LTREB Renewal: Experimental tests of alternative states on rocky intertidal shores.

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Unexpected events that tip an ecosystem from one type of community to another are poorly understood yet often have serious ecological and economic effects, exemplified by the sudden collapse of productive grasslands into desert in the southwestern US. This project will investigate the underlying causes of the switch between alternative communities, dominated by either seaweeds or mussels, on sheltered rocky shores in the Gulf of Maine. A unique long-term experiment will advance general understanding of the resilience of natural communities to ecological perturbations of different sizes, the extent to which ecological transitions are reversible when environmental conditions are restored, and the predictability of ecological communities when perturbed. The research team includes a high school teacher from Maine who has a long association with this project. Six to eight of her students will participate in the field research each year, and the teacher will develop classroom activities appropriate for high school students and K-12 teachers. Primary data, panoramic photographs of the experimental plots, and educational activities will be available electronically, and the investigators will post a YouTube video on data archiving and management. The researchers will test hypotheses that (1) switches between alternative community states of mussel beds and rockweed stands occur in response to infrequent, large-scale occurrences of ice scour during winter, (2) the community that becomes established in large clearings made by ice scour is not predictable, (3) the order of species arrival during re-establishment drives community development and (4) the alternative communities are stable. These hypotheses will be tested using a Before-After-Control-Impact (BACI) design consisting of experimental clearings of different sizes, uncleared controls that were created in 1996-97, and half of these controls that were cleared again in 2010-11. In addition to collecting data characterizing patterns of succession in these developing communities, the researchers will assay rates of predation on mussels, mortality rates of mussels in patches of different sizes, and recruitment of juveniles of four key species annually. Analyses of data will compare outcomes from the first run of the experiment (1997-2008) with those from the second run (2011-2021) to test if community development is predictable. The approach of repeating an experiment to test for alternative community states in a natural ecosystem is unique and has broad implications for studies of alternative states in other ecosystems.