ON DRILLING FREQUENCY AND MANLY'S ALPHA: TOWARDS A NULL MODEL FOR PREDATOR PREFERENCE IN PALEOECOLOGY

Abstract

In the paleoecological literature, drilling frequency—the percent of specimens in a prey taxon with complete drill holes—is commonly interpreted as an indication of a predator's preference. Such taxon-specific drilling frequencies are often compared with one another and related to underlying prey characteristics such as cost-benefit ratio or body size. Although this approach can demonstrate a predator's relative preference for one prey type over another, it fails to consider whether predation on any prey type is greater than would be expected by a predator without preference. Here we develop a null model for evaluating predator preference in paleoecology by considering drilling frequency in the framework of Manly's alpha, which is a well-established model in the ecological literature that has been used to evaluate predator preferences. In effect, Manly's alpha normalizes taxon-specific drilling frequencies with all potential prey types in the community, allowing for a statistically rigorous test of the null hypothesis that drilling predation, and therefore predator preference, on any given prey type is equivalent to all other available prey types in the community. After discussing the statistical basis for the model, we demonstrate the model's utility by applying it to a published dataset of drilling predation on Pliocene bivalves from Langenboom, Netherlands. The Manly's alpha approach, which uses the same data (i.e., drilling frequencies) that are commonly collected by paleoecologists, provides a null hypothesis to more rigorously assess predator preferences and inherently includes community context for predator-prey interactions in the fossil record.