Trends in Food & Habitat

These graphs show trends in availability  of elk and bison as well as consumption of terrestrial meat  by grizzly bears in the Greater Yellowstone Ecosystem (GYE)–all with consequences for levels of meat-related conflicts with. Humans. Graphs (A) and (B) show two indicators that reveal substantial increases in meat consumption by grizzly bears in the GYE coincident with terminal losses of whitebark pine seeds (the period denoted by vertical shaded orange).  Graphs (C-D) show trends in total numbers of bison and elk 1985-2019; graphs (E-F) show trends in total numbers of carcasses of each species available  to scavenging grizzly bears during spring on ungulate winter ranges. Note the concilient decline of both live and dead elk preceding and during the period of terminal whitebark pine  loss, and the paradoxical disconnect between numbers of live and dead bison. The decline in numbers of bison carcasses is of consequence given that bears consume almost all meat from bison by scavenging. The increasing reliance of bears on meat coincident with declines in availability of this resource from native ungulates has resulted in grizzlies increasingly exploiting livestock and the remains of elk killed by big game hunters, leading to the substantial increases in numbers of bears being killed by humans because of meat–related conflicts in both Wyoming and Montana, evident in graphs (g-h).

Trends in Availability & Consumption of Meat
Trends in Availability of Whitebark Pine Seeds in the NCDE

These maps show levels of whitebark pine mortality caused by white pine blister rust (a non-native fungal pathogen) in the Northern Continental Divide Ecosystem (NCDE) circa 1991 (A) and 2005 (B). Comparatively healthy forests are shown in tan; areas with near total loss of whitebark pine are shown in burgundy. Whitebark pine seeds were an important food of grizzly bears along the East Front of the NCDE prior to 2000. Loss of pine seeds in this area probably contributed to increased grizzly bear activity on lower elevation agricultural lands along the East Front. Estimated areas of whitebark pine mortality were taken from  Keane & Arno (1993), Keane et al. (1994), and Retzlaff et al. (2016).

Trends in Availability & Consumption of Whitebark Pine Seeds in the GYE

These graphics show losses of whitebark pine in the Greater Yellowstone Ecosystem (GYE) between 1988 and 2018 to wildfires (A) and an unprecedented outbreak of mountain pine beetles unleashed by a warming climate (B). Healthy whitebark pine forests are shown in tan, in contrast to areas of near-total mortality as dark brown to burgundy. Figures (C-E) show annual trends in whitebark pine forests (A), cone crops (i.e., cones per surviving tree; [B]), and total whitebark pine seed availability as a function of both tree and cone abundance (E). Losses of whitebark pine trees were temporarily offset by large cone crops during 2002-2009, after which landscape-level availability of cones/seeds  entered a phase of terminal decline. Maps (A) and (B) also show changes in distribution of grizzly bears during two different time periods, with little increase in distribution between 1990 and 1998, in contrast to a major increase during 1998-2018, coincident with major losses of whitebark pine, and far in excess of any possible increase in the bear population.

Trends in Availability of Fruit in the NCDE

These graphics show trends in forest conditions driven by wildfires in the Northern Continental Divide Ecosystem (NCDE), with implications for availability of fruit to grizzly bears. Maps (F) and (G) shows areas burned during 1985-1999 and 2000-2018, respectively. Figure (E) shows annual trends in cumulative area burned in gray. Large wildfires burned during 1988 followed by a period of stasis, followed in turn by a succession of years, 2002-2010, during which large areas burned. This latter period coincided with a sustained drought. Graphs (A-D) show relations between fruit crop sizes for huckleberry and buffaloberry relative to time since a stand-replacing wildfire (A-B) and percent overstory forest cover (C-D). The upshot is that fruit crops of both species are predictably small for roughly 20 years following wildfires, after which they peak before declining as forest cover increases. Annual trends in the cumulative area of these transient unproductive conditions are shown in (E) in burgundy, juxtaposed with  the cumulative area of productive conditions in blue. Notably, the effects of wildfires have been concentrated in remote portions of the NCDE, with the probable effect of accelerating increased distribution of grizzly bears on the periphery, especially along the East Front.

Trends in Availability & Consumption of Army Cutworm Moths

These graphics show locations of sites in the US Northern Rocky Mountains where grizzly bears consume moths while aggregated in alpine areas (in yellow in maps [A] and [B] ), as well as trends in use of moth sites during 1980-2018 for grizzly bears in the Greater Yellowstone Ecosystem (GYE; C-E). As illustrated in map (B), moth sites are located exclusively in alpine areas (shown in green) that are not subject to heavy snowfall (denoted by blue; areas of >50 dm snow water equivalent). Use of moth sites by grizzlies in the GYE dramatically increased from essentially nil in 1985 to the point where most bears in the Absaroka Mountains exploited these sites during the  2000s (C). Numbers of bears seen per site have varied over time (in D), reaching a nadir during 1992 and jumping substantially to high sustained levels after 2011 coincident with terminal losses of whitebark pine  seeds as a food source for grizzlies (denoted by vertical orange shading). The importance of moths to bears concentrated near moth sites is  illustrated in (F) by the comparative fractions of different foods consumed, with moths accounting for over 80% of the total.

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