DETECTING AVIAN PREDATION ON BIVALVE ASSEMBLAGES USING INDIRECT METHODS

Modern ecological studies suggest avian predation has significant ecological impacts on intertidal communities and can potentially alter the preservation of the subfossil record. To assess if bird predation can affect surficial mollusk deposits, life and death shell assemblages from lagoon, bar and gravel bar habitats known for bird predation on San Juan Island, WA were compared to a null model derived from the ecological literature: (1) minimal physical decay, (2) high shell fragmentation, (3) ubiquity of distinct fracture patterns, and (4) monospecific composition. In contrast to the highly degraded shells from the bay and lagoon, the gravel bar death assemblage displayed highly fragmented pristine shells, frequent presence (68%) of ligaments, a dominant (72%) of valve fracture pattern, and nearly monospecific composition. While overall fidelity between life and death assemblages was low, the similarities between the gravel bar death assemblage and life assemblages suggest the gravel bar approximates present day composition of the local mollusk fauna ecosystem better than either the bay or lagoon death assemblages. These results suggest bird predation can be identified indirectly using supratidal shell deposits, an attractive ecological tool for limited field observations, and the potential paleoecological perspectives of studying shell accumulations generated by molluscivorous birds.