Diet and Nutrition of grizzly bears in the Yellowstone Ecosystem
Seeking, finding and consuming food is the main factor which plays into grizzly bear movements, and where you find them. Other factors can include but are not limited to cover, bedding areas, den sites, and mating areas. Often times, the discussion of non-food related factors follows food factors (Herrero 2018). Hikers, backcountry users and people who live in bear country can limit and reduce their likelihood of bear encounters by recognizing food sources and key components of grizzly bear diets (Herrero 2018). Recognizing the abundance of a given food source(s) is important; the more food, the more likely there is going to be a bear there (Herrero 2018).
Grizzly bears are opportunistic, omnivore generalists (Bojarska & Selva 2011, Schwartz et al. 2013, Gunther et al. 2014). Their dietary plasticity and diversity amongst food sources provides a likely explanation to their worldwide distribution in comparison to other bear species (Gunther et al. 2014).
Between 1943 and 2009, grizzly bears were documented consuming more than 266 different species, in 200 genera and 4 kingdoms (Gunther et al. 2014). Plantae account for 61% of Greater Yellowstone Grizzly bear diets in this time frame, with graminoids accounting for 70% of Plantae. Over the nearly six decades of literature review on diet and food sources, the three most common graminoids in grizzly diets were bluegrass (Poapratensis), sedges (Carex spp.) and brome grass (Bromus spp.). These three graminoids accounted for 58.7% frequency of occurrence in scat collected from grizzly bears (Gunther et al. 2014). The top ten food sources by frequency of occurrence (in scat) are listed below:
Grizzly bears are opportunistic, omnivore generalists (Bojarska & Selva 2011, Schwartz et al. 2013, Gunther et al. 2014). Their dietary plasticity and diversity amongst food sources provides a likely explanation to their worldwide distribution in comparison to other bear species (Gunther et al. 2014).
Between 1943 and 2009, grizzly bears were documented consuming more than 266 different species, in 200 genera and 4 kingdoms (Gunther et al. 2014). Plantae account for 61% of Greater Yellowstone Grizzly bear diets in this time frame, with graminoids accounting for 70% of Plantae. Over the nearly six decades of literature review on diet and food sources, the three most common graminoids in grizzly diets were bluegrass (Poapratensis), sedges (Carex spp.) and brome grass (Bromus spp.). These three graminoids accounted for 58.7% frequency of occurrence in scat collected from grizzly bears (Gunther et al. 2014). The top ten food sources by frequency of occurrence (in scat) are listed below:
Grizzly bears will typically consume aboveground portions of green herbaceous plants, which are early in their growth stages. This includes easting the shoots, leaves, stems which are high in nutrients in comparison to later growth (flowering). Grizzlies will often times move into and forage in areas where preferred plants are in their early growth stages (Hamer & Herrero 1983). Early growth also is when plants yield the highest protein content. Plants which are at higher elevations typically grow quicker and more rapidly than those plants at lower elevation feeding sites; the greater percentage of protein available is greater in plant tissue at higher elevations than lower elevation feeding areas (Herrero 2018). The reason that grizzlies and other bear species are likely to consume plants which are early in their growth stages, is because bears lack a caecum, a large digestive organ which is present in most herbivorous mammals (bacteria, protozoans in the caecum assist the mammal in the digestion of cellulose) (Davis 1964).
Additionally, areas with southern exposures and slopes have a tendency to receive more energy via sunlight, allowing plant growth to occur early (Herrero 2018). The forest floor, generally greens up occurring in small meadows oriented to the south, along the edges of waterways and roads, often times attracting bears (Herrero 2018). Mountainous regions in late spring and summer (before berries) have increased moisture, encouraging prolonged growth. Grizzlies will forage in these moist meadows, similarly, in the same areas which attract people to recreate, and hike (Herrero 2018).
