THE INFLUENCE OF PREDATION ON COMMUNITY COMPOSITION AND EVENNESS: A CASE-STUDY FROM EOCENE MOLLUSCAN COMMUNITIES

The relationship between predation intensity and community composition and diversity has become of increasing interest as we attempt to improve our understanding of the factors that influence diversity in space and time. Several conflicting hypotheses have been suggested to explain how predation affects communities of prey. The indirect-competition hypothesis argues that predators will attack more abundant prey, and so will prevent any one prey species from achieving sufficient abundance to exclude rival species competing for similar resources. Thus, predation would maintain diversity and stabilize community composition. Although indirect competition has been documented frequently in the Recent, the subject has been little studied at the community scale or in the fossil record. If the hypothesis is correct, then prey communities experiencing high predation would have greater evenness, with no single species dominant.

To test the hypothesis, we examined a dataset of over 16000 specimens of 114 molluscan species from 25 localities within the late Eocene-age Moody’s Branch Formation of Louisiana and Mississippi (Hansen and Kelley, 1995). The localities cover a facies gradient from lagoon to outer-middle-shelf; molluscan communities experienced drilling predation frequencies (DF) of approximately 2–15% (mean = 6.9%). Consistent with the indirect-competition hypothesis, there were significant correlations between DF and community evenness (r = 0.47, p < 0.02) and between DF and proportion of most-abundant taxon (r = -0.60, p < 0.01). A Q-mode polar ordination (axes 1 and 2 account for > 80% of the variation) suggests that community structure was influenced by both environmental facies and by drilling predation. Although communities from the same facies tended to group together, communities with greater than average drilling frequency consistently grouped together in the center of the ordination plot, a pattern consistent with a relationship between predation and evenness.