CAN PREDATION METRICS BE RELIABLY CALCULATED FROM SIZE-BIASED TARGET COLLECTED SAMPLES?

Target sampling, or taxon-specific sampling, is one of the oldest sampling strategies employed by paleontologists and has been described as a method in which all specimens of a taxon of interest are collected without regard for size or preservation quality. Despite its history, this method is often dismissed as a means for collecting paleoecological data because of potential collecting biases. One of the acknowledged biases, that of larger specimen size in target samples relative to bulk samples, has been shown to be significant, however, the magnitude of this bias is unknown. Using two abundant gastropods, Eupleura muriciformis and Polinices chemnitzii, from the northern Gulf of California we attempted to quantify the magnitude of this bias for commonly used predation metrics including drilling frequency, drill hole stereotypy, prey effectiveness, and predator-prey size. In 26 tests on Eupleura, 24 indicated no difference between bulk and target sampling methods. Two tests did yield significant differences as a consequence of the size bias, yet, even when prey size was unequal the predation metrics were equivalent. In contrast, four of five preliminary tests on Polinices showed a significant difference in predation metrics between bulk and target samples as a result of the bias towards larger specimens in target samples. In this study, target sampled Eupleura and Polinices were both approximately 15% larger than those that were bulk sampled but the predation metric comparisons were considerably different. These results indicate a necessity to size-standardize Polinices data for comparisons between bulks and targets, but suggest that this may result in the unnecessary loss of data for Eupleura. We propose that the existence of a prey size refuge in Polinices explains the difference. While all sizes of Eupleura are susceptible to predation, greater than 50% of specimens in the Polinices size distribution exceed the maximum prey size for drilling predators. In effect, target samples of Polinices draw predominately from the portion of the size distribution that is undrilled, resulting in exaggerated differences between calculated bulk and target predation metrics.