It is important to understand that not all bears have the same accessibility to certain food sources. Grizzly bears in the Greater Yellowstone Ecosystem (GYE) use different food resources, largely dependent on where their home ranges are situated (Gunther et al. 2014). However, there is large scale seasonal variation in diet and food sources that are available (high vs. lower quality and quantity) (Mattson et al. 1990). Aside from seasonal variation, some diet items appear to be specifically sought after, and targeted by grizzlies. General literature findings concur with Knight et al. 1984, that bears exhibit “directed feeding behavior.” When appropriate foods resources are available. This means, bears will focus their efforts, energy, foraging and feeding activity towards food with high gross energy content (i.e. army cutworm moths, ungulate, cutthroat trout, clover, small mammals, springbeauty, yampa, whitebark pine seeds, biscuitroot, bistort, horsetail). Whenever these high gross energy nutrient/food sources are unavailable, bears will shift to the consumption of berries, forbs, ants and graminoids. Importantly, the consumption of graminoids in general may likely be due to their sheer abundance and distribution across the landscape in various habitats, as opposed to their selection based on caloric value or individual preference (Craighead et al. 1982). Moreover, during the spring months (May & June), there may be selection and preference towards graminoids as much of the new year growth has not yet taken form (Skolvin 1967, Pond & Smith 1971, Graham 1978).
Additionally, areas with southern exposures and slopes have a tendency to receive more energy via sunlight, allowing plant growth to occur early (Herrero 2018). The forest floor, generally greens up occurring in small meadows oriented to the south, along the edges of waterways and roads, often times attracting bears (Herrero 2018). Mountainous regions in late spring and summer (before berries) have increased moisture, encouraging prolonged growth. Grizzlies will forage in these moist meadows, similarly, in the same areas which attract people to recreate, and hike (Herrero 2018).
It is important to understand that not all bears have the same accessibility to certain food sources. Grizzly bears in the Greater Yellowstone Ecosystem (GYE) use different food resources, largely dependent on where their home ranges are situated (Gunther et al. 2014). However, there is large scale seasonal variation in diet and food sources that are available (high vs. lower quality and quantity) (Mattson et al. 1990). Aside from seasonal variation, some diet items appear to be specifically sought after, and targeted by grizzlies. General literature findings concur with Knight et al. 1984, that bears exhibit “directed feeding behavior.” When appropriate foods resources are available. This means, bears will focus their efforts, energy, foraging and feeding activity towards food with high gross energy content (i.e. army cutworm moths, ungulate, cutthroat trout, clover, small mammals, springbeauty, yampa, whitebark pine seeds, biscuitroot, bistort, horsetail). Whenever these high gross energy nutrient/food sources are unavailable, bears will shift to the consumption of berries, forbs, ants and graminoids. Importantly, the consumption of graminoids in general may likely be due to their sheer abundance and distribution across the landscape in various habitats, as opposed to their selection based on caloric value or individual preference (Craighead et al. 1982). Moreover, during the spring months (May & June), there may be selection and preference towards graminoids as much of the new year growth has not yet taken form (Skolvin 1967, Pond & Smith 1971, Graham 1978).
High Calorie Fat/Carbohydrate Breakdown: As mentioned previously, high calorie foods with high gross energy are especially important to grizzly bears. A mixture of food sources high in fat and high in carbohydrates allow for bears to maximize their ability to “pack on the pounds” during the year prior to hibernation.
Photos: Whitebark pine cross sectional cut (Josh Bronson, US Forest Service), Red-osier dogwood (Matt Lavin), Red baneberry (Arthur Chapman), Red elderberry (Glacier National Park, National Park Service), Creeping juniper (Jay Sturner), Beargrass (unknown, Public Domain), Glacier Lily (Glacier National Park, National Park Service), Springbeauty (Yellowstone National Park, National Park Service), Huckleberry (Glacier National Park, National Park Service), Biscuitroot (Matt Lavin), Yampa (J. Schmidt), Oniongrass bulbs (Matt Lavin)
Mammals: Grizzly bears most often use mammals as a food source during the spring and fall (Mattson et al. 1990). Ungulates (elk, bison, deer) comprise a huge portion of the mammals consumed. Importantly, grizzly bears would rather scavenge on winter-killed ungulates, or wolf killed ungulate carcasses rather than exert the energy to engage in predation (opportunistic). Grizzly bears will seek out winter-killed ungulates (wolf killed ungulates) and weakened animals during the early spring; during the fall, bears will seek out ungulates weakened by the rut (elk in September/October, bison in August/September) (Schleyer 1983).
