Pre-European diets I
If I were to take a comprehensive approach, much of what I would say about the diets of North American grizzly bears prior to the arrival of Europeans would be redundant to what I say about contemporary bear diets. Contemporary diets are probably not appreciably different from pre-historical diets in most of the places where grizzlies persist today. To avoid this redundancy, I have restricted myself in this section on pre-European diets to only those areas where we no longer have grizzlies, which is to say, most of the western US.
The challenge in reconstructing diets of grizzlies in places where they no longer live is the obvious one: we no longer have live bears to observe. But there is ample remedy. There are three sources of evidence that potentially provide a reliable basis for reconstructing diets: (1) written historical observations by those who observed grizzlies in places where they were extirpated; (2) observations of black bears in these same areas; and (3) judicious extrapolation of observations from populations of grizzlies along the southern margin of their current distribution.
A few words about these sources: For historical observations I relied on compendious works such as those by written by Storer and Tevis (California Grizzly) and David Brown (The Grizzly in the Southwest); books on the fauna of the western US; early US Biological Survey Reports, the most useful of which were by Vernon Bailey; numerous journals and reports produced by early European travelers; and ethnobotanical studies that included references to uses of human foods by animals such as grizzlies. Insofar as black bear diets are concerned, I relied upon numerous published studies, including a number of Master's theses that didn't make it into peer-reviewed journals. One important point insofar as black bear diets are concerned: I did not assume a 1:1 correspondance with grizzly bear diets given that grizzlies consistently eat more meat and excavated foods in place where these two bear species still coexist. Finally, I relied heavily on studies of current diets in the southermost regions still occurpied by grizzlies, principally the Yellowstone area and along the East Front of the Montana Rockies. Yellowstone in particular is a veritable museum of feeding behaviors that were probably once much more widespread in areas farther south. Of course, the key proviso to any extrapolations is that grizzly diets almost certainly varied in reflection of availability of different foods, as is evident in the gradient of diets among extant grizzly bear populations as you go south from the Arctic (see Current diets).
So, on to the fun stuff...
(To go directly to regional reconstructions of diet and habitat, click on the following link: )
Historical high-quality foods
I can confidently say that grizzly bears historically got most of their energy and nutrients from a handful of sources, including fleshy fruits (i.e., berries), tree mast (e.g., acorns and pine seeds), and meat from terrestrial and aquatic animals (e.g., bison, moose, and spawning salmoninds). Vegetal foods such as excavated roots and grazed foliage are now and probably always have been key to sustaining grizzlies through seasons of chronic dearth or years when higher-quality foods were not available. Although sometimes consumed in large quantities, these non-fruiting plant parts are only minimally digested by bears (see Nutrition).
With that in mind, it's worth looking at the distributions of known high-quality foods relative to the historical distribution of grizzly bears in the western United States, starting with spawning salmon. Salmon are noteworthy because of the demonstrable effects that this food has on growth and fecundity of individual grizzlies (see Nutrition), as well as densities of contemporary grizzly populations (see Bear density). The map at right shows the historical overlap of spawning salmon with grizzly bears in the western US, with the current distribution of grizzlies delineated by red lines. The white numbers denote the estimated past contributions of salmon to the energy content of local grizzly bear diets (thanks to research by Grant Hilderbrand). It is probably safe to say that salmon were a critical source of energy and nutrients for grizzlies as far inland as central Idaho and as far south along the Pacific coast as central California, including the Central Valley and western Sierra Nevada (for more on salmon see Spawning salmon under Current diets).
As elaborated on under The Bison Factor, bison were almost certainly a major grizzly bear food in many parts of what is now the western US. The map at left shows the overlap between historical distributions of grizzlies and bison (dark green), along with the location of most sites where First Peoples engaged in mass kills of bison (the area within the dashed line). These mass kills, comprised of jumps, pounds, and arroyos, were potential seasonal ecocenters for scavenging grizzly bears. Because of widespread extirpations, the only place where the dietary relationship between grizzlies and bison persists is in the Yellowstone region--a sad remnant. In contrast to bison, the other larger herbivores (elk, pronghorn, deer, and bighorn sheep) were probably never major grizzly bear foods. These herbivores were either smaller than is optimal for grizzlies (i.e., deer, pronghorn, and sheep); denizens of subpar grizzly habitats (i.e., sheep and pronghorn); and historically never that abundant (i.e., elk--see Yellowstone Paleoecology).
