It's a little like CSI: Yellowstone.
Researchers are using DNA analysis, atomic absorption spectrometry, and other techniques to analyze not just what Yellowstone grizzlies eat, but what they used to eat, based on study of museum specimens.
Download the paper: "Grizzly Bear Nutrition and Ecology Studies in Yellowstone National Park" (376kb PDF file)
It all bears (pun) on such issues as the displacement of cutthroat trout by introduced lake trout in Yellowstone Lake and diseases and bugs affecting whitebark pine trees.
The oldest grizzly bear bones that we found came from a 1,000-year-old packrat midden excavated from the Lamar Cave. Due to the efforts of this hard-working pack-rat that had a fetish for bones, we know that meat (everything from ants to trout and elk) provided 32% of the nourishment for those grizzly bears and 68% came from plants (everything from roots and leaves to berries and nuts. That distribution of dietary meat to plants is identical to what we found for five grizzly bears killed from 1856 to 1888 in eastern Montana and Wyoming.
From 1914 to 1918 when many hotels were feeding kitchen scraps to attract grizzly bears for tourist entertainment and local towns had open-pit garbage dumps, the park’s grizzly bears switched to 85% meat, 15% plants. After all such feeding ended by the early 1970s and bears were forced to return to natural foods, the diets of young bears of both sexes and adult females returned to the levels observed 1,000 years ago (~40% meat, 60% plants). Adult males have continued a more carnivorous life (~80% meat, 20% plants). Large males can prey more efficiently on the park’s elk and bison or claim the carcasses of animals that died from other causes. Bears that have been killed for preying on livestock outside the park had diets that were 85% meat, 15% plants. These levels of meat consumption are in contrast to those of grizzly bears in Glacier National Park and Denali National Park, where plant matter provides 97% of their nourishment. Thus, for grizzly bears, the opportunity to consume meat differentiates the Yellowstone ecosystem from many other interior ecosystems where bears must feed primarily on plants. Cutthroat trout are one of those meat sources.