Experimental evidence for the origin of alternative communities on rocky intertidal shores

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