Use of Sulfur and Nitrogen Stable Isotopes to Determine the Importance of Whitebark Pine Nuts to Yellowstone Grizzly Bears

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 survivial 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
that is distinctly different from those of all other grizzly bear foods.
Feeding trials with captive grizzly bears were used to develop relationships
between dietary sulfur-, carbon-, and nitrogen-isotope signatures and those of
bear plasma.

The sulfur and nitrogen relationships were used to estimate the importance of
pine nuts to free-rangin grizzly bears from blood and hair samples collected
between 1994 and 2001. During years of abundant cone availability,
approximately 8+ or - 10% of the bears made minimal use of pine nuts, while 67
+ or - 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.

The GYE includes Yellowstone and Grand Teton national parks and adjacent
federal, state, and private lands in portions of Montana, Wyoming, and Idaho.
The GYE contains the headwaters of three major continental-scale river systems:
the Missouri and the Mississippi, Snake and Columbia, and Green and Colorado.

Major plant and animal foods consumed by grizzly bears (Mattson et al., 1991)
were collected through the GYE to determine if whitebark pine nuts have a
unique isotopic signature relative to other foods. Plant samples were
collected at sites used by radio-collared grizzly bears and included whitebark
pine nuts; the foliage of clover (Trifolium spp.), horsetails (Equisetum
arvense), elk thistle (Cirsium scariosum), cow parsnip (Heracleum lanatum),
dandelion (Taraxacum spp.) , spring beauty (Claytonia lanceolata), sedges
(Carex raynoldsii and C. Praticola), and grasses (Bromus anomolous), Phleum
alpinum, Agropyron caninum, Poa spp. And Festuca idahoensis); and the bulbs or
roots of onion grass (Melica spectabilis) , biscuitroot (Lomatium spp.), and
yampa (Perideridia gairdneri) . Fleshy fruits or berries are not significant
grizzly bear foods in the GYE and therefore were not collected (Mealey 1975;
Kendall 1983; Mattson et al 1991). Collected animal matter included army
cutworm moths (Euxoa auxiliaris) at alpine aggregation sites, cutthroat trout
(Onchorhyncus clarki) in spawning streams around Yellowstone Lake, and bison
(Bison bison), elk (Cervus elaphus) , and mule deer (Odocoileus hemionus ) from
throughout the park where they had been killed in collisions with cars.