A COMMUNITY CONTEXT FOR NATICID PREDATION IN THE PRE-DAM ERA COLORADO RIVER ESTUARY USING MANLY’S ALPHA

The ecosystem in the Colorado River estuary (CRE) has changed in the last century after upstream dams and water diversions have redirected nearly all of the freshwater that once flowed into the estuary. One of the most abundant species, Mulinia modesta, has undergone population declines of approximately 94%, which has led to the hypothesis that predators relying on this species as a food source may also have experienced population declines, switched to alternative prey, or both. Although drilling frequency on Mulinia has been documented, it remains unknown how predation on this species compared to predation on other species in the community. Therefore, using a null model of predator preference, Manly’s alpha, we test whether Mulinia was preyed upon selectively by naticid gastropods as compared to other species in the community and the background drilling frequency for the community. A strong preference for Mulinia, and only this species, would suggest that predator populations likely declined in the post-dam era (post-1930s). If predators had strong preferences for multiple species, including or excluding Mulinia, then predators will likely have continued consuming non-Mulinia prey and maintained their population sizes.

Samples were taken from molluscan death assemblages at three sites in the CRE following the north-south salinity gradient that existed prior to water diversions (pre-1930s) in the Colorado River basin. Three to five bulk samples were collected at each site. All samples were sieved using a 5-mm mesh, identified to the species level, and the presence of naticid drillholes was noted. Only specimens judged to be at least 85% complete were included and bivalve counts were halved given that bivalves have two identifiable elements.

Application of the Bray-Curtis index of dissimilarity to randomized and observed drilling data at the three sites shows the existence of predator preferences at each site along the gradient. Resolution at the species level, by comparing each species’ alpha value to the null value of 1/# species, demonstrates that Mulinia was not the only species preferred by naticid predators. As naticids have demonstrable preferences for alternative prey species, we suggest that prey switching likely occurred.