Researchers performed isotope analysis of hair and bone samples to study Yosemite bears’ changing diets over the past century:
Yosemite National Park was established in 1890, and Hopkins obtained samples from bears killed between 1915 and 1919 to represent the earliest time period. In those early years, bears were attracted to garbage dumps in the park and were often killed when they became a nuisance. Visitors liked to see bears, however, and in 1923 the park began intentionally feeding bears where visitors could watch them. The last artificial feeding area closed in 1971. There was also a fish hatchery in Yosemite Valley, from 1927 to 1956, where bears once helped themselves to fresh trout from the holding tanks. But closing the hatchery and the feeding areas didn’t stop bears from eating human food.
“The bears just went back to the campgrounds and hotels and continued to find human food,” Hopkins said.
The average figures for the proportion of human food in bear diets during the four time periods in the study were 13 percent for the period from 1915 to 1919; 27 percent for 1928 to 1939; 35 percent for 1975 to 1985; and 13 percent again for 2001 to 2007.
These results are based on a kind of chemical forensics in which Koch’s lab specializes. Isotopic analysis of an animal’s tissues can yield clues to its diet because of natural variability in the abundance of rare isotopes of elements such as carbon and nitrogen. Isotope ratios (the ratio of carbon-13 to carbon-12, for example) are different in human foods than in the wild plants and animals that black bears naturally eat in Yosemite, partly due to the large amounts of meat and corn-based foods in our diets.
In order to analyze the data from Yosemite bears that ate a mixture of human and natural foods, Hopkins had to get samples from bears that did not eat any human food, and he had to track down samples of the non-native trout that had been raised in the hatchery. He also needed data representing a 100 percent human food diet, for which he turned to the Smithsonian Institution for samples of human hair from different periods over the past century.
“He searched far and wide to get the collection of samples we analyzed, and that collection made the study powerful enough to answer the question of how management practices affect bear diets,” Koch said.
According to Hopkins, the key to managing bear problems is to prevent bears from becoming conditioned to eat human food in the first place.