Black bears in California’s Yosemite National Park aren’t snacking as much on human food as they did decades ago, according to new research that traces changes in the diet of Yosemite bears over the past 100 years.
Researchers analyzed samples of bear bones from museums and bear hair collected from the field to determine the ratio of human-to-wild food that Yosemite bears ate as far back as 1915.
Not surprisingly, they found that the proportion of human food rose a lot after the park started feeding bears in 1923 to keep the animals away from developed areas. Lighted feeding platforms were even used to entertain park visitors until 1940.
The bears also took advantage of a park fish hatchery, dipping into its tanks for helpings of trout, which weren’t found in the park streams.
The hatchery closed in 1956, and the last feeding station was shut down in 1971. But the bears didn’t lose their appetite for human food. They raided campgrounds and concessions. From 1975 to ’85, human food made up about a third of the diet of the bears, according to the study, published in the March issue of Frontiers in Ecology and the Environment.
After the park installed bear-proof food lockers and started patrolling campgrounds for problem bears, the proportion of human food dropped to 13 percent of their diets, the level it was from 1915 to 1919.
“What we found was that the diets of bears changed dramatically after 1999, when the park [developed a plan] to keep human food off the landscape,” said Jack Hopkins, lead author of the study and a research fellow at the University of California at Santa Cruz who dealt with problem bears when he worked as a wildlife biologist in Yosemite.
Hopkins teamed with earth sciences professor Paul Koch to study bone and hair samples from park bears. The samples were obtained from museum collections and through field work.
The researchers compared samples from bears that ate only wild food, from bears known to forage for human food and from human hair, including some from the Smithsonian Institution that had been collected in 1940.
The meat- and corn-based products that people eat leave different chemical traces in bones and hair than wild food does. Those traces create a record of the bears’ culinary habits.
Hopkins “searched far and wide to get the collection of samples we analyzed,” Koch said.
All that work made the study powerful enough to answer the question of how management practices affect bear diets.