Diet & Habitat

of Grizzly Bears in the Northern Rocky Mountains 

This section of Mostly Natural Grizzlies is devoted to major foods eaten by grizzly bears in the northern US Rocky Mountains, as well as the habitats where bears consume them. The two are inextricably linked. This topic is particularly important because useful insights into demography (i.e., birth, death, immigration and emigration rates) are necessarily rooted in functional understandings of diet and habitat use. By derivation, effective management and conservation cannot occur in a vacuum, divorced from insight into how diet, nutrition, use of space, and configurations of habitats affect reproduction and survival.

Perhaps paradoxically, the focus of wildlife managers and researchers in the northern Rockies has shifted away from, rather than increasingly towards, better understandings of grizzly bear ecology. Obsessive attention is paid to population size and trend. There are numerous reasons for this trend, but one major driver for state wildlife managers is the political imperative to remove federal Endangered Species Act protections for grizzly bears and claim authority over management. Emphasis on numbers of bears has followed from the extent to which this metric is central to deliberations over removal of protections, divorced by these state managers as much as possible from ecological context. Because of all this, the honey-trail of research funding has led researchers to downplay complexities and details of ecological context, and instead dote on arcane estimates of demographic parameters featuring the latest and greatest statistical gimcrackery. 

So, the content of this section on Diet and Habitat reflects several motivations (as with all other sections). The first is simple curiosity. By compiling and synthesizing everything we know about what Rocky Mountain grizzly bears eat and where they eat it, I hope  to satisfy a burning curiosity about these bears and their lives. The second motivation is a desire to share insights that do justice to the bears and their environments. Put another way, I have no interest in sharing palliatives or partisan distortions to serve a political agenda. As a corollary, my third major motivation is to correct and otherwise remedy the impoverished narrative being served to the public by government representatives.

 

With that as context, what follows on this introductory page is a brief overview of conditions that shape diets of grizzly bears in the northern Rocky Mountains. Details related to specific foods or groups of foods can be found by following the links immediately above at right. I start with climate, and then provide a summary of bear diets in reference to regional distributions of key foods, emphasizing fruit, large mammals, and pine seeds which are, by and large, the most important high quality bear foods in this part of the world.

Climate, together with soils and geological substrate, determine biological productivity, including the form of dominant plants and the abundance of herbivores adapted to consume them. Forests of different climatic zones have a definitive biotic signature, as do herbaceous communities such as grasslands, forb meadows, and tundra. Humans can profoundly reconfigure certain aspects of all this by usurping primary productivity for anthropocentric purposes, but grasslands, for example, endure and, whether bison or domestic cattle, are productive environments for bovids. All of these effects ramify up through the trophic levels to dictate terms of existence for apex vertebrates such as grizzly bears.

  

The figures at left are thumbnail descriptors of climate in different ecoregions that host grizzly bears. The various recovery areas (AKA Primary Conservation Areas) are delineated in red or white, featuring those in the Cabinet-Yaak, Northern Continental Divide, and Yellowstone ecosystems. Current grizzly bear distributions are in beige in the map top left. The four inset diagrams show average monthly temperatures (top) and precipitation (bottom) for a number of weather stations in each of four regions, with each line representing monthly trends for an individual station. Average monthly temperatures that dip below freezing are shown in blue. The tryptick of maps at the bottom render annual temperature, annual precipitation, and percent of precipitation that falls during winter in spatial form. Paler blue denotes colder temperatures; orange less and blue more precipitation; and dark blue shading to purple, a higher percentage of winter precipitation.

