlorophyll a, turbidity) from long-term monitoring databases. Data were analyzed using structural equation models (SEM), and we chose the best models by Akaike's information criterion (AIC). Oyster abundance (R-2=0.74) and size (R-2=0.60) on prop roots were functions of mangrove habitat, salinity, sediment, and distance to freshwater discharges. Fetch and distance to inlet affected sediment and thus had indirect effects on oysters. Inlet distance also affected the 5 yr coefficient of variation of salinity. In a separate SEM, the abundance of ground oysters near mangroves was a strong, positive effect on oysters on prop roots, in part because prop root oysters can break off and seed oyster clumps to the ground. However, ground oysters were directly negatively affected by fetch and distance to inlet, which had no direct effects on oysters living on prop roots. Red mangroves facilitate oysters by providing protection from sedimentation and offering a stable refuge during storms. However, increasing numbers of mangroves near a site had negative effects on oysters on the ground, possibly because it provided an enhanced habitat for predators.