A HEARTY MEAL IN THE HOLOCENE: TROPHIC CONSEQUENCES OF ENVIRONMENTAL CHANGE ON THE COLORADO RIVER DELTA, MÉXICO

Before upstream dams and water diversions, Mulinia coloradoensis was the dominant mollusk in the shallow assemblages of the Colorado River Delta. More than 85% of the shells in adjacent beach deposits belong to this species. Nowadays, M. coloradoensis only occurs in small populations close to the river’s mouth. The increased salinity resulting from the diminished flow of Colorado River water to the delta likely caused the decline of this species. If M. coloradoensis was an important food resource for other organisms, its decline may have led to the decline of its predators and other species higher in the food web. We estimated the ecological importance of M. coloradoensis in the pre-dam delta ecosystem by examining shells for evidence of drilling predation and edge peeling. Laboratory experiments indicate that post-mortem physical damage to the shell margin is distinctively different from biological damage. Analyses of 300 shells from four sites indicate similar predation pressure by either crabs or gastropods throughout the delta. Five to 50% of the shells showed some type of predator damage. Five to 15% were attacked only by muricid or naticid gastropods, 35 to 50% were preyed upon only by crabs, two to ten percent were attacked by both. This paleoecological evidence for predation pressure by drilling snails and peeling crabs demonstrates that M. coloradoensis was an important source of food in the pre-dam Colorado Delta ecosystem. The decline in M. coloradoensis populations probably caused a decline in the population size of its predators and higher level predators such as birds and fish. Thus, paleoecological approaches can be used to assess the trophic consequences of human-caused environmental change even if the skeletal remains of predators are not available.