Effects of food on Demography & Distribution
Effects of Fruit on Demography & Distribution
These graphics show (A-C) variation in availability of major fruit crops to grizzly bears in the Cabinet-Yaak Ecosystem (CYE) and western portions of the Northern Continental Divide Ecosystem (NCDE); (D-F) demographic responses to this variation; and (G) associated changes in grizzly bear distribution in western portions of the NCDE. Annual variations in fruit crops were monitored by Wayne Kasworm and Bruce McLellan; associated changes in grizzly bear population density and growth rate were estimated by Bruce in the North Fork of the Flathead River Valley (E). The geospatial focus of these graphics is on areas where bears rely most heavily on fruit, shown is shades of red in (G). The most notable patterns are a synchronous dearth of fruit from huckleberry, serviceberry, and buffaloberry during a “berry famine” that lasted 1998-2008; related spikes in grizzly bear mortalities in (D) the CYE and (F) western portions of the NCDE (in striking contrast to in eastern portions, where mortality actually declined); associated declines in bear density and population growth rate in the North Fork of the Flathead (E); together with a major increase in distribution of bears westward from the NCDE (G). The berry famine clearly drove a spike of largely human-caused grizzly bear mortalities, declines in bear densities, and a rapid synchronous increase in bear distribution, presumably as bears ranged more widely in search of alternate foods or came into conflict with humans for the same reason.
Effects of Whitebark Pine on Conflict & Demography
These graphics summarize the documented effects of variation in availability of whitebark pine seeds on grizzly bears in the Greater Yellowstone Ecosystem during 1975-2002, prior to the loss of most mature whitebark pine to an unprecedented outbreak of mountain pine beetles. Figure (A) shows relative availability of whitebark pine with increasing distance to residential areas and roads in Yellowstone National Park (gray dots and lines) along with distributions of grizzly bears during years with good (tan) versus poor (burgundy) seed crops. This change in distribution reflected consumption of pine seeds by bears, exposure to people, and related levels of human-bear conflict, manifest in a negative relationship between consumption and conflict (B). More conflicts naturally led to more bear deaths during years of poor seed crops (C), along with reduced survival rates (D), and population growth rates (E). These results are from Mattson et al. (1992); Mattson et al. (2004) and unpublished data; Mattson (1998); Schwartz et al. (2006); and Pease & Mattson (1999).
These two graphics compare relations between availability of whitebark pine seeds (i.e., cones) and conflicts (indicated by conflict-related bear captures; [A]) and human-caused mortalities (B) in the Greater Yellowstone Ecosystem (GYE), contrasting relations for the period after most whitebark pine had died (2003-2018) compared to before (1983-2002). The basic nature of relations did not change between these two periods, although cone crops were perpetually smaller after 2002, simply because most mature whitebark pine had died. The tendency for comparatively greater numbers of bear to die at a given crop size during 2003-2018 is likely attributable to a dietary shift by bears to eating more meat, much of which led to increased conflicts with people.
Effects of Whitebark Pine & Army Cutworm Moths on Distribution
These three maps show increases in distribution of the Greater Yellowstone grizzly bear population relative to major environmental changes for three time periods : (A) 1990-1998; (B) 1998-2008; and (C) 2008-2018. Grizzly bear distributions are shown in green, with boundaries for beginning and end years of each transition differentiated by gray lines. Areas of major increase in distribution are identified by arrows. Per annum rates of growth in distribution for each period are also shown in the upper right-hand corner of each map. The white dashed lines delineate areas within which major environmental changes were concentrated. Areas burned during 1988 are denoted by dark red in (A), as is the area around Yellowstone Lake within which bears that consumed cutthroat trout tended to concentrate. Green dots denote sites where bears fed on army cutworm moths beginning in the mid-1980s. Foraging on moths increased dramatically between 1985 and 1995; cutthroat trout were driven to functional extirpation as a bear food between 1995 and 2005; and the 1988 fires burned extensive forest areas that had previously been used by bears to forage on whitebark pine seeds. The areas shaded red and burgundy in (C) denote heavy to near-total mortality of whitebark pine caused by an unprecedented outbreak of mountain pine beetle between 2002 and 2009. Key take-away points are, first, increases in distribution have accelerated over time, far in excess of any increases in population size, and, second, areas with the most rapid increases in distribution are associated with major changes in availability of important foods.