The information on this page only covers the distribution and taxonomic composition of vegetal foods that are currently common in the diets of interior North American grizzly bears. The behaviors exhibited by bears while feeding on these plants 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 grazing, root grubbing, and acquisition of fruits, which can be found by clicking on the following links:
Vegetal foods
Fruits & seeds
The graphs above left show trends in relative dietary importance of different berries 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." The maps above right show the joint distributions of the various berry-producing shrubs, differentiating those in the Arctic economy at top from those in the two interior economies at bottom. Parenthetically, these graphs and maps only feature those berry-producing species that are common in the diets of several grizzly bear populations.
To my eye at least, there are several interesting patterns. For one, there is clearly an ensemble of berries eaten by grizzlies in the arctic, comprised of bearberry (Arctostaphylos rubra), lingonberry (Vaccinium vitis-idaea), bilberry (V. uliginosum), and crowberry (Empetrum nigrum). All of these have impeccable credentials as arctic and alpine species evident in the map at top right. Straddling the latitudinal middle ground is huckleberry (Vaccinium membranaceum, but including V. myrtilloides), most prominantly in western areas with a maritime climatic influence. Farthest south, serviceberry (Amelanchier alnifolia) and chokecherry (Prunus virginiana) are well-represented in grizzly bear diets, with both very likely prominant in diets of now-extirpated grizzly bear populations as far south as Utah, Colorado, and the Sierra Nevada of California. Notice, though, that serviceberry is distributed as far north as Alaska and the southern limits of the Barren Grounds.
Buffaloberry (Sheperdia canadensis) is unique in that it is heavily consumed over the widest latitudinal range of any berry-producing species, from the arctic to the current southern limits of grizzly bear distribution, but primarily in drier interior regions. This widespread use is reflective of the wide latitudinal and longitudinal distribution of buffaloberry evident in the map above in the lower right quadrant (corresponding to map legends 1, 2, 3, & 4).
What about preferences? These are evident in contrasts between availability and levels of consumption for the various berry-producing species. Huckleberry seems to trump all of the other berries. In most places where they are available, grizzlies heavily consume huckleberries, regardless of whether any of the other important berry-producing shurbs are present. Second to this, the berries typical of an arctic diet are rarely consumed farther south wherever any other berries are abundant, even though bilberry, crowberry, and lingonberry are available well into boreal regions. This may be partly attributable to an increasingly patchy distribution of these three species at lower latitudes, typically at increasingly high elevations in the Rocky Mountains the farther south you go.
Whitebark pine
I threw whitebark pine in with the fruits, even though, technically speaking, the food item sought by bears is a seed, not a fruit. As you will find by looking at Pre-European diets, whitebark pine has probably always been important only within narrow a latitudinal zone centered on Montana, Idaho, and Wyoming. The limited extent of heavy consumption of pine seeds by bears is evident in the map at left, which shows the distribution of whitebark pine in the northern US and adjacent Canadian Rockies, which is the only area where noteworthy pine seed consumption has been documented in North America. Levels of consumption are denoted by the intensity of green shading, with concentrations in colder drier areas east of any substantial maritime climatic influence. Red squirrels are an important intermediary for grizzlies when consuming whitebark pine seeds. The squirrels harvest seed-containing cones from tree canopies, and then cache the cones in middens at ground level. At that point, the bears then to only need to seek out the squirrel middens and excavate the abundant cones cached within (see Mattson & Reinhart 1997 for more details).
Foliage & roots
Other vegetal foods include stuff that is grazed and excavated by grizzly bears--foliage and roots. As with fruits, above, I summarized the contribution of different grazed and excavated plants to the diets of North American grizzlies, ordinated as a gradient from north to south--from the arctic to interior temperate regions. The results are in the figure to the right, with grazed foods in shades of green at top and root foods in shades of brown at bottom. Because not all diet items were differentiated by species in the studies that I reviewed, I aggregated information to the lowest taxonomic category feasible. In the case of grasses and sedges, this was at the garbage can level of "graminoids," primarily because most analyses of scat content report these grassy items in such coarse-grain terms. As with fruits, I only feature those grazed and excavated items that were relatively important in multiple grizzly bear diets.
Of all the vegetal foods, graminoids and horsetail (Equisetum sp.) are ingested over the widest latitudinal range. For graminoids, this is largely an artifact of a coarse-grained taxonomic treatment. Turn-over at the level of individual species is high from one study area to another. But, even so, some genera are widely represented in grizzly bear diets, most prominantly bluegrasses (Poa sp.). Other commonly consumed taxa include wheatgrasses (which go under several generic names these days), bromes (Bromus sp.), reedgrasses (especially Calamagrostis canadensis), sedges such as water sedge (Carex aquatilis), and cottongrasses (Eriophorum sp.) in the far north.
Horsetails are an interesting case. The most commonly consumed species is field horsetail (Equisetum arvense), which, not coincidently, grows throughout arctic and temperate regions of the Northern Hemisphere. Horsetail contains abundant silica, which one might assume would deter consumption. Yet it doesn't. It may be that bears seek out horsetail for pharmaceutical properties engendered not only by silica, but also by high concentrations of potassium and calcium, all of which could benefit bears during spring, after hibernation, which is when most horsetail grazing occurs.
Insofar as the remaining grazed foods are concerned, the most striking patterns are, first, the prevalence of grazing on boykinia (Boykinia richarsonii) in the arctic; second, the widespread heavy use of cow parsnip (Heracleum sphondylium) throughout the wetter interior mid-latitudes; and, third, the grazing of angelica (Angelica sp.) and sweet cicely (Osmorhiza sp.) in temperate regions. The remaining gazed forbs (clover [diverse species of Trifolium], glacier lily [Erythronium grandiflorum], and dandelion [mostly Taraxacum officinale]) are consumed by grizzlies over a relatively wide latitudinal range and gradient of moisture regimes.
Turning to the roots, there are only two taxa consumed heavily enough by grizzlies in enough different regions to warrant mentioning here: sweetvetch (Hedysarum sp.), in the legume family, and biscuitroot (mostly Lomatium cous), in the carrot family. Which is not to say that grizzlies don't commonly consume roots. They do. It is just that the exploited taxa can vary substantially from one area to another. Of the two featured taxa, sweetvetch stands out as being heavily consumed by grizzlies over an exceptionally wide latitudinal gradient that includes diverse environments ranging from arid and cold (the high arctic) to wet and relatively temperate (interior British Columbia). Two species of sweetvetch account for most of the observed use: alpine sweetvetch (H. alpinum) in the north and yellow sweetvetch (H. sulphurescens) in the south. The one transitions to the other in southern Canada, with alpine sweetvetch excavated at progressively higher elevations the farther south one goes. Boreale sweetvetch (variously, H. boreale, H. mackenzii, or H. boreale spp. mackenzii) is also occasionally excavated by grizzlies, but with unclear consequences for the involved bears given that the roots are mildly (or even, cumulatively, severely) toxic to humans. Finally, there is a bit more information on biscuitroot under Pre-European diets. Although currently consumed by grizzlies only in the southernmost parts of their current distribution, I speculate that biscuitroot was once an important food of grizzlies in much of the interior western US.
