Proceedings of the South Dakota Academy of Sciences, vol. 75 (1996).
STABLE ISOTOPE VALUES FOR TWO MODERN BLACK BEARS FROM PRINCE OF WALES ISLAND, ALASKA

Korey B. Champe and Timothy H. Heaton
Department of Earth Sciences
University of South Dakota
Vermillion, South Dakota 57069

ABSTRACT

In southeast Alaska, stable isotope values indicate dietary patterns according to a simple two-food-source model. Exclusively terrestrial plant eaters, such as rodents and ungulates, exhibit an average del-13C value of -22.7% and an average del-15N value of 5.0% while exclusively marine fish eaters, such as otters and seals, exhibit an average del-13C value of -12.4% and an average del-15N value of 15.8%. Marine fish from the region have an average del-13C value -13.5%. Some coastal Alaskan bears are mixed terrestrial and marine feeders and therefore exhibit intermediate values.

Until now only fossil specimens from Prince of Wales Island have been studied and only del-13C values measured. Prior to 7,000 years ago brown and black bears coexisted on the island, but only black bears are present today. We examined the del-13C values on collagen and bioapatite and the del-15N values on collagen of two large male black bears that were recently killed on the island. The del-13C values are -19.8% and -20.0% and the del-15N values are 7.9% and 7.8% on collagen, and the del-13C values are -18.3% and -18.2% on bioapatite.

Fossil black bears from Prince of Wales Island and living inland black bears have average del-13C values of 21.0% and average del-15N values of 4.8% on collagen, indicating that they are almost exclusively terrestrial plant eaters. The slightly higher values exhibited by the modern black bears from Prince of Wales Island suggest a marine component to their diet, and this is confirmed by citings of bears fishing for salmon in island streams. Fossil brown bears from the island have an average del-13C value -17.6%, indicating a significant marine component to their diet. We postulate that the extinction of brown bears allowed black bears access to salmon streams and resulted in the higher stable isotope values for the modern population. Corresponding differences can be found between coastal and inland populations of modern brown bears. The primary black bear diet on Prince of Wales Island (at least 90%) is still terrestrial plants, however, as indicated by the stable isotope values and examination of feces.

Differences (spacings) between del-13C values on collagen and bioapatite have been suggested to indicate the tropic level of an organism with a spacing of 7% for herbivores and 3% for carnivores. Spacings of 1.5% and 1.7% for the two modern black bears from Prince of Wales Island are in substantial conflict with our conclusion that these bears are primarily herbivores. These values contrast with a 3.9% spacing for an inland black bear, 6.1% for a British brown bear, and a mean of 6.6% for nine European cave bears. The only low spacings we have found are from a polar bear (1.1%), two seals (2.1%), and 35 ancient coastal humans (2.6%). While unusually low spacings are typical for marine carnivores, such values in primarily herbivorous bears are especially perplexing. Until we have obtained spacings on a wider variety of island animals, we cannot speculate on this apparent contradiction.


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