Consumer-controlled community states on Gulf of Maine rocky shores

Mussel beds and algal canopies on Gulf of Maine (USA) rocky shores have been hypothesized to represent stochastically determined community states, but we recently found that they were highly deterministic, consumer-controlled states in a tidal river. Mussel beds dominated high-flow habitats with high larval delivery and low consumer pressure, whereas algal canopies dominated low-flow habitats with high consumer pressure. In this paper we examine the alternative-community-state hypothesis on rocky shores on the open coast of Maine. We hypothesized that community states would be less deterministic due to reduced consumer pressure and more stochastic flow patterns. At 10 mussel-bed and 10 seaweed-canopy sites on the open coast of central Maine, we created 9-m(2) and 1-m(2) clearings and an unmanipulated control area, each containing control (no caging), caged, and cage-control quadrats. Independent of disturbance patch size, mussel beds and seaweed canopies rapidly began to return to their original community type in the absence of consumers (crabs and snails). With consumers present, recovery of the original communities did occur, but at a much slower pace. Our results suggest that mussel beds and algal canopies in the Gulf of Maine are deterministic consumer-controlled states, rather than being stochastically determined. If stochastic alternative community states exist in this system, they occur in a very restricted subset of habitats where the strong consumer control and determinism we have found in both tidal river and open coast habitats are relaxed.