Diet ,
Alaska Although grizzlies are of the order
Carnivora and have the digestive system of carnivores, they are normally
omnivores: their diets consist of both plants and animals. They have been known to prey on large mammals, when available, such as
moose,
elk,
caribou,
white-tailed deer,
mule deer,
bighorn sheep,
bison, and even
black bears, though they are more likely to take calves and injured individuals rather than healthy adults. Grizzly bears feed on fish such as
salmon,
trout, and
bass, and those with access to a more
protein-enriched diet in coastal areas potentially grow larger than inland individuals. Grizzly bears also readily
scavenge food or carrion left behind by other animals. Grizzly bears will also eat
birds and their eggs, and gather in large numbers at fishing sites to feed on spawning salmon. They frequently prey on baby
deer left in the grass, and occasionally they raid the nests of raptors such as
bald eagles. Coastal Canadian and Alaskan grizzlies are larger than those that reside in the
Rocky Mountains. This is due, in part, to the richness of their diets. In
Yellowstone National Park in the United States, the grizzly bear's diet consists mostly of
whitebark pine nuts,
tubers, grasses, various
rodents,
army cutworm moths, and scavenged carcasses. None of these, however, match the fat content of the salmon available in Alaska and British Columbia. With the high fat content of salmon, it is not uncommon to encounter grizzlies in Alaska weighing . Grizzlies in Alaska supplement their diet of salmon and
clams with
sedge grass and
berries. In areas where salmon are forced to leap waterfalls, grizzlies gather at the base of the falls to feed on and catch the fish. Salmon are at a disadvantage when they leap waterfalls because they cluster together at their bases and are therefore easier targets for the grizzlies. Grizzly bears are well-documented catching leaping salmon in their mouths at
Brooks Falls in
Katmai National Park and Preserve in Alaska. They are also very experienced in chasing the fish around and pinning them with their claws. At such sites such as Brooks Falls and
McNeil Falls in Alaska, big male grizzlies fight regularly for the best fishing spots. Grizzly bears along the coast also forage for
razor clams, and frequently dig into the sand to seek them. During the spring and fall, directly before and after the salmon runs, berries and grass make up the mainstay of the diets of coastal grizzlies. Inland grizzlies may eat fish too, most notably in Yellowstone grizzlies eating
Yellowstone cutthroat trout. The relationship with cutthroat trout and grizzlies is unique because it is the only example where Rocky Mountain grizzlies feed on spawning salmonid fish. Grizzly bears occasionally prey on small mammals, such as
marmots,
ground squirrels,
lemmings, and
voles. The most famous example of such predation is in
Denali National Park and Preserve, where grizzlies chase, pounce on, and dig up
Arctic ground squirrels to eat. In some areas, grizzly bears prey on
hoary marmots, overturning rocks to reach them, and in some cases preying on them when they are in
hibernation. Larger prey includes
bison and
moose, which are sometimes taken by bears in
Yellowstone National Park. Because bison and moose are dangerous prey, grizzlies usually use cover to stalk them and/or pick off weak individuals or calves. Grizzlies in Alaska also regularly prey on moose calves, which in Denali National Park may be their main source of meat. In fact, grizzly bears are such important predators of moose and elk calves in Alaska and Yellowstone that they may kill as many as 51 percent of elk or moose calves born that year. Grizzly bears have also been blamed in the decline of elk in
Yellowstone National Park when the actual predators were thought to be
gray wolves. In northern Alaska, grizzlies are a significant predator of
caribou, mostly taking sick or old individuals or calves. Several studies show that grizzly bears may follow the caribou herds year-round in order to maintain their food supply. In northern Alaska, grizzly bears often encounter
muskox. Despite the fact that muskox do not usually occur in grizzly habitat and that they are bigger and more powerful than caribou, predation on muskox by grizzlies has been recorded. Grizzlies along the Alaskan coast also scavenge on dead or washed up whales. Although the diets of grizzly bears vary extensively based on seasonal and regional changes, plants make up a large portion of them, with some estimates as high as 80–90%. Various berries constitute an important food source when they are available. These can include
blueberries, blackberries (
Rubus fruticosus), salmon berries (
Rubus spectabilis), cranberries (
Vaccinium oxycoccos), buffalo berries (
Shepherdia argentea), soapberries (
Shepherdia canadensis), and huckleberries (
Vaccinium parvifolium), depending on the environment. Insects such as
ladybugs, ants, and bees are eaten if they are available in large quantities. In Yellowstone National Park, grizzly bears may obtain half of their yearly caloric needs by feeding on
miller moths that congregate on mountain slopes. When food is abundant, grizzly bears will feed in groups. For example, many grizzly bears will visit meadows right after an avalanche or glacier slide. This is due to an influx of
legumes, such as
Hedysarum, which the grizzlies consume in massive amounts. When food sources become scarcer, however, they separate once again.
Interspecific competition The relationship between grizzly bears and other predators is mostly one-sided; grizzly bears will approach feeding predators to steal their kill. In general, the other species will leave the carcasses for the bear to avoid competition or predation. Any parts of the carcass left uneaten are scavenged by smaller animals.
