Do Alternate Stable Community States Exist in the Gulf of Maine Rocky Intertidal Zone

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It has recently been suggested that intertidal mussel beds and seaweed canopies represent alternate community states on rocky shores in the Gulf of Maine. This theoretically appealing idea hypothesizes that these shoreline habitats are disturbance patch mosaics dominated by either seaweed or mussel communities, and that the community occurring in a given habitat is stochastic and dependent on the size of the original disturbance and recruit availability. Large disturbances are postulated to be dominated by mussel beds and barnacles which have widely dispersed larvae,while smaller disturbances are dominated by seaweeds, which have more limited dispersal. Positive feedbacks are proposed to maintain the stability of these two distinct community types. Dr. Bertness tested this hypothesis on the shores of a tidal estuary in central Maine. These habitats are conducive to exploring this idea because discrete mussel beds and seaweed canopies occur in close proximity and are easily manipulated. The results did not support the alternate stable state hypothesis. Instead, they suggest that the occurrence of mussel beds and seaweed canopies is highly deterministic. Largely independent of the size of artificial disturbances, they found that mussel beds and seaweed canopies rapidly returned to their original community type, but only inthe absence of grazers. Seaweed canopies dominate habitats with little water movement, whereas mussel beds dominate habitats with high flows. In the absence of grazers, these communities predictably and rapidly returned to their original state, but with grazers present, neither community showed signs of recovery even after two years. In this project Dr. Bertness and collaborators will examine the robustness and consequences of these findings. In particular, they will: I. Continue monitoring experimental clearings to examine the time course and trajectory of natural disturbances with grazers present and examine the role of consumer refuges in the recovery of naturally occurring mussel beds and seaweed canopies; II. Explore the hypothesis that intertidal mussel and seaweed communities can representalternate community states, but in a narrower range of intermediate flow habitats; and III. Extend the generality of the findings to coastal, wave-exposed habitats where the idea that mussel beds and seaweed canopies can represent alternate states was initially developed. The notion that natural communities can represent stochastically determined alternate stable states is an intellectually stimulating idea that has important implications for understanding the structure and dynamics of natural systems and for the management of marine and terrestrial ecosystems. Nonetheless, the very existence of alternate stable community states in nature has been difficult to establish. The work outlined in this project will critically evaluate the alternate community stable state hypothesis in the Gulf of Maine rocky intertidal zone, elucidate mechanisms underlying the stochastic and/or deterministic nature of alternate stable states in this system and establish the conditions where they are most likely to occur.