Competitive displacement and predation between introduced and native mud snails.

Experimental field and laboratory studies indicate that Cerithidea californica, a native mud snail, is restricted to only a portion of its normal habitat range in San Francisco Bay as a result of direct interactions with an introduced ecological equivalent, Ilyanassa obsoleta. The native snail typically inhabits marsh pans, tidal creeks and mudflats in estuaries along the Pacific coast. However, in San Francisco Bay it is confined to pans for most of the year, while the non-native snail inhabits the creeks and mudflats. Experiments and field monitoring demonstrate that this abnormal distribution pattern is caused by 1) interference competition for space in the form of an adult-adult behavioral avoidance by C. californica in the presence of invading I. obsoleta, and 2) predation by I. obsoleta on the eggs and juveniles of C. californica. The competitive exclusion of C. californica by I. obsoleta has not led to the extinction of the native snail because of the existence of a refuge for C. californica in pan habitats, beyond the physiological tolerances of I. obsoleta. As a consequence of the seasonal migrations of both species and changes in abiotic factors along the habitat gradient, repeated competitive displacements, rather than a one-time competitive exclusion, are observed between these two species. This is the first documented case of the competitive displacement of an endemic marine intertidal species by an introduced ecological equivalent.