Climates

Seminal features of climate for grizzly bears include absolute amounts of precipitation, the seasonality of when it falls, and whether warmer or colder. All areas occupied by grizzly bears in the northern US Rockies are comparatively cold, but the broad sweep of high-elevations in the Yellowstone ecosystem are coldest of all. Average monthly temperatures for most stations in this area dip below freezing six months of the year, in contrast to only three to four months for stations in the Cabinet-Yaak ecosystem and along the western flanks of the Northern Continental Divide (NCDE). The eastern slopes of both Yellowstone and the NCDE are distinctly dry (8-15 inches per annum), with relatively pronounced peak moisture early in the growing season, during May and June. This seasonality promotes herbaceous vegetation and the grazers that subsist on it; e.g., bovids and elk. By contrast, precipitation is more abundant (20-40+ inches) and arrives in both a winter and early summer pulse elsewhere in occupied grizzly bear habitat. This winter-heavy precipitation promotes the growth of woody vegetation, resulting in more profuse fruit-producing shrubs as well proportionately more numerous browsers, notably moose and deer--especially in the NCDE and Cabinet-Yaak. All of this manifests in regional grizzly bear diets, past and present.

The map at left shows the distribution of five historically-important grizzly bear foods in the northern Rocky Mountains: Bison, in beige; Spawning salmonids, west of the blue dashed line; Whitebark pine, in purple; Gambel oak, in brown; and Fruit-producing shrubs, in various shades of green. These shrubs include globe huckleberry (Vaccinium globulare), serviceberry (Amelanchier alnifolia), buffaloberry (Shepherdia canadensis), and chokecherry (Prunus virginiana). Bison were most abundant to the east, along with whitebark pine and oak, whereas berries and salmon were most abundant to the west. These regional differences translated into different economies for grizzly bears, including a salmon-berry economy from northwestern Montana west; a bison-berry-whitebark pine economy to the east; and an acorn-berry-bison economy to the south. Of the berries, huckleberry progressively dropped out farther east, shifting to greater prominence of serviceberry and chokecherry on or near the Great Plains. (For more, follow this link to AllGrizzly).

Pre-European Economies

Contemporary Diets

Contemporary grizzly bear diets in the northern US Rockies are differentiated primarily by amounts of fruit and herbaceous vegetation consumed compared to the amounts of meat and pine seeds. Insofar as meat sources are concerned, bison are gone everywhere other than the Yellowstone ecosystem, replaced by domestic cattle, but elk remain abundant in the mountains and nearby lowlands, as do deer.

The maps at right feature contemporary distributions of major foods, as above, along with synoptic representations of five grizzly bear diets each represented by a pie-diagram. These diagrams are based on the results of analyzing literally thousands of scats from different study areas, corrected to represent ingested diets. Diagrams in the top figure are highlighted to show contributions of fruits (orange) and grazed foods (shades of green), whereas those in the bottom figure are highlighted to show meat (dark brown), roots (next-darkest brown), pine seeds (brown), fish (pink), and insects (gray).

 

The basic trend is from fruit being the single largest dietary component to the north and west (roughly 35-40%), to meat and pine seeds being dominant to the south and east (20% pine seeds and near 50% meat). Yellowstone represents the extreme end of this gradient, with the East Front farther north intermediate.

These trends manifest differences in abundance of berry-producing shrubs (greatest in northwestern Montana), whitebark pine (historically greatest in the Yellowstone region), and numbers of elk and bison. Elk habitat is denoted by areas shaded tan. Estimated elk populations are given for each region, with fewest in northwestern Montana (c. 4,300) and most in Yellowstone (over 67,000). These differences in numbers of elk reflect differences in grazing resources, which reflect, in turn, amounts and timing of precipitation (see above).

 

One key proviso to all of this is that the world of grizzly bears has not remained static, even during recent decades. Estimates of diet all come from data collected primarily during the 1970s, 80s, and 90s. As I cover in more detail elsewhere, since 2000 whitebark pine has been functionally eliminated as a bear food in most places by disease and insects. The cutthroat trout that comprised roughly 12% of ingested foods in Yellowstone (the pink slice) are also functionally gone (see the entry on trout). Likewise, kokonee salmon in Flathead Lake, which grizzlies consumed while spawning in McDonald Creek, are functionally extinct, which means that the 9% contribution of fish to the diet of Glacier grizzly bears has diminished to nothing. 

To be completed