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Variation in δ13C and δ15N diet–vibrissae trophic discrimination factors in a wild population of California sea otters

January 1, 2010

The ability to quantify dietary inputs using stable isotope data depends on accurate estimates of isotopic differences between a consumer (c) and its diet (d), commonly referred to as trophic discrimination factors (TDFs) and denoted by Δc-d. At present, TDFs are available for only a few mammals and are usually derived in captive settings. The magnitude of TDFs and the degree to which they vary in wild populations is unknown. We determined δ13C and δ15N TDFs for vibrissae (i.e., whiskers), a tissue that is rapidly becoming an informative isotopic substrate for ecologists, of a wild population of sea otters for which individual diet has been quantified through extensive observational study. This is one of the very few studies that report TDFs for free-living wild animals feeding on natural diets. Trophic discrimination factors of 2.2‰ ± 0.7‰ for δ13C and 3.5‰ ± 0.6‰ for δ15N (mean ± SD) were similar to those reported for captive carnivores, and variation in individual δ13C TDFs was negatively but significantly related to sea urchin consumption. This pattern may relate to the lipid-rich diet consumed by most sea otters in this population and suggests that it may not be appropriate to lipid-extract prey samples when using the isotopic composition of keratinaceous tissues to examine diet in consumers that frequently consume lipid-rich foods, such as many marine mammals and seabirds. We suggest that inherent variation in TDFs should be included in isotopically based estimates of trophic level, food chain length, and mixing models used to quantify dietary inputs in wild populations; this practice will further define the capabilities and limitations of isotopic approaches in ecological studies.

Publication Year 2010
Title Variation in δ13C and δ15N diet–vibrissae trophic discrimination factors in a wild population of California sea otters
DOI 10.1890/09-1502.1
Authors Seth D. Newsome, Gena B. Bentall, M. Tim Tinker, Olav T. Oftedal, Katherine Ralls, James A. Estes, Marilyn L. Fogel
Publication Type Article
Publication Subtype Journal Article
Series Title Ecological Applications
Index ID 70003962
Record Source USGS Publications Warehouse
USGS Organization Western Ecological Research Center
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