Yellowstone National Park is in bear country. The National Park Service, at its Yellowstone Web site, states the following about camping and hiking in bear country: Do not leave packs containing food unattended, even for a few minutes. Allowing a bear to obtain human food even once often results in the bear becoming aggressive about obtaining such food in the future. Aggressive bears present a threat to human safety and eventually must be destroyed or removed from the park. Please obey the law and do not allow bears or other wildlife to obtain human food. What negative externality does obtaining human food pose for bears? What negative externality do bears obtaining human food pose for future campers and hikers? Source: National Park Service, Yellowstone National Park, "Backcountry Camping and Hiking," June 7, 2013, ww.nps.gov/yell/plan your visit/backcountryhiking.htm.