Grizzly bears (Ursus arctos) and American black bears (U. americanus) are sympatric in much of Yellowstone National Park. Three primary bear foods, cutthroat trout (Oncorhynchus clarki), whitebark pine (Pinus albicaulis) nuts, and elk (Cervus elaphus), have declined in recent years. Because park managers and the public are concerned about the impact created by reductions in these foods, we quantified bear diets to determine how bears living near Yellowstone Lake are adjusting. We estimated diets using: 1) stable isotope and mercury analyses of hair samples collected from captured bears and from hair collection sites established along cutthroat trout spawning streams and 2) visits to recent locations occupied by bears wearing Global Positioning System collars to identify signs of feeding behavior and to collect scats for macroscopic identification of residues. Approximately 45 AE 22% (x AE SD) of the assimilated nitrogen consumed by male grizzly bears, 38 AE 20% by female grizzly bears, and 23 AE 7% by male and female black bears came from animal matter. These assimilated dietary proportions for female grizzly bears were the same as 10 years earlier in the Lake area and 30 years earlier in the Greater Yellowstone Ecosystem. However, the proportion of meat in the assimilated diet of male grizzly bears decreased over both time frames. The estimated biomass of cutthroat trout consumed by grizzly bears and black bears declined 70% and 95%, respectively, in the decade between 1997-2000 and 2007-2009. Grizzly bears killed an elk calf every 4.3 AE 2.7 days and black bears every 8.0 AE 4.0 days during June. Elk accounted for 84% of all ungulates consumed by both bear species. Whitebark pine nuts continue to be a primary food source for both grizzly bears and black bears when abundant, but are replaced by false-truffles (Rhizopogon spp.) in the diets of female grizzly bears and black bears when nut crops are minimal. Thus, both grizzly bears and black bears continue to adjust to changing resources, with larger grizzly bears continuing to occupy a more carnivorous niche than the smaller, more herbivorous black bear. ß 2012 The Wildlife Society.

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...

Recently, brown bear (Ursus arctos) viewing has increased in coastal Alaska and British Columbia, as well as in interior areas such as Yellowstone National Park. Viewing is most often being done under conditions that offer acceptable safety to both people and bears. We analyze and comment on the underlying processes that lead brown bears to tolerate people at close range. Although habituation is an important process influencing the distance at which bears tolerate people, other variables also modify levels of bear-to-human tolerance. Because bears may react internally with energetic costs before showing an overt reaction to humans, we propose a new term, the Overt Reaction Distance, to emphasize that what we observe is the external reaction of a bear. In this paper we conceptually analyze bear viewing in terms of benefits and risks to people and bears. We conclude that managers and policy-makers must develop site-specific plans that identify the extent to which bear-to-human habituation and tolerance will be permitted. The proposed management needs scientific underpinning. It is our belief that bear viewing, where appropriate, may promote conservation of bear populations, habitats, and ecosystems as it instills respect and concern in those who participate.