Patterns of Mortality

Regional Patterns of Grizzly Bear Mortality

The pie diagrams in this figure represent proportional causes of grizzly bear mortality for different regions of the US Northern Rocky Mountains, with percentages given for dominant causes. The key for causes is shown in the bottom left-hand corner; gray denotes miscellaneous other causes, including natural ones such as predation and disease. Causes vary substantially from one region to another, but with shared patterns moving from west to east. Grizzly bears in the Montana and Wyoming portions of the Greater Yellowstone (GYE) and along the East Front of the Northern Continental Divide (NCDE) die much more often because of conflicts with big game hunters and livestock producers compared to bears farther west. Grizzly bears in the Selkirk Mountains and Cabinet-Yaak die proportionately much more often from poaching (i.e., malicious killing) and because of mistaken identifications by black bear hunters. Bears on the west side of the NCDE, in areas near the Flathead and Mission Valleys, die far more often because of collisions with vehicles and conflicts over attractants at human residences—both of which are associated with high densities of people living in and travelling through this region.

Trends in Grizzly Bear Mortality

These graphs show trends in total numbers of known and probable grizzly bear mortalities, broken down by major causes, for the Greater Yellowstone (GYE) and Northern Continental Divide (NCDE) Ecosystems : in the GYE for (A) Wyoming, (B) Montana, (C) Idaho, and (D) National Park Service jurisdictions; in the NCDE for (D) the entire ecosystem, 1978-2018, differentiating deaths on the west and east sides, 1999-2018; and (E) by major causes for the entire ecosystem, 1999-2018. Gray lines and darker gray dots denote total mortality whereas each major cause is shown by different colored bands. Periods of major food shortages are shown by vertical bands of orange-colored shading, including the terminal decline in availability of whitebark pine seeds in the GYE and a period of widespread berry shortages (“berry famine”) in the NCDE. Note the substantial increase in bear mortalities in  Wyoming and Montana portions of the GYE coincident with loss of whitebark pine seeds , driven largely by conflicts with humans over meat, and the spike in bear mortalities coincident with the berry famine, but driven largely by increases on the west side of the NCDE.  Berries are a more important food source on the west compared to east sides of the NCDE. Data are from datasets obtained under Open Documents and Freedom-of-Information Act requests for 1959-2014 in the GYE and for 1999-2018 in the NCDE. Other data on mortalities come from Interagency Grizzly Bear Study Team Annual Reports for 2015-2018 and from Dood et al. (2006). Data for estimating abundance of whitebark pine seeds come from Macfarlane et al. (2013), Van Manen et al. (2016), and Interagency Grizzly Bear Study Team Annual Reports for 1979-2018. Data for estimating berry abundance come from McLellan (2015) and Kasworm et al. (2018).

Distributions of Mortalities in Greater Yellowstone

These three maps show the distribution of grizzly bear mortalities in the Greater Yellowstone Ecosystem during 2012-2018 relative to different causal factors. Map (A) identifies clusters of mortalities that are shown in (B) and (C). Map (B) shows clusters relative to areas with heavy whitebark pine mortality during 2002-2009 (denoted by red and burgundy) and sites where grizzlies fed on army cutworm moths (denoted by green dots). Map (C) shows mortality clusters relative to the human footprint (in shades of yellow to orange) and public-land grazing allotments differentiated by whether these allotments experienced chronic bear depredations (red), something less than chronic conflicts (orange), or had been retired (green). The major points to be taken away from these maps are, first, many mortality hotspots are spatially associated with heavy losses of whitebark pine that were not offset by nearby availability of moth sites (hotspots 1, 3, 9, 10, 11); second, a heavier human footprint was associated with only a handful of hotspots (2, 4, 5); and, third, most hotspots were associated with public-land grazing allotments in areas with heavy losses of whitebark pine, but with the Upper Green River area (hotspot 11) standing out from all of the rest. Notably virtually no grizzly bears died in grazing allotments that had been retired.

Distributions of Mortalities in the Northern Continental Divide

These maps show (A) the distribution of grizzly bear mortalities in the Northern Continental Divide Ecosystem during 2004-2018, superimposed on (B) different causal factors. Map (A) also shows mortality hotspots. Map (B) shows the distribution of mortalities relative to the human footprint (shades of yellow to orange), major highways (red lines), and roaded US Forest Service lands (tan-brown), with lands devoted to timber harvest  shown in dark brown. The human footprint represents any permanent appropriations of land for human purposes, including croplands on plains to the east of the main distribution of grizzly bears. The main takeaways are, first, the striking concentration of mortalities along major highways (hotspot 13), especially when superimposed on agricultural lands (hotspots 8 & 9); and, second, an equally striking concentration of mortalities on or near roaded US Forest Service lands (hotspots 1, 2, 10, 11, and 12). Increasing numbers of bears are also dying along riparian corridors passing through agricultural lands on the plains to the east of the Rocky Mountain Front.

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