Estimating the strength of predator-prey interactions is a fundamental goal of ecology. We compared the results of two methods for inferring predator diets: stable isotope analysis and a feeding survey method developed by Novak and Wootton. Stable isotope analysis estimates diet proportions with mixing models using the ratios of naturally-occurring heavy and light carbon and nitrogen isotopes in the tissue of predators and their prey. The Novak and Wootton method estimates feeding rates, and hence diet proportions, using snapshot feeding surveys of a focal predator’s population along with estimates of prey abundances and handling times. To assess their congruence, we applied both methods to quantify how the diets of dog whelks, Nucella ostrina, responded to variation in prey abundances across replicate mussel-free patches in the Oregon marine intertidal.
At the site-wide scale, the two methods of inferring predator diets gave prey-specific estimates of diet contributions that were positively correlated with one another. However, no correlation was observed at the patch-specific scale, with isotopic inferences suggesting that the whelk's diets entailed a significantly higher proportion of some prey species that were rarely observed being fed upon during feeding surveys. Our results therefore suggest that the spatial resolution of the two methods may be governed by their differing ability to capture diet breadth and by variation in prey abundances across space, with the method of Novak and Wootton being more sensitive to fine-scale variation in prey availability and selection by whelks.