Millions of people visit Yosemite National Park each year, resulting in an influx of human-derived foods. Some bears seek out these foods on a daily basis, leading to conflicts with people and/or their property. Developing management strategies to prevent human-bear conflicts is essential for the protection of bears and people. In 1999, Yosemite began receiving $500,000 to implement a management strategy that focused on preventing bears from acquiring human foods, while managing bears involved in conflict. Although recorded incidents have decreased over the past decade, it was unclear if the availability of human food (the main problem) has decreased throughout the Park and if the current management methods should continue to be used. In this study, we addressed four important questions related to the management of black bears conditioned to forage for anthropogenic foods in Yosemite.
How many bears forage for human foods in Yosemite?
1) Measure the abundance of stable isotopes in the tissues of bears that are known to forage for human foods, bears known to forage for plants and animals exclusively, and bears sampled throughout Yosemite (via hair-snare) with unknown diets
2) Model the relationship between bears with known diets and stable isotopes derived from their hair
3) Predict the diets of bears sampled throughout Yosemite with unknown diets
Wildlife Genetics identified 109 and 115 bears in 2006 and 2007, respectively (n = 195; 29 recaptures). We conducted carbon and nitrogen isotope analysis on 167 hair samples from 144 bears (23 recaptures). We used stable isotopes from bear hair to detect 33 different bears that forage for human foods and 111 that foraged for plants and animals exclusively. Download PDF
Has the amount of human food decreased in the diets of bears through time?
1) Develop a stable isotope mixing model used to estimate the diets of bears Download PDF
2) Estimate dietary contributions (% human foods, % trout, % plants and animals) for bears over the past century using stable isotopes derived from bear bone and hair sampled from the museum and field, respectively
3) Compare the proportion of human food in bear diets through time to determine if the recent management program was successful in reducing the amount of human food available to bears
The average proportion of human food in the diets of bears increased from 13% (<1923) to 27% (1924–1970) due to the increased number of open-pit garbage dumps and feeding areas in Yosemite. The average contribution of human foods (35%) in the diets of these bears did not decrease in 1971–1998 even though garbage dumps and feeding areas had closed in Yosemite. Bears likely compensated for the loss of this consistent food source by foraging for human foods in developed areas such as campgrounds and hotels. The contribution of human food in bear diets decreased significantly to 13% in 1999–2007, suggesting the park was recently successful at reducing the amount of human food available to the bear population. Download PDF
Do bears learn to forage for human foods from their mothers?
1) Estimate relatedness coefficients and most probable relationship (parent–offspring, full-siblings, half-siblings, and unrelated) for each combination of 2 independent bears (n = 150) sampled in 2004–2007
2) Test predictions deduced from 4 hypotheses that each explain a primarily mechanism responsible for black bears foraging for human foods in Yosemite
Evidence suggests that mother-offspring social learning is the primary mechanism responsible for black bears foraging for human foods in Yosemite. Evidence suggests that some bears learn to forage for human foods as innovators, but no evidence supports the claim that bears inherit food-conditioned foraging behavior from their parents. Download PDF
What happens to bears that are transported from developed areas to other locations in the Park?
1) Determine if bears that forage for human food and bears that do not forage for human food (juveniles and adults) are equally likely to return to developed areas following transport
2) Determine the ultimate fate of transported bears
We found that bears that foraged for human foods (n = 20) were more likely to return to developed areas than bears that did not forage for human foods (n = 9). Of the 16 returning bears, 15 foraged for human foods (9 juveniles, 6 adults) and one did not. The other 8 bears that did not forage for human foods never entered developed areas. By 2011, 65% of bears that foraged for human foods (13 of 20) were euthanized by wildlife management personnel (n = 10) or harvested near developed areas (n = 3).