The grizzly bear (Ursus arctos horribilis) has rebounded from near extinction to stable populations in several recovery zones in the contiguous United States. Despite this recovery, the species remains protected under the Endangered Species Act in the lower 48 states. Tracking annual grizzly bear mortalities is a fundamental component of management, providing data to assess population health and inform conservation strategies. The annual death count indicates the challenges the species faces as human development encroaches on bear habitat.
Annual Mortality Rates and Geographic Context
Determining a single, national annual mortality number for the grizzly bear is not practical because management and population density vary widely across North America. In the United States, the focus is on the two main recovery areas in the lower 48 states: the Greater Yellowstone Ecosystem (GYE) and the Northern Continental Divide Ecosystem (NCDE). These areas provide the most detailed and consistent mortality data, which is tracked by federal and state agencies.
The Greater Yellowstone Ecosystem (GYE), centered around Yellowstone National Park, has seen annual mortality figures ranging between 45 and 75 known and probable deaths. High conflict years have recorded mortalities substantially above the 10-year average of roughly 54 deaths annually. The Northern Continental Divide Ecosystem (NCDE), which includes Glacier National Park, also tracks high mortality. NCDE analysis often focuses on the six-year running average for independent bears, which has hovered around 40 deaths within the Demographic Monitoring Area.
The fluctuations in these figures are often tied to environmental factors, such as drought or the availability of natural food sources, which can push bears into greater contact with human communities. These regional numbers are in sharp contrast to figures from Alaska, where a large, unthreatened population allows for regulated hunting, resulting in an order of magnitude higher annual mortality. In the lower 48, the management goal is to keep mortality within prescribed limits to ensure continued population growth and genetic stability.
Primary Mechanisms of Human-Caused Mortality
The vast majority of documented grizzly bear deaths in the recovery zones—typically between 85 and 90 percent—are directly attributable to human activity. Human-caused mortalities are primarily categorized into three mechanisms. The first category is Management Removals, where wildlife officials euthanize or permanently remove bears due to chronic conflicts with humans, often involving livestock depredation.
Management removals are typically taken only after a bear has become food-conditioned or exhibited repeated problem behavior that poses a threat to public safety or property. Bears repeatedly drawn to livestock are often the targets of these actions, especially outside core protected areas like national parks. Increased removals associated with livestock conflicts have contributed to recent record mortality years in the GYE.
The second mechanism is Defense of Life or Property, where private citizens kill a bear. These incidents frequently occur during hunting seasons for other game, with bears being shot in perceived self-defense when they approach a hunter or a carcass. Every such death must be reported to the authorities and is subject to an investigation to determine if the action was justifiable under state and federal law.
The final category is Accidental Mortality, which includes vehicle collisions and train strikes. As human infrastructure expands, the risk of vehicle collisions and train strikes increases, particularly near major highways and rail lines that bisect bear habitat. This category also includes illegal kills, or poaching, where a bear is intentionally shot but the incident is not reported or is falsely attributed to another cause.
Natural Causes and Conservation Implications
Natural causes of death account for a relatively small percentage of the overall mortality, often less than 15 percent. The most common natural cause of death is intraspecific conflict, which occurs when one bear kills another. This is particularly frequent among males, who may kill cubs or smaller males during territorial disputes or while seeking to breed with a female.
Younger bears, such as cubs and yearlings, face higher natural mortality rates than adults, with a significant portion of their deaths resulting from these aggressive encounters with larger bears. Tracking these deaths helps biologists understand the density and social dynamics of the bear population. However, the conservation focus remains firmly on the human-caused deaths because they are largely preventable and directly impact population recovery.
Wildlife managers use the annual mortality data to ensure the total number of deaths does not exceed the Maximum Allowable Mortality (MAM) thresholds, which are established limits designed to maintain a stable or growing population. Grizzly bear populations are particularly sensitive to the loss of reproductive-age females due to their slow reproductive rate. Consequently, the MAM for females is set much lower than for males, making the death of every adult female a serious management concern.