Spatial Patterns

in consumption of meat by grizzly bears

As you might expect, grizzly bears in different areas eat different amounts of meat from large herbivores. Moreover, the sources of this meat vary one place to another, certainly at a continental scale, but even among and within ecosystems in a given region. In general, bears living in drier, less productive, more open regions eat more meat from terrestrial sources (follow this link for more details). Sources vary from caribou and ground squirrels in the Arctic, to moose in boreal forests, to elk, deer, and bison farthest south. For good or ill, livestock is on the menu wherever grizzlies have ready access to sheep and cows, most often at mid-latitudes where grizzly range includes higher-elevation pastures and grasslands. Such is the case along the East Front of the Rocky Mountains in Montana and, increasingly, along the eastern margins of grizzly bear range in the Greater Yellowstone Ecosystem of Wyoming (see below).

The map at left, derived from research by Garth Mowat, provides a continent-spanning overview of meat consumption by grizzly bears. Terrestrial sources such as ungulates and rodents are featured, excluding fish such as salmon. Darker shades of red or burgundy denote larger fractions of terrestrial meat in the bear diet, with fractions exceeding 50% in the arctic and eastern interior boreal and temperate regions. At the opposite extreme, mid-latitude interior areas where fruit is the dominant source of energy are delineated by a blue dotted line, and coastal areas where salmon are the dominant source, by a white dotted line. As I point out in the Introduction to DIET & HABITAT, the Selkirk, Cabinet-Yaak, and western US Northern Continental Divide ecosystems are parts of a fruit-centric region that is anomalous at a continental scale. Eating lots of meat is actually more the norm for grizzlies. (Click on the following highlighted text to find more information on consumption of fruit, salmon, and terrestrial meat).

Continental Patterns

Regional Patterns

Patterns of meat consumption by grizzlies in the northern US Rocky Mountains reflect broader continental trends. The map immediately above shows the fractional composition of ingested grizzly bear diets in 5 regional ecosystems, with Yellowstone farthest south, the Cabinet-Yaak farthest northwest and, east of the continental divide, the Rocky Mountain East Front. Dark red-brown denotes the contribution of meat from large herbivores, at least as estimated from bear fecal matter corrected to account for the effects of detectability and digestion. Parenthetically, tan denotes the contribution of whitebark pine seeds, pink the contribution of fish, dark brown the contribution of insects, and gray the contribution of roots. Yellow represents contributions of vegetal foods including fruit.

 

Amounts of dietary meat are comparable (around 24-27%) among Cabinet-Yaak, East Front, and North Fork of the Flathead grizzlies, but with the proviso that amounts of meat consumed by grizzlies increases dramatically with distance from the crest of the continental divide east onto the high plains (see below). Yellowstone stands out in the Northern Rockies as an area where grizzlies uniformly eat a lot of meat from terrestrial sources, comprising nearly 50% of the ingested diet. This greater consumption of meat is a direct reflection of more numerous large herbivores--roughly 67,600 elk in the Yellowstone ecosystem versus 4,300 in all of northwest Montana and 6,000 along or near the East Front, plus Yellowstone has supported somewhere between 3,000 and 5,000 bison during the last two decades. As context, beige denotes the extent of habitat suitable for elk (although not necessarily occupied nor of equal carrying capacity), and, purple, the extent of alpine and high subalpine environments--of relevance to the potential occurrence of army cutworm moth aggregation sites.     

The graphs at right show seasonal variation in consumption of meat by grizzly bears (in brown) in four ecosystems. Emblematic of patterns throughout the northern US Rocky Mountains, meat consumption as a proportion of total diet peaks during spring and to a lesser extent during late fall. Of greater relevance to the theme of spatial variation, the bar graphs immediately to the right of each seasonal graph show the contribution of different ungulate species to total meat in grizzly bear diets. The differences are substantial. Elk account for the majority (70%) of dietary meat in the North Fork of the Flathead River. Immediately south, in the Glacier National Park area. elk make a major contribution (39%), but second to white-tailed deer (50%). East of the continental divide, livestock--primarily cattle--take first place (65%), followed by deer (25%). 

Farthest south, in the Yellowstone ecosystem, elk historically contributed the majority (53%) of dietary meat, followed by bison (24%). Notably, Yellowstone is the only place on Earth where grizzly bears still have bison on the diet (see here for more on the bison-grizzly bear connection). Of the remaining ungulate species, moose make (or made) a significant dietary contribution on in Yellowstone (18%) and the North Fork of the Flathead (15%).

 

In general, deer, especially white-tailed deer, tend to be a more prominent source of meat for grizzlies in heavily forested regions such as the Cabinet-Yaak and Glacier ecosystems. Much of this deer is consumed by bears as the remains of animals killed by hunters during fall. By contrast, bison and cattle--both bovids--are more important sources of meat in drier more open ecosystems, and both exploited primarily through scavenging rather than outright predation with the exception of cow calves during their first 3 months of life. 

Variation within Ecosystems

Not surprisingly, consumption of terrestrial meat by grizzlies varies within ecosystems largely as a function of the local abundance of large herbivores. Emblematic of such variation, the map at left shows core distributions of the largest elk herds in the Yellowstone ecosystem based on telemetry locations of radio-marked animals. State boundaries and the boundary of Yellowstone Park are shown for reference; each elk herd is denoted by a different pastel color (blue-green, green, blue, red, and yellow); bison by tan and brown (core distribution). I've also overlain, as black dots, all of the locations where grizzly bears were known to obtain meat from an ungulate carcass during 1977-1993.

 

The basic points here are, first, that elk and bison are not uniformly distributed, but rather concentrated along travel corridors or in highly productive habitat; and, second, that grizzly bear exploitation of ungulates is likewise concentrated in ways that reflect concentrations of elk and bison. Notice, though, that grizzly bear activity more closely reflects distributions of bison than elk, which reflects the disproportionate contribution of bison to dietary meat in this ecosystem, which has likely been the case wherever and whenever bison and grizzlies overlapped historically.

Interestly and, for managers, problematically, the largest local fractions of meat in grizzly bear diets coincide with concentrations of livestock, which also tend to be areas with a dearth of other rich foods. The maps above illustrate this pattern for the Yellowstone ecosystem (left) and the the Northern Continental Divide (NCDE; right).

 

The Yellowstone-focused map shows the current approximate distribution of grizzly bears in green, the distribution of livestock depredations by grizzly bears as red or reddish-brown dots, public land grazing allotments with chronic depredations in orange, and allotments that were closed prior to 2012 (i.e., "retired") in yellow. I show depredations only for 2012-2013 because spatial locations for other years are (for some reason) not publicly available. The main patterns are (1) a concentration of depredations on the periphery of grizzly bear distribution in areas where cattle densities are highest; (2) an absence of depredation in areas where livestock are not present (i.e., inside National Parks and on retired grazing allotments); and pronounced depredation hotspots, even on the periphery where depredations are generally more common. These hotspots tend to coincide with areas where livestock husbandry is lax and livestock producers both hostile and politically well-connected.

 

The NCDE-focused map shows the gradient of dietary meat for this ecosystem based on isotopic analysis of tissues from grizzly bears captured for research or management purposes. Dietary fractions of meat increase dramatically from northwest to east and south--from less than 12% to greater than 75% on the high plains. Grizzlies tend to eat the least meat where that meat is obtained primarily from elk and deer and the most meat where it is obtained from cattle, coincident with where grizzly bear distribution overlaps high plains environments comprised largely of private ranch lands converted from supporting bison to raising cattle for profit (for example, see this book by Michael Wise).