Rodents make up a small portion of grizzly bear diets in the GYE. Based on the average volume sampled in scat, peak use is during spring and late summer entering fall. Rodents like Uinta ground squirrels, are consumed typically when bears are excavating root caches (Mattson et al. 1990). It is important to understand that they are not specifically targeting the rodents, but rather taking advantage of the opportunity to maximize their foraging and feed strategies (again, demonstrating that grizzlies are opportunistic).
Rodents make up a small portion of grizzly bear diets in the GYE. Based on the average volume sampled in scat, peak use is during spring and late summer entering fall. Rodents like Uinta ground squirrels, are consumed typically when bears are excavating root caches (Mattson et al. 1990). It is important to understand that they are not specifically targeting the rodents, but rather taking advantage of the opportunity to maximize their foraging and feed strategies (again, demonstrating that grizzlies are opportunistic).
Photos: Mule deer (Zion National Park, National Park Service), Elk (MOGO), Bison (Jack Dykinga)
Fish: Cutthroat trout (Oncorhynchus clarki) experience peak use by grizzly bears in the GYE during June and July; they are most commonly used as a food source while spawning in streams and tributaries which connect into Yellowstone Lake, Yellowstone National Park (YNP) (Mattson et al. 1990). Historically, cutthroat trout use to supply one of the most calorically dense and concentrated food sources for grizzly bears in the GYE (Mealey 1975, Pritchard & Robbins 1990, Mattson & Reinhart 1995). Over the past several decades, however, trout use and consumption has sharply declined (Strapp & Hayward 2002). For example: Clear Creek, YNP is a tributary to Yellowstone Lake. Historically, in 1988, this area saw a migration of 54,000 cutthroat trout. In 2007, that number was estimated to be just over 500 fish. Lake Trout have principally been the species in the spotlight to blame solely for the decline in cutthroat fisheries. However, cutthroat have also been affected by both drought, and the infection of whirling disease (French 2013). During 2007-2009, there was 70% less trout biomass (1060 kg vs. 314 kg) and 87% fewer trout in comparison to the previous decade (Felicetti et al. 2004, Koel et al. 2005, Fortin et al. 2013). In the long run, loss of native cutthroat trout as a food source means added additional pressures on other meat resources (Fortin et al. 2013, Middleton et al. 2013).
There exists a connection between cutthroat trout and migratory ungulates in the GYE. Essentially, as cutthroat fisheries decline and population numbers plummet, it could be a driving reason for elk herds migrating out of Yellowstone National Park (YNP) (Middleton et al. 2013, Fortin et al. 2013). The connection lies between the decline in spawning trout, and a shift to grizzly bear diets incorporating more elk calves (ungulates) (Middleton et al. 2013). This dietary change has created a reduction in elk calf recruitment (4-16%) and created an impediment to population growth (migratory elk herds that use YNP Lake area decreased 2-11%) (Middleton et al. 2013).
Similarly to cutthroat trout, YNP elk herds, particularly the northern range herd, have seen a significant decrease in population. During 1988, the northern range herd was estimated to consist of 19,000 elk. During the winter of 2013, the herd was estimated to consist of 3,900 (French 2013). Just like trout, elk have also had issues with disease (brucellosis) in combination with hunting harvest, and overall increase in populations of predators (wolves, bears, cougars).
There exists a connection between cutthroat trout and migratory ungulates in the GYE. Essentially, as cutthroat fisheries decline and population numbers plummet, it could be a driving reason for elk herds migrating out of Yellowstone National Park (YNP) (Middleton et al. 2013, Fortin et al. 2013). The connection lies between the decline in spawning trout, and a shift to grizzly bear diets incorporating more elk calves (ungulates) (Middleton et al. 2013). This dietary change has created a reduction in elk calf recruitment (4-16%) and created an impediment to population growth (migratory elk herds that use YNP Lake area decreased 2-11%) (Middleton et al. 2013).
