"Wild fruit is typically an important part of grizzly bear diet, especially in late summer when they are trying to gain weight as rapidly as possible before winter hibernation," Oregon State forest ecosystems Professor William Ripple said. "Berries are one part of a diverse food source that aids bear survival and reproduction, and at certain times of the year can be more than half their diet in many places in North America."

When wolves were removed from Yellowstone early in the 1900s, increased browsing by elk herds caused the demise of many food plants, including many berry-producing shrubs.

Re-introducing wolves has helped reduce elk numbers in Yellowstone and allowed tree and shrub recovery, the researchers said.

At the end of 2011, there were at least 98 wolves in 10 packs in Yellowstone's 3,468 square miles, they said.

In 2012, bear numbers were estimated at more than 600.

The park's website states about 10,000 to 20,000 elk from six or seven herds spend their summers in Yellowstone. The number dwindles to fewer than 5,000 during the winter.