Grizzly Bears: Ultimate Omnivores of the Greater Yellowstone Ecosystem

Kerry A. Gunther, Rebecca R. Shoemaker, Kevin L. Frey, Mark A. Haroldson, Steven L. Cain, Frank T. van Manen, & Jennifer K. Fortin

From Yellowstone Science 23(2): 2015, pages 7-11.

Brown bears are widely distributed throughout Europe, Asia, and North America (Schwartz et al. 2003). In North America, grizzly bears once occupied diverse biomes from Northern Alaska south to Mexico and from the Great Plains west to the Pacific Coast. Although historic grizzly bear range is much reduced, grizzly bears continue to survive in vastly different landscapes including treeless arctic tundra, boreal and coastal forests, mountain forest/grasslands, shrub steppe, and prairie/riparian habitats. Their ability to use such a wide variety of habitats is attributed to their intelligence, adaptability, and opportunistic, omnivore generalist lifestyle (Schwartz et al. 2003). Yellowstone grizzly bears occupy alpine, subalpine, montane, foothill, and even the edges of prairie vegetation zones encompassing the Yellowstone Plateau of the Central Rocky Mountains, referred to as the Greater Yellowstone Ecosystem (GYE). As is typical of mountain ecosystems, food resources for grizzly bears in the GYE are very dynamic, changing from season to season, year to year, and from location to location. In recent decades, there have been substantial changes in the distribution and availability of several high-calorie foods used by Yellowstone grizzly bears, such as cutthroat trout and whitebark pine seeds. With potential impacts from climate change and other human influences on the landscape, managers want to better understand how grizzly bears may respond to future changes in availability of food resources. To do so, one needs to know what foods are currently being consumed and how diverse their diet is under present-day environmental conditions. Research on grizzly bears in the GYE has been conducted continuously for over 50 years, likely making them the most studied bear population in the world; although no single study has compiled all data on foods consumed by GYE grizzly bears. Since no synthesis existed of all foods consumed by grizzly bears in the GYE, we conducted a review of 49 published papers, 17 books, 4 doctoral dissertations, 11 master's theses, and 97 state and federal agency administrative reports that have documented grizzly bear food habits in the GYE during the 124-year period from 1891 to 2014. From this literature we compiled a list of all the reported foods consumed by grizzly bears into one comprehensive document. Grizzly bears will consume almost any fresh, processed, frozen, canned, dried, boxed, or packaged foods sold for human consumption, including camp foods, groceries, beverages, grease, and garbage. We did not include the almost endless list of anthropogenic foods potentially available to GYE bears because, although these foods can provide nutrition for bears, their use most often leads to bears' lethal removal from the ecosystem.

Dietary Breadth of the Grizzly Bear
Our literature review indicated GYE grizzly bears eat an incredibly diverse array of foods, from those as large as bison and moose, to those as small as ants and midges. Grizzly bears of the GYE prey on fast and agile animals like elk, but also on slow and relatively immobile species such as earthworms. Some prey species fight back and are quite formidable to subdue, which we have observed when grizzly bears kill adult black bears or even other grizzly bears. Some insects, such as yellow jackets, put up a stinging defense but are not much of a threat to bears. Other species like ladybird beetles and cutthroat trout are relatively defenseless. In addition to prey species, grizzly bears eat many plants and mushrooms. Plants are consumed through grazing, digging, plucking, stripping, and peeling. Grasses, sedges, and clover are grazed; whereas, biscuitroot, yampa, and thistle roots are dug from the soil. Berries are plucked and stripped from branches and cambium is peeled from tree trunks. The foreclaws of grizzly bears are as long as 10 cm (4 inches) (Herrero 2002), and can be used to delicately dig up an individual yampa root or to 'rototill' acres of meadows to catch pocket gophers and dig up their food caches of plant roots. The average energy values of the ungulates (6.8 kcal/g), trout (6.1 kcal/g), and small mammals (4.5 kcal/g) eaten by grizzly bears were higher than those of the berries (3.2 kcal/g), forbs (2.9 kcal/g), and graminoids (2.5 kcal/g) they consumed (Craighead et al. 1995). At 7.9 kcal/g, army cutworm moths had the highest reported gross energy value of any food consumed (French et al. 1994). Some of the plant foods consumed, such as springbeauty, cow parsnip, and clover, are relatively high in protein. Other foods are rich in fats, like whitebark pine seeds and beargrass seed pods; whereas, others provide plenty of carbohydrates, such as oniongrass bulbs, yampa tubers, and roots of biscuitroot.