Similarly to cutthroat trout, YNP elk herds, particularly the northern range herd, have seen a significant decrease in population. During 1988, the northern range herd was estimated to consist of 19,000 elk. During the winter of 2013, the herd was estimated to consist of 3,900 (French 2013). Just like trout, elk have also had issues with disease (brucellosis) in combination with hunting harvest, and overall increase in populations of predators (wolves, bears, cougars).
Photos: Elk calf (Yellowstone National Park, National Park Service), Cutthroat Trout (Yellowstone National Park, National Park Service), Uinta Ground Squirrel (Frank Schulenberg).
ADD-ON: Male grizzly bears have a greater proportion of meat in their diets as opposed to their female counterparts. Additionally, grizzlies in general have a greater proportion of meat in their diets compared to black bears; larger bears typically seek more energy dense diets (Welch et al. 1997, Rode et al. 2001, Robbins et al. 2004). Female grizzly bears in the Yellowstone Lake area had demonstrated the same proportion of meat in their diets between 1997-2000 as in 2007-2009; these levels also were observed to not differ or fluctuate based on ecosystem based estimates from 1977-1996 during grizzly bear expansion (Jacoby et al. 1999, Felicetti et al. 2003, Fortin et al, 2013). However, the proportion of meat documented in subadult and adult male grizzly bear diets decreased. Grizzly bear males in the Lake area in 2007-2009 had less proportion of meat in their diets than during 1997-2000. Both levels were less on an ecosystem basis from 1977-1996 during grizzly expansion (Jacoby et al. 1999, Felicetti et al. 2003, Fortin et al. 2013). From this information, it can be concluded that male bears are the first to experience the consequences associated with declining meat biomass and resources (Fortin et al. 2013).
Fungi: Grizzly bears do feed on mushrooms and puffballs. Their peak time of use is during the month of September. Grizzlies will eat Basidiocarps, which can be found predominantly in dry lodgepole pine forests (Mattson et al. 1990). Grizzlies have been documented consuming three types of gilled mushrooms and four types of non-gilled mushrooms. One of the more frequently consumed, non-gilled mushrooms are false truffles (Rhizopogon spp.)(Podruzny 1999, Mattson 2002, Felicetti et al. 2004, Fortin 2011).
Ferns: Grizzly bears in the GYE have been documented consuming four types of ferns, fern allies as food sources in their diets (Gunther et al. 2014). One frequently consumed type of fern, is the common horsetail (Equistem arvense). Horsetails are consumed most during their peak season in June and July (Mattson et al. 1990). In most cases, horsetails are generally obtained by grazing stems in wet, marshy like areas (Mattson et al. 1990).
Graminoids: The peak time of use for graminoids is during May-June. Two of the most frequently consumed types of graminoids include bluegrass (Poa pratensis) (early season) and sedge (Carex spp.) (late season). Most of the graminoid materials gathered via scat analysis have been categorized and identified based on the prevalence of their leaves (Mattson et al. 1990).
Photos: False truffle (Bjorn S.), Common Horsetail (Aiaraldea Gaur eta Hemen), Dandelion (Ryan Hodnett), Fireweed (Kallerna), Thistle (Matt Lavin)
Insects: Grizzlies primarily feed on insects during their peak season of use during July-August. The most frequently consumed insect for GYE grizzlies is ants (Mattson et al. 1990). Grizzly bears frequently consume a total of 10 different species of ants.
Forb Foliage: Peak season for consumption of forbs by grizzly bears is during the month of July (Mattson et al. 1990). Dandelion (Taraxacum spp.) reaches peak consumption during June, whereas Clover (Trifolum spp.) peaks during August and Elk Thistle (Cirsium scariosum) peaks during July. Some other minor components of their forb diet include springbeauty (Claytonia lanceolate), fireweed (Epilobium angustifolium), and salsify (Trapopogon dubius).
