Experimental Evidence for the Origin of Alternative Communities on Rocky Intertidal Shores

Ecological theory of alternative stable communities suggests the switch between species assemblages occurs when environmental conditions alter species so that it crosses a critical threshold and enters a different basin of . The theory implies that once the threshold is crossed, rates of key processes, such as predation and competition, will change. This conjecture tested on sheltered intertidal shores of New England that are dominated by mussel (Mytilus edulis) beds or algal (Ascophyllum nodosum) stands. Twelve of clearings of different sizes, which mimicked effects of ice scour, were created algal stands, and mortality of transplanted mussels was monitored. Clearings were 1, 2, 4 and 8 m in diameter. Mussel mortality due to the predatory snail Nucella declined in a step-like manner in clearings > 4 m in diameter.I nitial densities the predator did not vary among experimental clearings or sites and were not with mussel mortality. Mussel mortality was correlated with predatory densities during and at the end of the experiment. These results provide evidence supporting conjectures about the switch between alternative in ecological communities.