Use of sulfur and nitrogen stable isotopes to determine the importance of whitebark pine nuts to Yellowstone grizzly bears

Abstract

Whitebark pine (Pinus albicaulis) is a masting species that produces relatively large, fat-and protein-rich nuts that are consumed by grizzly bears (Ursus arctos horribilis). Trees produce abundant nut crops in some years and poor crops in other years. Grizzly bear survival in the Greater Yellowstone Ecosystem is strongly linked to variation in pine-nut availability. Because whitebark pine trees are infected with blister rust (Cronartium ribicola), an exotic fungus that has killed the species throughout much of its range in the northern Rocky Mountains, we used stable isotopes to quantify the importance of this food resource to Yellowstone grizzly bears while healthy populations of the trees still exist. Whitebark pine nuts have a sulfur-isotope signature (9.2 ± 1.3‰ (mean ± 1 SD)) that is distinctly different from those of all other grizzly bear foods (ranging from 1.9 ± 1.7‰ for all other plants to 3.1 ± 2.6‰ for ungulates). Feeding trials with captive grizzly bears were used to develop relationships between dietary sulfur-, carbon-, and nitrogenisotope signatures and those of bear plasma. The sulfur and nitrogen relationships were used to estimate the importance of pine nuts to free-ranging grizzly bears from blood and hair samples collected between 1994 and 2001. During years of poor pine-nut availability, 72% of the bears made minimal use of pine nuts. During years of abundant cone availability, 8 ± 10% of the bears made minimal use of pine nuts, while 67 ± 19% derived over 51% of their assimilated sulfur and nitrogen (i.e., protein) from pine nuts. Pine nuts and meat are two critically important food resources for Yellowstone grizzly bears.Résumé : Le pin albicaule (Pinus albicaulis) est un arbre à glandée qui produit des noix relativement grosses et riches en graisses et en protéines, dont se nourrissent les grizzlis (Ursus arctos horribilis). Les arbres donnent des récoltes abondantes certaines années, pauvres d'autres années. La survie des grizzlis de l'écosystème du Grand Yellowstone est étroitement liée aux variations de la disponibilité des noix de pin. Le pin albicaule est sujet aux infections de rouille vésiculaire du pin (Cronartium ribicola), un champignon exotique qui a décimé les populations sur une grande partie de la répartition de l'espèce, dans le nord des Rocheuses; c'est pourquoi nous avons utilisé des isotopes stables pour quantifier l'importance de cette ressource alimentaire pour les grizzlis tandis qu'il existe encore des populations de pins en bonne santé. Les noix du pin albicaule ont une signature d'isotopes du soufre de 9,2 ± 1,3 ‰ (moyenne ± 1 écart type) qui diffère de celles que l'on retrouve dans les autres aliments du grizzli dont les signatures de soufre se situent entre 1,9 ± 1,7 ‰ pour les autres plantes et 3,1 ± 2,6 ‰ pour les ongulés. Des expériences alimentaires sur des grizzlis en captivité ont servi à identifier les relations entre les signatures d'isotopes du soufre, du carbone et de l'azote dans la nourriture et celles du plasma des ours. Les relations du soufre et d...

Elk Calf Survival and Mortality Following Wolf Restoration to Yellowstone National Park

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Diversity in trophic interactions of green sea turtles Chelonia mydas on a relatively pristine coastal foraging ground

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Defining the predator landscape of northeastern British Columbia.

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