In all, we documented more than 266 species in 200 genera from four different kingdoms (Plantae, Animalia, Fungi, and Protista) consumed by GYE grizzly bears (Gunther et al. 2014; see a list of the scientific names of all food items mentioned in this article at: nps.gov/yellowstonescience). Grizzly bears consumed more than 162 different plant species (149 native and 13 non-native), including at least 85 forbs, 31 shrubs and berries, 25 grasses, 4 sedges, 2 rushes, 4 aquatic plants, and 4 different species of ferns and fern allies as well as cambium, catkins, and nuts from 7 tree species. The primary forbs eaten were clover, dandelion, thistle, horsetail, yampa, and biscuitroot. Frequently consumed berries included whortleberry, huckleberry, and strawberry. The most frequently consumed graminoids were Kentucky bluegrass, sedges, and brome grasses. Seven species of mushrooms were consumed, including false truffles, puffballs, and morels. We also documented bears feeding on at least 26 mammal, 4 fish, 3 bird, and 1 amphibian species. Additional animal species are undoubtedly consumed opportunistically, but have not been documented. The primary mammals consumed were bison, moose, elk, mule deer, pocket gophers, voles, and ground squirrels; but mountain goats, marmots, mice, and pikas were also eaten. Grizzly bears consumed at least 36 species of invertebrates: primarily ants, army cutworm moths, yellow jackets, and earthworms. Twenty-four different species of ants were eaten including mound, ground, and log-dwelling species. It is easy to underestimate the importance of these insect species; however, ants are one of very few species that have been documented in every single grizzly bear scat-based diet study in the GYE from 1943 to 2009. Consumption of one algae (Knight et al. 1978) and one soil type (geothermal; Mattson et al. 1999) were also documented. Although the reason bears eat geothermal soil remains somewhat of a mystery, the soil may serve to restore mineral deficiencies because it contains high concentrations of potassium, magnesium, and sulfur (Mattson et al. 1999). Finally, grizzly bears consumed 26 species of domesticated plants and animals, including 13 species of cultivated agricultural, garden, and ornamental plants; 7 species of domestic livestock; and 4 species of domestic poultry, as well as domestic dogs and honey bees.

Foraging Strategy
Because of their need to accumulate large fat reserves for hibernation, grizzly bears are energy maximizers (Erlenbach et al. 2014). In grizzly bears, a diet of approximately 20% protein and 80% non-protein energy achieves maximum weight gain per unit of energy intake (Erlenbach et al. 2014, Coogin et al. 2014). To achieve this nutrient target and maximum weight gain, grizzly bears select mixed diets that maximize energy intake while optimizing macronutrient intake. This has been demonstrated in both wild and captive grizzly bears (Erlenbach et al. 2014, Coogin et al. 2014). The diverse diets of grizzly bears provides them the opportunity to eat appropriate proportions of nutritionally complementary foods during different seasons and in different geographic locations.

Whenever available, grizzly bears seek foods of high caloric value that are concentrated on the landscape and can be efficiently foraged (Schwartz et al. 2003). Accordingly, frequently used foods included ungulates (bison, elk, moose, mule deer), cutthroat trout, army cutworm moths, and whitebark pine seeds. Bears make seasonal movements within their home ranges to areas where these foods are abundant, such as ungulate winter ranges, elk calving areas, spawning streams, moth aggregation sites on remote talus slopes, and forest stands containing whitebark pine. Most of these foods are subject to seasonal, annual, and geographic variation in availability, and therefore are not abundant or available during all seasons, every year, or within every individual bear's home range. If some of these concentrated high-caloric foods are not readily available, grizzly bears can usually shift their diet to other items, which may have lower caloric value but are widely distributed across the landscape and readily available most years. These lower-calorie foods include a wide variety of plants (clover, spring beauty, yampa, biscuitroot, bistort, other forbs, horsetail, grasses, and sedges), colonial insects (ants), fungi (false truffles and mushrooms), berries (huckleberry, whortleberry, and gooseberry), and small mammals (voles, ground squirrels, and pocket gophers). Spatial and temporal abundance and annual predictability of these foods can compensate for their lower caloric value;and, consequently, these foods can comprise a large proportion of grizzly bear annual diets (Craighead et al. 1982).

