Alternative stable states in a Pacific Northwest estuary

Alternative stable states refer to distinct communities with positive feedbacks that prevent gradual transitions among communities, each of which could persist under similar environmental conditions. Their existence has prompted recent attention to the extent that some communities provide more ecosystem services than others, but the factors controlling transitions among alternative states are not well understood. One of the hallmarks of alternative states is that the current state should be difficult to “invade” by another option, except when the current state is perturbed. We tested this idea with three possible alternative communities dominated by eelgrass (Zostera marina), burrowing shrimp (Neotrypaea californiensis), or exotic oysters (Crassostrea gigas) in the lower intertidal zone of a coastal estuary. Each state was transplanted into every other state, with and without first removing the existing state. These species effectively exert positive feedbacks by restricting water flow, bioturbating sediments, and creating hard substrate, respectively. Short-term survival of the transplants was strongly affected by the presence or absence of the current state in two cases (oysters and eelgrass), but effects of shrimp were only evident over a longer period. Overall, the transplants may have been too small to result in shifts to alternative states in all cases.