Forb and Graminoid roots: During the early spring, grizzly bears consume forb and graminoid roots via the excavation of pocket gopher root caches. During the late season, stretching into late summer/fall, bears seek out and actually excavate individual root complexes and systems (Mattson et al. 1990). Some of the frequently consumed forb and graminoid roots include:
Oniongrass bulbs (Melica spectabilis) which is consumed primarily during April, May and October, inadvertently while digging out rodent root caches.
Sweet cicely (Osmorhiza spp.), Pondweed (Potamogeton spp.), Yampa (Perideridia gairdneri or P. montana) all receive peak use during September and October; Yampa also receive increased use during July and August). These forbs can be found middle elevation, mesic forest and mesic non-forest, or in seasonally inundated areas (Mattson et al. 1990). Biscuit-root (Lomatium spp.) is individually sought after and excavated during the months of June, July and August. Biscuit-root can be found in lithic, high elevation ridge areas (Mattson et al. 1990).
Oniongrass bulbs (Melica spectabilis) which is consumed primarily during April, May and October, inadvertently while digging out rodent root caches.
Sweet cicely (Osmorhiza spp.), Pondweed (Potamogeton spp.), Yampa (Perideridia gairdneri or P. montana) all receive peak use during September and October; Yampa also receive increased use during July and August). These forbs can be found middle elevation, mesic forest and mesic non-forest, or in seasonally inundated areas (Mattson et al. 1990). Biscuit-root (Lomatium spp.) is individually sought after and excavated during the months of June, July and August. Biscuit-root can be found in lithic, high elevation ridge areas (Mattson et al. 1990).
Photos: Yampa (J. Schmidt), Biscuitroot (Matt Lavin), Oniongrass bulbs (Matt Lavin), Huckleberry (Glacier National Park, National Park Service), Whortleberry (Walter Siegmund), Buffaloberry/Soapberry (Yellowstone National Park, National Park Service), Creeping Oregon grape/ Creeping barberry (Matt Lavin)
Shrubs: Most of the shrubs grizzly bears seek out are fleshy fruits (berries). They receive peak use during the month of August. Some of the frequently consumed shrubs are whortleberry (Vaccinium scoparium), huckleberry (Vaccinium globulare) and buffalo/soapberry (Shepherdia canadensis). Creeping Oregon grape is also consumed by grizzly bears, found typically in understory vegetation (Knight et al 1977, 1980; Craighead et al. 1995). Ironically, grizzly bears also use whortleberry during the month of June, where they have been observed and documented eating their leaves (Mattson et al. 1990).
Trees: Grizzlies themselves do not typically consume trees, but they rather consume the products that trees create (i.e. pine cones, seeds, etc.). Whitebark pine is especially important to grizzly bears; its peak use is during September and October, where pine seeds are extracted from cones dug out of red squirrel caches (Mattson & Jonkel 1990). Like cutthroat trout and elk, whitebark pine is on the decline. The two major ailments which have plagued the species have been blister rust, and mountain pine beetle infestation. It is important to recognize that the mountain pine beetle is native to the rocky mountains. However, due to less harsh and severe winters, and energetics affiliated with climate change, beetles and their larvae are able to survive and endure winters, and even sometimes move into higher elevation stands of trees. As whitebark pine continue to decrease and die, they have been shifted away from during their peak season; female grizzly bears and black bears have started to develop more of a dependence on false truffles, as demonstrated during poor pine cone production years (Mattson et al. 1990, Haroldson & Podruzny 2010). Grizzly bear mortality and management actions have been documented to vary based on the availability and production of pine cones from whitebark pine (Picton et al. 1986, Knight et al. 1988).
