Of the Northern Rocky Mountains
For everything on the politics and policy of grizzly bear conservation
go to Grizzly Times
For everything on the ecology of Ursus arctos, go to All Grizzly
Bears eat berries. In fact, in most people's minds, "berries" are the definitive bear food. Yet berries are but one kind of fruit that bears eat, including fruits with a fleshy outer covering as well as fruits with ones that are hard. This outer covering is called a pericarp, unique to flowering plants, e.g., angiosperms. Put another way, fruit includes all kinds of fleshy types called berries, drupes, pomes, and aggregates, as well as hard ones commonly called nuts. Yet they are all fruit.
But fruits do not include the seeds borne in cones produced by conifers (i.e., gymnosperms), notably whitebark pine and pinyon pine. In other words, pines do not produce nuts, which, as a fruit, are only produced by flowering plants. Rather, conifers produce seeds, which also happen to be eaten by bears (see this entry).
The upshot is that I refer to fruit here, not just "berries," and because nut-producing flowering plants (e.g., oak, hazelnut, and beech) are rare to non-existent in the northern Rocky Mountains, fruit as used here refers exclusively to the fleshy sort.
I belabor this referential nomenclature because the scientific literature is littered with sloppy usage by bear researchers trained as zoologists who seem to delight in displays of ignorance about plant anatomy. They use the term "berry" to refer to all fleshy fruits, and "nut" to refer to pine seeds. Both usages are incorrect. Hence, when I write fruit, I mean fruit, and when I write berry, I mean a specific kind of fleshy fruit.
The Mythic Importance of Fruit
Contrary to popular perception, grizzly bears and their brown bear kin in Eurasia (both Ursus arctos) typically do not rely on fleshy fruits as a dietary mainstay. More often these bears are sustained principally by meat from mammals, meat from spawning salmonids (see this entry), seeds from stone pines, or even roots--with fleshy fruits as dietary augmentation.
The map of left is illustrative, thanks to research by Garth Mowat. Garth estimated the proportion of grizzly bear diets comprised of meat from mammals in northwestern North America, denoted by different-colored isopleths. Darker red denotes a greater portion, >50%, and pink or salmon a lesser portion, <20%. Grizzly bears in many parts of North America subsist largely on terrestrial meat. Notable exceptions are coastal areas, where they get most of their energy and nutrients from spawning salmon (delineated by the white dotted line), and interior regions west of the Rocky Mountain east front, where they get much or most of their energy and nutrients from fruit (delineated by the blue dotted line).
Of relevance here, the northern US Rocky Mountains and adjacent Canada are anomalous compared to most of North America, primarily because of the extent to which grizzly bears rely on fruit. Parenthetically, this heavy consumption of fruit here heightens competition between grizzlies and black bears to levels unlike anywhere else (see this entry).
Regional Patterns I.
Although fruit is a major food of grizzly bears in many parts of the northern Rocky Mountains of the US and adjacent Canada, seasonal patterns of consumption vary, as do the species most heavily consumed. Differences are greatest between west and east of the Continental Divide and secondarily from north to south.
The maps and figures above are illustrative. The map shows distributions of fruit-bearing species used most heavily by bears in the northern Rocky Mountains, which, as a geomorphic region, is delineated in red. Brown denotes the exclusive distribution of buffaloberry (Shepherdia canadensis), dark dusky green the overlapping distribution of buffaloberry and serviceberry (Amelanchier alnifolia), light green the exclusive distribution of chokecherry (Prunus virginiana), and blue-green, highlighted and delineated, the distribution of huckleberry (Vaccinium membranaceum). The white dashed line demarcates the eastern limit of maritime climatic influence signaled principally by winter-heavy precipitation (see this entry). The peripheral diagrams show proportional monthly contributions of various foods to grizzly bear diets in six regions, with the contribution of fruit highlighted in blue. The bar diagrams immediately to the right of each show the proportional contributions of various species to the fruit diet. Notably, light blue represents huckleberry, dark red, buffaloberry, pink, kinnikinnick (Arctostaphylos uva-ursi), and, bright red, chokecherry (Prunus virginiana).
Without being exhaustive, the principle patterns are (1) a greater preponderance of dietary fruit in areas with a greater maritime climatic influence (west), (2) later consumption of fruit to the east, in colder continental climates, compared to the west, in comparatively milder maritime climates; (3) greater consumption of buffaloberry to the east versus greater consumption of huckleberry to the west; and (4) less consumption of buffaloberry in the south compared to the north, with compensatory increases in consumption of fruit from species such as chokecherry, serviceberry, and hawthorn (Crataegus chyrsocarpa). At a broader scale yet (as I describe in the Introduction to diet), overall consumption of fruit drops off dramatically at the southernmost limit of current grizzly bear distribution in the Yellowstone ecosystem, reflective of diminished consumption of fruit a bit farther north along the East Front of the Rocky Mountains in Montana.
Regional Patterns II.
The dietary gradient of grizzly bears in the Northern Continental Divide ecosystem of northwestern Montana provides a finer-scale picture of how consumption of fruit varies with environment, especially in contrast to consumption of meat from mammals. Rick Mace collected tissue from a number of grizzlies in this ecosystem and then analyzed it for isotopes signifying the relative contribution of meat versus vegetation, primarily fruit. The result was a map of isopleths representing the proportion of digested nutrients derived from meat, as per the map at left.
The heart of fruit consumption occurs west of the Continental Divide, especially farther north, consistent with the map and figures shown immediately above. By contrast, consumption of meat increases the farther east and south one goes. A particularly steep gradient characteristics the eastern verge of the Rocky Mountains and, for grizzlies out on the plains, meat can comprise 90% of the nutrients they digest and metabolize. All of this is consistent with the trend towards increasing consumption of fruit by grizzlies in areas with greater maritime climatic influence (barring areas with spawning salmon) versus increasing consumption of meat in areas that are more continental, at least at mid-latitudes (see the Introduction to diet).