Editor's note: The following story by Kerry Gunther, Yellowstone National Park's bear management biologist, appears in the latest edition of Yellowstone Science. You can find the entire edition here. Copyright Yellowstone Association, used with permission.

Although grizzly bear attacks on people in Yellowstone National Park are rare, they draw world-wide media attention and can be quite traumatic for park visitors, staff, and the general public both locally and nationwide when they happen. One of these rare attacks occurred in the park during the 2015 summer season, resulting in a human death, killing of the adult grizzly bear, and placement of two cubs in a zoo. This event was tragic, but also very unusual in the ecosystem, especially in light of the number of grizzlies and humans that could overlap in time and space.

Park managers strive to make the park as safe for visitors as possible, while still maintaining the park's wilderness character and protecting its resources. As part of these efforts, the park has an extensive Bear Safety Messaging Program that uses face-to-face interactions, social media, web pages, video, printed handouts, park newspaper articles and inserts, restaurant table tents, and roadside and trailhead signs to convey bear safety messages to park visitors. Documenting trends in bear attacks is one method managers use to gauge the efficacy of the park's Bear Safety Messaging Program. The following information includes statistics through the 2014 calendar year.

The number of grizzly bear-inflicted human injuries peaked in the 1940s at two injuries per one million park visits, but declined thereafter (figure 1, table 1). The number of bear-inflicted human injuries has averaged less than one injury per one million park visits each decade from 1970 to 2014. The year 1970 is considered the beginning of modern-day bear management in Yellowstone because previously most bears were conditioned to human foods and garbage. Food-conditioned bears are often involved in bear-human conflicts. In 1970, Yellowstone implemented a new bear management program. The foundation of the program was to prevent bears from obtaining human foods, garbage, or other anthropogenic (human) attractants to reduce bear-human conflicts (Meagher and Phillips 1983). By 1979, sources of anthropogenic attractants had been made bear-proof, most food-conditioned bears had been removed from the population (i.e., killed or sent to zoos), and bear-human conflicts declined significantly thereafter. This period in the late 1970s and early 1980s was most likely the low point for numbers of grizzly bears in the park and the ecosystem. From the mid-1980s through early-2000s, bear numbers increased at a fairly robust rate of 4-7% per year. Since the early 2000s, evidence suggests that the rate of increase for grizzly bear numbers in the park and the ecosystem has slowed, due primarily to density-dependent effects.

Grizzly Bear Attack History

Grizzly bear-inflicted injuries to humans in developed areas averaged approximately one per year during the 1930s through the 1950s, then increased to four per year during the 1960s. Grizzly bear-caused human injuries in developed areas decreased to one injury every two years (0.5 per year) during the 1970s. During the last 35 years (1980-2014), there have been only two (0.1 per year) grizzly bear-caused human injuries in developed areas (an average of approximately 1 every 18 years). The reduction in bear-inflicted human injuries within park developments during the period of increasing grizzly bear numbers is likely attributable to implementation of the new bear management program in 1970, which has been successful at preventing bears from obtaining human foods and garbage and keeping bears from becoming conditioned to anthropogenic attractants. From 1980 to 2014, one bear inflicted human injury occurred along a service road and one occurred on a boardwalk trail. Bear attacks along road corridors (3%) and boardwalk trails (3%) comprise very small proportions of total bear attacks.

During the 35-year period from 1980 to 2014, there were 33 human injuries caused by grizzly bears in backcountry areas of the park, an average of one per year. The vast majority of these were attributable to defensive aggression by bears during surprise encounters with hikers. Thirty-two of the 33 (97%) injuries occurred while people were traveling. Only 1 of the 33 (3%) backcountry bear attacks occurred in a backcountry campsite.

Risk of Grizzly Bear Attack

From 1980 to 2014, 37 people were injured by grizzly bears in Yellowstone (an average of 1.1 injuries per year). During that time period, the park recorded over 100 million visits. For all visitors combined, the chances of being attacked by a grizzly bear are approximately 1 in 2.7 million visits. The risk of grizzly bear attack is significantly lower for those visitors who do not leave park developments or roadsides, but significantly higher for those hiking in backcountry areas (table 2).

From 1980 to 2014, there were 100,436,902 visits recorded in Yellowstone. During that same time period, four people were injured in frontcountry areas of the park including developments (n = 1), roadside campgrounds (n = 1), roadside corridors (n = 1), and roadside boardwalk trails (n = 1). Therefore, the chances of being injured by a grizzly bear while in frontcountry areas of Yellowstone is approximately 1 in 25.1 million visits. Of the four people injured in frontcountry areas, one occurred in a roadside campground. From 1980 to 2014, there were 22,824,762 overnight stays in roadside campgrounds. Therefore, the chances of being injured by a grizzly bear while staying in a roadside campground in Yellowstone is approximately 1 in 22.8 million overnight stays.

Of the 33 people attacked in backcountry areas since 1980, 7 were on multi-day overnight trips and 26 were on day-trips. From 1980 to 2014, there were 1,396,299 multi-day overnight stays in the backcountry. Therefore, the chances of being injured by a grizzly bear while on a multi-day overnight trip in Yellowstone's backcountry is approximately 1 in 200,000 overnight stays. The park does not have statistics on how many park visitors day-hike in the backcountry, so the chances of being attacked by a grizzly bear while day-hiking in backcountry areas cannot be precisely calculated.

During the 143-year (1872-2014) history of Yellowstone National Park, 6 people are known to have been killed by grizzly bears inside the park (table 3), and one additional person was killed by a bear that was not identified to species. More people have died in the park from drowning (n = 119), falling (n = 36), suicide (n = 24), airplane crashes (n = 22), thermal burns (after falling into boiling thermal pools, n = 20), horse related accidents (n = 19), freezing (n = 10), and murder (n = 9) than have been killed by grizzly bears (n = 6). In fact, the frequency of people being killed by grizzly bears in the park (6 incidents in 143 years) is the same as being killed by a falling tree (n = 6) or in an avalanche (n = 6), and only slightly higher than the frequency of being struck and killed by lightning (n = 5) while visiting the park.

Challenges of Bear Safety Messaging

Bear safety messaging is especially challenging because, even though the consequences of bear attack can be quite severe (severe mauling and even death), the risk of attack for most park visitors is extremely small. This makes it difficult for visitors to understand the need for adhering to bear safety recommendations. The low rate of bear attack among frontcountry recreationalists suggests that bear management and safety messages for this type of recreationalist are effective, especially given the increases in grizzly bear numbers over the past several decades. Backcountry hikers have the highest risk of bear attack (approximately 1 in 200,000), but adherence to bear spray and hiking group size recommendations among this user group is low. In a 2011-2014 survey, only 13% of day hikers and 52% of backpackers carried bear spray (see "Visitor Compliance with Bear Spray and Hiking Group Size in Yellowstone National Park," this issue). The most common party size for both day-hikers and backpackers was two people per party, indicating that many day-hikers and backpackers did not follow the park's recommended group size of three people for hiking in bear country. Although backcountry hikers are probably more accepting of the inherent risks associated with recreating in bear country than other park visitors, new innovative messaging strategies may be needed to reduce the frequency of bear attacks on this recreational group.

There are no guarantees of safety when recreating in bear country. However, an awareness of the hazards can often mitigate the potential dangers. To learn more about safety in bear country, visit the park's web page at: go.nps.gov/yell/bearsafety