Peter Petraitis

ecology of marine intertidal communities

I am interested in the ecological processes responsible for the origin and maintenance of natural communities and my current research in New England focuses on the ecological mechanisms that maintain stands of algae and beds of mussels and barnacles in the Gulf of Maine as distinct, self-sustaining communities. Mussels, barnacles, and macroalgae, which are known as rockweeds, are the major structural components of the natural communities that line the shore. Stands of rockweed and beds of mussels and barnacles are distinct and rarely overlap even though they occupy very similar places on the shore. Currently, it is thought that local site-specific differences in ecological processes such as predation, competition, and recruitment, control the species composition of these two communities.

I am in the process of testing an alternative hypothesis that these two communities represent different stable states in the same environment. The theory of multiple stable states is well understood, but whether multiple stable states actually exist in nature has remained a hotly debated subject because natural variation and historical events can blur what stability, equilibrium, and habitat mean in nature. These issues pose difficult problems for community ecology as a whole, and not surprisingly, definitive examples of multiple stable states in nature continue to be elusive.

This work is being carried out on sheltered shores in the Gulf of Maine, which are dominated by either canopy-forming fucoid rockweeds (Ascophyllum nodosum and Fucus vesiculosus) or beds of barnacles (Semibalanus balanoides) and mussels (Mytilus edulis). Petraitis and Latham (1996) hypothesized that infrequent ice scour events could initiate the formation of either communitY by opening up new clearings, and allowing divergent successional events to pave the way for one or the other community states. This is most likely to occur in large clearings where much of a patch may lie beyond the edge effects of the surrounding community, and thus recruits, which arrive in opportunistic fashion, may have sufficient time and space to become established and develop into a divergent assemblage. Experimental clearings of different sizes were established in A. nodosum stands to test this hypothesis in 1996 at 12 sites on Swan=s Island, Maine. The set of clearings at each site included four clearings (1, 2, 4 and 8 m in diameter), and a control plot that was not cleared for a total of 60 plots. These plots have been sampled at least once a year since 1996, and about 40% of the 4 and 8 m clearings are now mussel beds.

This work has important implications for the management of marine ecosystems. The premise of many environmental management strategies is that there is a one-to-one correspondence between the magnitudes of cause and effect. Small changes in conditions give rise to small changes in communities. However, the theory of multiple stable states suggests small changes in conditions can push communities past thresholds and cause dramatic shifts. Moreover, these changes may not be reversible. If the natural communities found on the shores of the Gulf of Maine exist as multiple stable states, then management strategies dealing with fisheries, development, and man-made disasters will need to be altered to account for the existence of thresholds and the potential of small environmental insults to cause large irreversible changes.

This sort of research requires good, long-term baseline data, and as a result, I am also involved in applied problems of long-term monitoring and the impacts of near-shore fishing. I have helped develop sampling protocols for the International Long-Term Ecological Research (ILTER) site at Lake Hovsgol in Mongolia and for the National Park Service in Acadia National Park in Maine. I have also been a consultant to private industry on the ecological impacts of harvesting invertebrates, such as clams and sea urchins.