There are numerous unambiguous references to the importance of acorns to grizzly bears in the western US, ranging from those produced by the numerous oak species in woodlands and forests surrounding California's Central Valley, to Gambel's oak in the central and southern Rockies, to the white oak of Oregon's Willamette Valley, to the various species of oak along the Mogollon Rim and among the Sky Islands of the Southwest. Whitebark pine seeds were the dominant tree mast consumed by grizzlies to the north of the oaks, and remain one of the most important foods of grizzly bears in the Yellowstone region as well as (at least historically) bears along the East Front of the Montana Rockies. Seeds of whitebark and other pines (including pinyon pines) were eaten by grizzly bears in areas farther west and south, but probably not to the extent that whitebark pine seeds are (and were) eaten by grizzlies in parts of the Northern Rockies. With this as background, the two maps immediately above show the historical overlap between whitebark pine and grizzly bears (at left, in light green), and between grizzlies and vegetation communities dominated by oaks (right, in dark brownish-green). As you can see, acorns were probably a major grizzly bear food in valleys and ranges along and near inland from the Pacific coast, in the central Rockies, and in the Southwest. Whitebark pine seeds were probably important to varying degrees throughout the Rocky Mountains of Idaho, Montana, and Wyoming.
Additional evidence for the importance of some of the foods I describe here can be found in the extent to which they were correlated with pre-historic distributions of grizzly bears in the western US. Three of these corrrelations are illustrated immediately above in x-y diagrams that show the probability that grizzly bears would have been present in any given 900-km2 area (the vertical or y-axis) relative to the local extent of a given food (the horizontal or x-axis). As you can see, there was a very strong relation between the occurrence of grizzlies and the extent of oak-dominated vegetation types (the figure farthest to the left), as well as a weaker relation between grizzly distribution and the abundance of pinyon pines (the central figure). In fact, the joint occurrence of oaks and pinyon pines pretty much defines the southern boundary of where grizzly bears occurred, at least within the contiguous US. Finally, to the east, there was a positive but not very compelling correlation between the distributions of bison and grizzlies, consistent with the importance of bison as a food, but also suggestive of other factors operating along this eastern margin; i.e., competition with people and black bears, amplified by human predation (see The human factor and The black bear factor).
Some excavated foods
A few excavated foods warrant mention here: biscuitroots (Lomatium cous), yampa roots (Perideridia gairdneri), and pocket gophers (Thomomys talpoides) and their food caches. At times and in places, these excavated foods were probably important to grizzlies, either seasonally or annually. Unfortunately, historical records rarely identify the foods that an observed grizzly was digging for. As a result, my inspiration for focusing on these three excavated foods is drawn from contemporary grizzly bear diets, but principally the diet of Yellowstone's grizzly bears. Habitats and foods in Yellowstone are probably the closest of any to those that exist farther south in areas where grizzlies are now extirpateed.
As with the maps above, the maps to the right show the historical overlap between grizzly bears and the three excavated foods that I high-light here: biscuitroot at top, yampa in the middle, and pocket gophers at the bottom.
Biscuitroots show up as an important root food among several extant grizzly populations at the southern limit of current distributions (see Current diets), and were probably commonly consumed by grizzlies throughout the interior northwestern US. Biscuitroots are dug by grizzlies on rocky ridges at progressively higher elevations as the growing season advances (see Mattson 1997 for more details).
The starchy roots of yampa are dug by Yellowstone's grizzly bears in moist meadows at mid to high elevations from July through as late as October. Not surprisingly, grizzlies tend to select for clumps of plants with the largest roots, in microsites that are easiest to dig. Although Yellowstone is currently the only region where grizzly bears still excavate yampa roots, this behavior could have been widespread north of the central Rockies and throughout Pacific coastal and near inland regions.
Finally, pocket gophers are another food source, like yampa roots, that are currently heavily exploited only by grizzlies in the Yellowstone ecosystem. Most bear excavations for pocket gophers occur during the spring, shortly after snowmelt, when the gophers are at their most vulnerable and when their winter caches of roots are still well-stocked. Not surprisingly, excavations tend to be concentrated in the well-watered gently-sloping meadows favored by gophers (for more details see Mattson 2004). I suspect that exploitation of pocket gophers by grizzly bears historically occurred throughout the central Rocky Mountains--an ecoregion that is especially well-endowed with gophers and habitats that would favor exploitation.