Photos: Whitebark pine seeds (Richard Sniezko), whitebark pine cone cross-sectional (Josh Bronson, US Forest Service), whitebark pine tree w/ pinecone (Crater Lake National Park, National Park Service), alive and dead whitebark pine (Yellowstone National Park, National Park Service), verbenone packets on whitebark (Crater Lake National Park, National Park Service)
Table 1: Break down of frequently consumed food sources by grizzly bears in the Greater Yellowstone Ecosystem (GYE) by parts consumed, caloric value, period of use and peak use.
Works Cited
- Bojarska, K., Selva, N., (2011). Spatial patterns in brown bear Ursus arctos diet: The role of geographical and environmental factors. Mammal Review 42:120–143.
- Craighead, J.J., Mitchell, J.A., (1995). The grizzly bears of Yellowstone, their ecology in the Yellowstone ecosystem, 1959–1992. Island Press, Covelo, California, USA.
- Craighead, J.J., Sumner, J.S., Scaggs, G.B., (1982). A definitive system for analysis of grizzly bear habitat and other wilderness resources. Wildlife–Wildlands Institute Monograph No. 1. University of Montana Foundation, University of Montana, Missoula, Montana, USA.
- Davis, D. D. (1964). The giant panda: a morphological study of evolutionary mechanisms. Fieldiana (Zoology Memoirs), 3, 1-339.
- Felicetti, L.A., Schwartz, C.C, Rye, R.O., Gunther, K.A., Crock, J.G., Haroldson, M.A., Waits, L., Robbins, C.T., (2004). Use of naturally occurring mercury to determine the importance of cutthroat trout to Yellowstone grizzly bears. Canadian Journal of Zoology 82:493–501.
- Felicetti, L.A., Schwartz, C.C., Rye, R.O., Haroldson, M.A., Gunther, K.A., Robbins, C.T., (2003). Use of sulfur and nitrogen stable isotopes to determine the importance of whitebark pine nuts to Yellowstone grizzly bears. Canadian Journal of Zoology 81:763–770.
- Fortin, J.K., (2011). Niche separation of grizzly (Ursus arctos) and American black bears (Ursus americanus) in Yellowstone National Park. Dissertation, Washington State University, Pullman, Washington, USA.
- Fortin, J.K., (2013). Management and Conservation Dietary Adjustability of Grizzly Bears and American Black Bears in Yellowstone National Park. Journal of Wildlife Management 77(2):270-281.
- French, B., (2013). Study: Yellowstone trout decline pushes bears to elk calf diet. Billings Gazette. Accessed 17 June 2019. Billings, Montana. USA.
- Graham, D.C., (1978). Grizzly bear distribution, use of habitats, food habits and habitat characterization of Pelican and Hayden Valleys, Yellowstone National Park. MSc. Thesis, Montana State University, Bozeman, Montana. USA
- Gunther, K.A, Shoemaker, R.R., Frey, K.L., Haroldson, M.A. Cain, S.L., van Manen, F.T., Fortin, J.K., (2014). Dietary breadth of grizzly bears in the Greater Yellowstone Ecosystem. Ursus 25(1):60-72.
- Hamer, D., & Herrero, S. (1983). Ecological studies of the grizzly bear in Banff National Park: Final Report. Parks Canada. Calgary, Alberta.
- Haroldson, M.A., Podruzny, S., (2010). Whitebark pine cone production. Pages 33–34 in C. C. Schwartz and M. A. Haroldson, editors. Yellowstone grizzly bear investigations: annual report of the Interagency Grizzly Bear Study Team, 2009. U.S. Geological Survey, Bozeman, Montana, USA.
- Herrero, S. (2018). Bear attacks: their causes and avoidance. Rowman & Littlefield.
- Jacoby, M.E., Hilderbrand, G.V., Servheen, C., Schwartz, C.C., Arthur, S.M., Hanley, T.A., Robbins, C.T., Michener, R., (1999). Trophic relations of brown and black bears in several western North American ecosystems. Journal of Wildlife Management 63:921–929.
- Knight, R.R., Blanchard, B.M., Eberhardt, L.L., (1988). Mortality patterns and populations sinks for Yellowstone grizzly bears, 1973-1985. Wildl. Soc. Bull. 16:121-125.
