EMERGENT EFFECTS OF MULTIPLE PREDATORS ON PREY IN THE FOSSIL RECORD

Most prey are exposed to risks from more than one predator. Ecological theory predicts that, over the long term, multiple predators may produce emergent impacts on prey (i.e., effects are not simply the sum of the effects of individual predators). One key factor that is likely to govern the potential for emergent effects of multiple predators is an overlap among predators in their timing of foraging on a shared prey resource, and hence opportunities for interference and/or intraguild predation among them.

The shallow infaunal bivalve Mercenaria rileyi, from the Moorehouse Member of the Pliocene Yorktown Formation of Virginia, was a shared prey resource for three different predatory gastropods: shell-chipping busyconine whelks, shell-drilling Ecphora quadricostata (both of which are epifaunal), and infaunal shell-drilling naticid gastropods. All three predators leave distinctive predation traces on their victims. The three predators may or may not have had opportunities to interact and compete for prey depending on the time of year each was most active.

We used oxygen isotope profiles to determine the season of attack by whelks, Ecphora, and naticids on Mercenaria. Seven adult prey shells bearing one or more successful or unsuccessful predation traces by these predators were serially sampled at 1 – 3 mm intervals along the axis of shell growth. All shells yielded clear cyclical isotopic signals. Preliminary results suggest that all three predators were active during the same seasons of the year, and thus had the potential to interact and compete for prey with each other.

An understanding of the emergent effects of multiple predators is important to assess the likely strength of evolutionary response within predator-prey systems. For instance, from the prey’s perspective, the presence of more than one predator species has the counterintuitive effect of reducing predation pressure. The evolutionary implication of this emergent effect for the prey species is that multiple predators may have reduced, rather than intensified, selection pressure for antipredatory response. Work in progress compares the seasonal frequency of predation traces of each Mercenaria predator.