Facilitation between two dominant ecosystem engineers extends their footprints and degree of overlap

Ecosystem engineers influence the habitat, diversity, and productivity of ecosystems. However, multiple ecosystem engineers may coexist, and because of their dominant influence, how they interact can affect the entire system. In the southeastern US, the eastern oyster, Crassostrea virginica (Gmelin) and coarse cordgrass, Spartina alterniflora (Loisel) are two prominent ecosystem engineers that affect estuarine ecosystem functioning. Despite both species' importance and their proximity to one-another, few studies have focused on bidirectional interactions between these two species. First, we assessed the potential effect of fringing oyster reefs on adjacent cordgrass stands. We found that when tidal creek banks are occupied by oysters, cordgrass horizontally extends towards the water 5.25 m more (often overlapping with the oysters) than adjacent bank areas that were lacking oysters. Ostensibly, the positive effect of oysters to increase cordgrass occupancy is due to known abilities of oysters to stabilize shoreline, baffle hydrodynamic energy, and accrete sediment. Second, we experimentally examined the effects of cordgrass on oysters using the presence and absence of cordgrass and cordgrass-mimic treatments to parse the mechanistic effects of structure and shade provided by cordgrass. We found that oysters recruited 2.4 times more and increased biomass 3.2 times more underneath natural cordgrass and shade mimics compared to plots with cordgrass removal and structure-only mimics. Thus, cordgrass shade is an important mechanism of facilitation, especially on small recruiting oysters that are vulnerable to desiccation. Collectively, the mutualism between these two engineers expands their footprints and amount of overlap, strengthening their presence and thus their ecosystem services to the estuary.