title: Distance measures and optimization spaces in quantitative fatty acid signature analysis.
abstract: Quantitative fatty acid signature analysis has become an important method of diet estimation in ecology, especially marine ecology. Controlled feeding trials to validate the method and estimate the calibration coefficients necessary to account for differential metabolism of individual fatty acids have been conducted with several species from diverse taxa. However, research into potential refinements of the estimation method has been limited. We compared the performance of the original method of estimating diet composition with that of five variants based on different combinations of distance measures and calibration-coefficient transformations between prey and predator fatty acid signature spaces. Fatty acid signatures of pseudopredators were constructed using known diet mixtures of two prey data sets previously used to estimate the diets of polar bears Ursus maritimus and gray seals Halichoerus grypus, and their diets were then estimated using all six variants. In addition, previously published diets of Chukchi Sea polar bears were re-estimated using all six methods. Our findings reveal that the selection of an estimation method can meaningfully influence estimates of diet composition. Among the pseudopredator results, which allowed evaluation of bias and precision, differences in estimator performance were rarely large, and no one estimator was universally preferred, although estimators based on the Aitchison distance measure tended to have modestly superior properties compared to estimators based on the Kullback-Leibler distance measure. However, greater differences were observed among estimated polar bear diets, most likely due to differential estimator sensitivity to assumption violations. Our results, particularly the polar bear example, suggest that additional research into estimator performance and model diagnostics is warranted.
authors: U.S. Geological Survey, Alaska Science Center 4210 University Drive, Anchorage, Alaska, 99508.
authors: Jeffrey F
authors: JF
authors: Bromaghin
authors: U.S. Geological Survey, Alaska Science Center 4210 University Drive, Anchorage, Alaska, 99508.
authors: Karyn D
authors: KD
authors: Rode
authors: Process Engineering and Applied Science, Dalhousie University Halifax, Nova Scotia, B3H 4R2, Canada.
authors: Suzanne M
authors: SM
authors: Budge
authors: Faculty of Environmental Studies, York University 4700 Keele St., Toronto, Ontario, M3J 1P3, Canada.
authors: Gregory W
authors: GW
authors: Thiemann
keywords: Aitchison; Chukchi Sea; Halichoerus grypus; Kullback–Leibler; QFASA; Ursus maritimus; diet composition; gray seal; polar bear