Use of stable isotopes to estimate the dietary responses of black bears to changing management regimes in Yosemite National Park

Background/Question/Methods

We used carbon (d¹³C) and nitrogen (d¹⁵N) stable isotopes to investigate the dietary plasticity of black bears (Ursus americanus) that foraged on anthropogenic foods (human foods, non-native trout) over the past century in Yosemite National Park. The purpose of our study was to estimate proportional dietary contributions for bears through time and to use these parameter estimates to evaluate the human-bear management program. We conducted stable isotope analysis on museum specimens (hair from bear pelts, and bone collagen from bear skulls and trout fins) collected in 1915–1985. We used these isotope values as well as isotope values derived from human hair collected over the past century and bear hair collected in 2001–2007 to estimate proportional dietary contributions for bears using the stable isotope mixing model, IsotopeR. We assumed that the average proportion of anthropogenic foods in the diets of bears was a proxy for the availability of these foods to the bear population. We used a novel Bayesian approach to compare anthropogenic dietary contributions among four time periods when different anthropogenic foods were available to bears in the park. We considered a decrease of human foods in the diets of bears between time periods as an indicator that management was successful in reducing the amount of human foods available to bears.

Results/Conclusions

The diets of black bears changed significantly through time, suggesting a high level of dietary plasticity. The average contribution of human foods increased in diets of bears (13% to 27%) from 1915–1919 to 1928–1939 due to the park intentionally feeding bears at designated areas; the average contribution of trout did not increase (3% to 4%) during these time periods. Some bears might have overcompensated for the loss of protein-rich trout from local fish hatcheries (1895-1956) and human foods at feeding areas (1923–1940) and open pit garbage dumps (1915-1970) by foraging on human foods in developed areas such as campgrounds and hotels (35%; 1971–1998). In 1999–2007, the average proportion of human food decreased in bear diets (13%), suggesting the human-bear management program was successful at reducing the availability of human foods to levels similar to those in the early 1900s. This result is remarkable considering the park received several thousand visitors each year in the early 1900s and nearly four million visitors each year today.