The Economist explains
PADDINGTON loved marmalade. Winnie went for honey from a pot. But ecologists and rangers worry most about bears like Yogi, famous for stealing picnics from visitors to Jellystone Park. A new study led by Jack Hopkins from Montana State University and the University of California Santa Cruz, recently published in Frontiers in Ecology and the Environment, reveals what black bears in California’s Yosemite Park actually eat.
The research estimates the varying proportions of both human-derived food sources (including anthropogenic items and non-native trout) and natural food sources in the diet of bears over the past century. It proves that attempts to get bears eating less human grub are working.
Black bears in the park cause costly trouble in the pursuit of high calorie human treats. In the past 20 years, 12,000 incidents involving them have been reported: 50 people have been hurt and $3.7m worth of property damage has been caused (bears will smash car windows for tasty morsels locked inside according to Dr Hopkins). No one has been killed, however.
It makes biological sense for bears to seek provisions meant for people. Those that do so in Yosemite are larger, on average, have bigger litters and reproduce earlier than those that do not.
In an attempt to reduce the number of bear-related incidents at Yosemite, and prevent the animals from eating human foods, Congress has funded the interdivisional human-bear management programme since 1999. It has cost around $7.5m, but the study suggests that the park has successfully reduced the amount of human food available to bears over the past decade (mainly through the cooperation of most of its 4m seasonal visitors).
The team collected specimens from museum-housed Yosemite black bear skulls and pelts, and from hairs of modern bears using hair-snaring devices installed throughout the park. They then analysed the rare carbon and nitrogen stable isotopes within them. Specimens collected were partitioned by time periods: 1890-1922; 1923-1971; 1972-1998 and 1999-2007. Each denotes a major shift in human-bear management strategies.
Because stable isotopes do not undergo radioactive decay they can serve as reliable tracers in food chains. Researchers related isotopes from bear foods to the isotopes assimilated in their bone and hair. This provided dietary information specific to the time frame in which their tissues grew.
Relatively high rates of rare carbon and nitrogen isotopes in black bear tissues from Yosemite suggest a plant-based diet of corn and a protein rich diet of meat—eating habits that show the creatures in question were used to dining on human vittles.
Ecologists then estimated the amount of human-derived food in bear diets through time by constructing models that revealed how the isotopic composition of bear hair would have appeared had a creature been feeding on each source of food exclusively. For example, they used human hair from the Smithsonian (collected in 1940) to represent what the isotopic composition of bear hair would have looked like if the scrutinised creature had foraged only for human fare. This allowed the team to estimate the contribution of human items to bears’ diets through time.
Restricting the availability of anthropogenic foods in the park should reduce the amount of them in bear diets. Because it is impossible to measure the exact quantity of human food available to bears historically, the authors used the average contribution of human food in ursine diets as an indicator of how much was available to the animals on the ground during each time period. The team then compared the differing amounts of human foods in bear diets over the past century to determine how they changed in response to different management practices.
An increase in rare stable nitrogen and carbon isotopes was discovered in deceased black bears from 1915-1939, a time when, alongside the Happy Isles Fish Hatchery (which farmed non-native trout), artificial feeding sites for bears operated. The two kept ursine tummies well fed, and the sites, opened in 1923, 