Grizzly bears also supplement their diet with many foods consumed opportunistically. In fact, most foods we identified (including a variety of plants, fungi, vertebrates, and invertebrates) were consumed opportunistically with consumption varying annually based on availability and other factors. Opportunistic foods include a variety of species of plants, fungi, vertebrates, and invertebrates. Some opportunistic foods are consumed only during a short period each year (e.g., earthworms in wet meadows at the edge of the receding snowline during spring snow melt), while some are available only in small, localized areas (e.g., pondweed rhizomes from small ephemeral ponds within the Yellowstone caldera), and others are available only during sporadic periods of abundance (e.g., yellow jackets, grass hoppers, Mormon crickets, and midges). Many opportunistic foods are eaten primarily during periods with shortages of more preferred foods (e.g., yellow salsify and fernleaf licorice-root) or when randomly encountered while foraging for other species (e.g., mountain goats, grouse, boreal chorus frogs, and Utah suckers). Some foods of lower caloric value, such as grasses, sedges, and many forbs, may be consumed in areas between concentrations of higher-quality foods, thereby subsidizing travel and search costs (Mattson et al. 1984).

Benefits of a Flexible Diet
Grizzly bears in the GYE exhibit diet flexibility, consuming different foods depending on where their home ranges are located (figure 1). Some of the highest-quality foods are not found within all parts of the ecosystem and thus are likely not available to all GYE bears (figure 1). For example, army cutworm moths, bison, and cutthroat trout have limited distributions in the GYE, and even whitebark pine is only available to about two-thirds of all GYE grizzly bears (Costello et al. 2014). Because occupied grizzly bear habitat in the GYE is managed by many different state and federal agencies (figure 1), bears often cross jurisdictional boundaries to forage different food resources. Consequently, interagency cooperation is critical for successful, long-term conservation of grizzly bears and may be particularly important in the face of uncertainties such as climate change and expanding human occupation of the area. The diet flexibility demonstrated by Yellowstone grizzly bears greatly enhances their ability to occupy diverse habitats over a large geographic area. This diet flexibility likely also enhances their ability to cope with seasonal, annual, and longer-term perturbations in the abundance of high-calorie foods. Knowledge of the dietary breadth of grizzly bears helps managers of grizzly bears and their habitat document future changes in patterns of food consumption. This increased understanding provides managers with a strong foundation for making decisions about future grizzly bear management in the GYE.

Changing climate and introduced non-native species will probably change the species composition and distribution of the plants and animals consumed by GYE grizzly bears. However, the adaptability and omnivore generalist lifestyle characteristics that allow grizzly bears to occupy diverse habitats throughout the world give us optimism that grizzly bears are capable of adjusting to these changes, provided that sufficient habitat is protected to allow grizzly bears to adjust to the changes, and human-caused mortalities are kept at sustainable levels.

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Costello, C.M., F.T. van Manen, M.A. Haroldson, M.R. Ebinger, S.L. Cain, K.A. Gunther, and D.D. Bjornlie. 2014. Influence of whitebark pine decline on fall habitat use and movements of grizzly bears in the Greater Yellowstone Ecosystem. Ecology and Evolution 4:2004-2018.

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Figure 1. Distribution of four concentrated high-caloric grizzly bear foods (army cutworm moths, bison, cutthroat trout, and whitebark pine) within occupied grizzly bear range under different land management agency jurisdictions in the Greater Yellowstone Ecosystem (adapted from Gunther et al. 2014)