- Knight, R.R., Mattson, D.J., Blanchard, B.M., (1984). Movements and habitat use of the Yellowstone Grizzly Bear. Interagency Grizzly Bear Study Team, Bozeman, Montana, USA.
- Koel, T.M., Bigelow, P.E., Doepke, P.D., Ertel, B.D., Mahony, D.L., (2005). Nonnative lake trout results in Yellowstone cutthroat trout decline and impacts to bears and anglers. Fisheries 30:10–19.
- Mattson, D.J., Jonkel, C., (1990). Stone pines and bears. In Proceedings of a Symposium on Whitebark Pine Ecosystems: Ecology and Management of a High-Mountain Resource. U.S. For. Serv. Gen. Tech. Rep. INT-270, pp. 223-236.
- Mattson, D.J., Podruzny, S.R., Haroldson, M.A., (2002). Consumption of fungal sporocarps by Yellowstone grizzly bears. Ursus 13:95–103.
- Mattson, D.J., Reinhart, D.P., (1995). Influences of cutthroat trout (Oncorhynchus clarki) on behaviour and reproduction of Yellowstone grizzly bears (Ursus arctos), 1975–1989. Canadian Journal of Zoology 73:2072–2079.
- Mealey, S.P., (1975). The natural food habits of free ranging grizzly bears in Yellowstone National Park, 1973-74. MSc. Thesis, Montana State University, Bozeman, Montana. USA.
- Middleton, A.D., Morrison, T.A., Fortin, J.K., Robbins, C.T., Proffitt, K.M., White, P.J., ... & Kauffman, M.J., (2013). Grizzly bear predation links the loss of native trout to the demography of migratory elk in Yellowstone. Proceedings of the Royal Society B: Biological Sciences, 280(1762), 20130870.
- Picton, H.D., Mattson, D.J., Blanchard, B.M., Knight, R.R., (1986). Climate, carrying capacity, and the Yellowstone grizzly bear. U.S. For. Serv. Gen. Tech. Rep. INT-207. Pp. 129-135.
- Podruzny, S.R., Reinhart, D.P., Mattson, D J. (1999). Fire, red squirrels, whitebark pine, and Yellowstone grizzly bears. Ursus, 131-138.
- Pond, F.W., Smith, D.R. (1971). Ecology and management of subalpine ranges on the Big Horn mountains of Wyoming. Res. J. No. 53, Agric. Exp. Stn., University of Wyoming, Laramie.
- Pritchard, G.T., & Robbins, C T. (1990). Digestive and metabolic efficiencies of grizzly and black bears. canadian Journal of Zoology, 68(8), 1645-1651.
- Robbins, C.T., Schwartz, C.C, Felicetti, L.A., (2004). Nutritional ecology of ursids: a review of newer methods and management implications. Ursus 15:161–171.
- Rode, K.D., Robbins, C.T., Shipley, L.A. (2001). Constraints on herbivory by grizzly bears. Oecologia, 128(1), 62-71.
- Schleyer, B.O., (1983). Activity patterns of grizzly bears in the Yellowstone ecosystem and their reproductive behavior, predation and the use of carrion (Doctoral dissertation, Montana State University-Bozeman, College of Letters & Science).
- Schwartz, C.C., Haroldson, M.A., Gunther, K.A., Robbins, C. (2013). Omnivory and the Terrestrial Food Web: Yellowstone Grizzly Bear Diets: Chapter 7.
- Skolvin, J.M., (1967) Fluctuations in forage quality on summer range in the Blue Mountains. U.S. For. Serv. Res. Rap. PNW-44.
- Stapp, P., Hayward, G.D., (2002). Estimates of predator consumption of Yellowstone cutthroat trout (Oncorhynchus clarki bouvieri) in Yellowstone Lake. Journal of Freshwater Ecology, 17(2), 319-329.
- Welch, C.A., Keay, J., Kendall, K.C., Robbins C.T., (1997). Constraints on frugivory by bears. Ecology 78:1105–1119.