Small changes in environmental conditions can unexpectedly tip an ecosystem from one community type to another, and these often irreversible shifts have been observed in semi-arid grasslands, freshwater lakes and ponds, coral reefs, and kelp forests. A commonly accepted explanation is that these ecosystems contain multiple stable points, but experimental tests confirming multiple stable states have proven elusive. Here we present a novel approach and show that mussel beds and rockweed stands are multiple stable states on intertidal shores in the Gulf of Maine, USA. Using broad-scale observational data and long-term data from experimental clearings, we show that the removal of rockweed by winter ice scour can tip persistent rockweed stands to mussel beds. The observational data were analyzed with Anderson's discriminant analysis of principal coordinates, which provided an objective function to separate mussel beds from rockweed stands. The function was then applied to 55 experimental plots, which had been established in rockweed stands in 1996. Based on 2005 data, all uncleared controls and all but one of the small clearings were classified as rockweed stands; 37% of the large clearings were classified as mussel beds. Our results address the establishment of mussels versus rockweeds and complement rather than refute the current paradigm that mussel beds and rockweed stands, once established, are maintained by site-specific differences in strong consumer control.

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Changes in the shell architecture of marine snails enhance defenses and greatly improve survival against predators. In the northwest Atlantic Ocean, shorter and thicker shells have been reported for several species following the introduction of predatory Carcinus maenas crabs early in the 20th century. But we report that the shell lengths of Nucella lapillus actually increased by an average of 22.6% over the past century, with no evidence of shell thickening after correcting for shell length. The increases in shell length were greatest on sheltered shores, highlighting the interaction between wave exposure and the sampling period. Comparisons were based on archived shells collected in 1915-1922 from sites that were resampled in 2007. N. lapillus is an important member of North Atlantic marine ecosystems, and our results suggest that the impacts of historical changes in species' key morphological traits on marine ecosystems remain underappreciated.morphology ͉ museum collections ͉ natural history ͉ rocky shores ͉ trait variation

Abstract. Experimental clearings in macroalgal (Ascophyllum nodosum) stands were made in 1996 to determine if mussel beds and macroalgal stands on protected intertidal shores in New England represent alternative community states. Uncleared control plots and four sizes of circular clearings, which mimicked ice scour events, were established in A. nodosum stands at 12 sites on Swan's Island, Maine, USA. The purpose of this data set is to provide access to data on densities and percent cover in the 60 experimental plots from 2003 to 2007 and to update data from 1996 to 2002 that are already published in Ecological Archives. Data include densities of mussels (Mytilus edulis and Modiolus modiolus), an herbivorous limpet (Tectura testudinalis), herbivorous snails (Littorina littorea, Littorina obtusata), a predatory snail (Nucella lapillus), a barnacle (Semibalanus balanoides), and fucoid algae (Ascophyllum nodosum and Fucus vesiculosus), and percent cover by mussels, barnacles, fucoids, and other sessile organisms.

Experimental clearings in macroalgal (Ascophyllum nodosum) stands were made at 12 sites in 1996 to determine if mussel beds and macroalgal stands on sheltered intertidal shores of New England represent alternative community states. Sites were located on Swan's Island, Maine, USA. At each site an uncleared control plot and four sizes of circular clearings, which mimicked ice scour events, were established. The purpose of this data set is to provide access to recruitment data collected in the experimental plots from 1997 to 2007.