Terrestrial meat
The information on this page only covers the distribution and taxonomic composition of terrestrial mammal foods that are currently common in the diets of interior North American grizzly bears. The behaviors exhibited by bears while feeding on these foods are also obviously of interest. However, rather than trying to cover characteristic behaviors as well as dietary content and distributions all on the same page, I feature behaviors on separate pages, one each devoted to predation and exploitation of rodents, which can be found by clicking on the following links:
The taxonomy of mammals in grizzly bear diets
The graph above left shows trends in relative dietary importance of different terrestrial mammals as you move from the arctic to temperate latitudes of the contiguous US--from the Barren Grounds to Yellowstone. I've identified what appear to be relatively discrete breaks in consumption of different groups of foods, which I call "economies." These economies are based on differences in the vegetal composition of diets (see Vegetal foods) as well as differences in consumption of different mammals. The map above right shows these economies along with the joint distributions of terrestrial mammals that are prominant in grizzly bear diets, including not only ungulates, but also the widely consumed arctic ground squirrel. These figures only feature species that are common in the diets of several grizzly bear populations.
Perhaps the most striking pattern evident in grizzly bear consumption of terrestrial mammals is the dietary dominance of caribou and ground squirrels at high latitudes--in the Barren Grounds. These regions are typified by large migratory caribou herds that provide grizzlies with vulnerable prey in the form of calves or animals distracted by the rigors of river-crossing. Ground squirrels are not only abundant but also vulnerable at these high latitudes because permafrost limits them to shallow depths where they are accessible to grizzly bears. South of widespread permafrost and migratory caribou, in the depths of boreal forests, moose rather than caribou become the main source of meat for grizzlies, again often in the form of newborn calves. Farther south yet, in temperate latitudes, deer and elk become the most heavily exploited sources of meat, along with moose in interior British Columbia. Although elk are an abundant source of meat in places such as Yellowstone (as carrion, calves, or adults that are killed outright), deer are rarely so anywhere. The comparatively small-bodied deer are too agile for grizzlies to efficiently prey on outright and are not readily available even as carrion because of intense competition from other smaller scavengers such as coyotes. That being said, most deer meat is, in fact, consumed by grizzlies as carrion either scavenged from winter kills or kills made by human hunters.
Bison are currently important only in the diet of grizzly bears in Yellowstone, which is the only ecosystem that now supports both species. As I observe under Pre-European diets, there is ample evidence for bison being one of the most important foods of grizzly bears throughout the Great Plains and in much of the interior western US prior to the arrival of Europeans and the resulting widespread extirpations of both grizzlies and bison. Which is to say, Yellowstone's grizzlies provide us with a rare window onto a relationship with bison that was once widespread and perhaps profoundly important.
Determinants of dietary meat
Garth Mowat undertook an analysis of grizzly bear diets in North American, with an emphasis on consumption of terrestrial meat (follow this link to his paper). The map at left immediately above is modified from one presented in his paper, and shows the relative amounts of meat in diets of interior grizzly bears, with darker hues representing greater amounts. I've added dotted lines to this map that delineate areas with scant terrestrial meat in bear diets, where either berries (areas within the blue-dotted line) or spawning salmonids (within the white-dotted line) are the dominant sources of energy. As you can see, terrestrial mammals are a major, if not majority, dietary element in the Barren Grounds and in interior regions at the eastern extremities of current grizzly bear distribution. As you move out onto the Plains or deeper into interior boreal forests, meat from mammals becomes all the more important. Unfotunately, in temperate latitudes much of this meat currently comes from livestock.
The x-y graphs at upper right show two of the strongest relations between terrestrial meat consumption and landscape features, again thanks to data provided by Garth Mowat (these graphs represent the results of my own analysis of these data). As you can see, grizzlies consume more terrestrial meat in drier flatter regions, which coincides with the Barren Grounds, interior boreal forests, and the Great Plains. Why this pattern? Perhaps because meat from ungulates in particular is comparatively more abundant under these conditions. By contrast, one might expect there to be a combination of greater vegetal productivity, more diverse habitats and foods, and fewer hooved herbivores in wetter mountainous ecosystems such as interior British Columbia where berries dominate grizzly bear diets.
This expectation is confirmed by looking at the diets of four grizzly bear populations arrayed progressively from the foothills of Alberta, through Alberta's mountains, south and coastward to interior BC, and then farther south and coastward yet to the Cabinet-Yaak ecosystem of Montana. These diets are shown in the inset diagrams above in the middle, with shifts in seasonal diet shown from spring (left) to fall (right; the top three graphs are adapted from a paper by Lopez-Alfaro and colleagues). The contributions of berries are shown in blue and of meat in red. Berries tend to be less important in the Alberta diets compared to the diets of grizzlies in BC and far northwestern Montana along the gradient of declining meat and increasing berry consumption. Farthest south and coastward, most meat is consumed during spring and late fall, primarily as carrion from winter-killed ungulates (spring) or ungulates killed by hunters (fall). Importantly, these early and late seasons coincide with lower overall levels of feeding activity, which considerably diminishes the contribution of spring and late fall foods to the overall energetic budget (see Nutrition). Notice, also, that peak consumption of foliage (shown in green) occurs progressively earlier in the year as average temperatures warm--from the Alberta mountains to the Cabinet-Yaak.
The gradient in consumption of meat by grizzlies, going from the more clement and maritime west to the more arid, flat, and continental east is perhaps most dramatically evident in the Northern Continental Divide ecosystem of north-central Montana, just east of the Cabinet-Yaak. The map at right is centered on this roughly 40,000 km2 ecosystem, and shows the relative abundance of berry-producing shrubs as different hues of green (darker means more berries) as well as a series of different-colored dashed lines that correspond to isopleths of meat consumption for grizzly bears in this part of the world (the isopleths come from recent work published by Rick Mace). The numbers associated with each isopleth are percentages of total dietary energy contributed by meat, based on analyses of carbon and nitrogen isotopes in grizzly bear tissues. The bottom line: Within a short distance you go from a diet that contains very little meat in the west to a diet that is comprised, energetically, of >80% meat in the east--on the high plains. This dramatic transition clearly reflects differences in the availability of berries as well as, perhaps, the abundance of large hooved herbivores. If nothing else, there are certainly more cattle to the east.
