bear black
March 3, 2014

Diets Of Dumpster Diving Yosemite Bears Have Changed Over Time

Brett Smith for - Your Universe Online

People living or vacationing in rural parts of the northern US and Canada know not to leave food waste lying around at night or else they may receive an unwanted visitor in the form of a scavenging black bear.

A new study on black bears living in Yosemite National Park, which was published in Frontiers in Ecology and the Environment, has found that the diets of these scavengers have changed over the past two decades as bear-related policies by the park have changed.

"Yosemite has a rich history of bear management practices as a result of shifting goals over the years," said study author Jack Hopkins, a research fellow at the University of California, Santa Cruz. "What we found was that the diets of bears changed dramatically after 1999, when the park got funding to implement a proactive management strategy to keep human food off the landscape."

Park changes implemented in 1999 included enforcing the use of bear-resistant food containers, staffing more employees to manage aggressive bears and establish a "bear crew" to boost visitor compliance in campgrounds and area hotels.

The study, which focused entirely on bears that ate human food or food waste, discovered the amount of human food in their diets reduced by about 63 percent after the new strategies had been enacted. Regrettably, once a bear gets accustomed to eating ‘people food,’ it has a taste for it and seeks it out, Hopkins said. Also, there will always be some people who mistakenly leave food or food waste out that bears could get at it.

In the study, the researchers conducted a radioactive analysis of hair and bone samples. Modern-day hair samples were taken during encounters between bears and bear-management staff, as well as from wire hair-traps deployed throughout the park.

Historical samples were taken from archived specimens and included bears killed between 1915 and 1919 – the earliest time period in the study. In the early 20th century, bears were drawn to garbage dumps in the park and were often slaughtered when they became a pest. Interestingly, the park began intentionally feeding bears in 1923 - so that visitors could watch them. The practice lasted until 1971. From 1927 to 1956, a fish hatchery in Yosemite allowed the bears to feast on fresh trout straight out of the holding tanks.

The researchers found that the amount of human food in the bears' diets was about 13 percent from 1915 to 1919, 27 percent from 1928 to 1939, 35 percent from 1975 to 1985 and 13 percent again for 2001 to 2007.

"This study shows the power of using museum specimens and archived historical material to reconstruct the ecology of a species and to answer pressing management questions," said study author Paul Koch, a professor of Earth sciences and dean of physical and biological sciences at UC Santa Cruz. "The remarkable thing is that the bears that eat human food are now back to the same level of dumpster diving as in 1915, despite the fact that there are now millions of visitors in Yosemite every year and presumably a lot more garbage."

"The bears just went back to the campgrounds and hotels and continued to find human food," Hopkins added. "People like to see bears, and they don't like to hear about bears being killed. But the bears they often see in visitor-use areas like Yosemite Valley are the ones that are conditioned to eat human food, and those are the ones that become problems and have to be killed.”

Image 2 (below):  Researcher Jack Hopkins used barbed-wire snares to collect hair samples from bears in Yosemite National Park. Analysis of isotope ratios in hair samples showed how much of the bears' diets came from human food. Credit: Photo courtesy of Jack Hopkins