Using stable isotopes to assess dietary changes of American black bears from 1980 to 2001

^§Contribution to Special Section ‘Stable Isotopes in Mammals’.

Abstract

We measured stable carbon and nitrogen isotope ratios in 117 hair samples from American black bears (Ursus americanus) in Great Smoky Mountains National Park, Tennessee, during 1980–2001 from live-trapped bears. We also collected hair from bears with known diets to compare with the wild bears. We hypothesized that biological factors (age, mass, and sex), food availability (hard mast and wild hogs (Sus scrofa)), and nuisance status would influence food selection by black bears and changes in their feeding history would be measureable using stable isotopes. We developed a set of a priori models using nine variables to examine changes in black bear stable isotope values. We found no support for changes in δ¹³C values associated with any of the nine variables we analyzed. Bears had enriched ¹⁵N in years with low white oak mast production and depleted ¹⁵N when white oak mast was abundant. Subadults had enriched ¹⁵N compared with adults and older adults. Variation in δ¹⁵N increased from 1980–1991 to 1992–2000 when hard mast production had greater fluctuations. Bears in a better physical condition appeared more likely to access foods with higher protein content. In years of low white oak acorn production, larger bears and subadults likely turned to alternative food sources. The long-term variation detected in this study was important in identifying which bears were potentially more susceptible to changes in availability of hard mast.

Keywords:

• carbon-13
• diet
• food sources
• hair samples
• isotope ecology
• long-term data
• mammals
• modelling
• nitrogen-15
• Ursus americanus

Acknowledgements

We thank the Stable Isotopes Lab at the UT Department of Earth and Planetary Sciences for support and use of equipment. We appreciate the help and support of B. Stiver (NPS); F.T. van Manen and J. Clark (USGS). This study would not have been possible without the work of numerous technicians capturing black bears in GSMNP. We thank the following people for donating bear hair and food samples to the project: K. Delozier, B. Stiver, and J. Yarkovich (NPS); E. Ramsey (UT School of Veterinary Medicine); L. Stewart (Appalachian Bear Rescue); D. Brandenburg (Tennessee Wildlife Resources Agency); L. McKay (UT Department of Earth and Planetary Sciences).

Funding

We thank the University of Tennessee (UT) Department of Forestry, Wildlife and Fisheries and Great Smoky Mountains Conservation Association's Carlos C. Campbell Fellowship for funding.

Notes

^§ Contribution to Special Section ‘Stable Isotopes in Mammals’.