Wolves With the reintroduction of
gray wolves to Yellowstone, many visitors have witnessed a once common struggle between a
keystone species, the grizzly bear, and its historic rival, the gray wolf. The interactions of grizzly bears with the wolves of Yellowstone have been under considerable study. Typically, the conflict will be in the defence of young or over a carcass, which is commonly an
elk killed by wolves. The grizzly bear uses its keen sense of smell to locate the kill. As the wolves and grizzly compete for the kill, one wolf may try to distract the bear while the others feed. The bear then may retaliate by chasing the wolves. If the wolves become aggressive with the bear, it is normally in the form of quick nips at its hind legs. Thus, the bear will sit down and use its ability to protect itself in a full circle. Rarely do interactions such as these end in death or serious injury to either animal. One carcass simply is not usually worth the risk to the wolves (if the bear has the upper hand due to strength and size) or to the bear (if the wolves are too numerous or persistent). While wolves usually dominate grizzly bears during interactions at wolf dens, both grizzly and black bears have been reported killing wolves and their cubs at wolf dens even when the wolves were acting in defence.
Big cats Cougars generally give the bears a wide berth. Grizzlies have less competition with cougars than with other predators, such as coyotes, wolves, and other bears. When a grizzly descends on a cougar feeding on its kill, the cougar usually gives way to the bear. When a cougar does stand its ground, it will use its superior agility and its claws to harass the bear, yet stay out of its reach until one of them gives up. Grizzly bears occasionally kill cougars in disputes over kills. There have been several anecdotes, primarily from the late 19th and early 20th centuries, of cougars and grizzly bears killing each other in fights to the death. The other
big cat present in the United States which might pose a threat to bears is the
jaguar; however, both species have been extirpated in the regions of the Southwest where their former habitats overlapped, and grizzlies remain so far absent from the regions along the U.S.-Mexico border, where jaguars appear to be returning.
Other bears hybrid in
Yukon Territory, Canada
Black bears are not strong competition for prey because they have a more herbivorous diet. Confrontations are rare because of the differences in size, habitats, and diets of the bear species. When this happens, it is usually with the grizzly being the aggressor. The black bear will only fight when it is a smaller grizzly such as a yearling or when the black bear has no other choice but to defend itself. There is at least one confirmed observation of a grizzly bear digging out, killing, and eating a black bear when the latter was in hibernation. The segregation of black bear and grizzly bear populations is possibly due to competitive exclusion. In certain areas, grizzly bears outcompete black bears for the same resources. For example, many Pacific coastal islands off British Columbia and Alaska support either the black bear or the grizzly, but rarely both. In regions where they coexist, they are divided by landscape gradients such as the age of forest, elevation, and land openness. Grizzly bears tend to favor old forests with high productivity, higher elevations and more open habitats compared with black bears. Encounters between grizzly bears and
polar bears have increased in recent times due to
global warming. In encounters the grizzly is usually the more aggressive one and often dominate in fight. However, healthy polar bears seem to be dominant over the grizzly. However, conflict is not the only result of the two bears meeting; in some instances
grizzly–polar bear hybrids (called grolar bears or pizzly bears depending on the sex of the parents) are produced.
Various small predators Coyotes, foxes, and
wolverines are generally regarded merely as pests to grizzlies rather than competition, though they may compete for smaller prey, such as ground squirrels and rabbits. All three will try to scavenge whatever they can from the bears. Wolverines are aggressive enough to occasionally persist until the bear finishes eating, leaving more scraps than normal for the smaller animal. However, the removal of wolves and grizzlies in California may have greatly reduced the abundance of the endangered
San Joaquin Kit Fox.
Ecological role The grizzly bear has several relationships with its ecosystem. One such relationship is a mutualistic relationship with fleshy-fruit bearing plants. After the grizzly consumes the fruit, the seeds are excreted and thereby dispersed in a germinable condition. Some studies have shown germination success is indeed increased as a result of seeds being deposited along with nutrients in feces. This makes grizzly bears important seed distributors in their habitats. While foraging for tree roots, plant bulbs, or ground squirrels, bears stir up the soil. This process not only helps grizzlies access their food, but also increases species richness in alpine ecosystems. An area that contains both bear digs and undisturbed land has greater plant diversity than an area that contains just undisturbed land. Nitrogen cycling is not only facilitated by grizzlies digging for food, it is also accomplished via their habit of carrying salmon carcasses into surrounding forests. It has been found that spruce tree (
Picea glauca) foliage within of the stream where the salmon have been obtained contains nitrogen originating from salmon on which the bears preyed. These nitrogen influxes to the forest are directly related to the presence of grizzly bears and salmon. Grizzlies directly regulate prey populations and also help prevent overgrazing in forests by controlling the populations of other species in the food chain. This, in turn, changed the structure and density of plants in the area, which decreased the population sizes of migratory birds. When grizzly bears fish for salmon along the coasts of Alaska and British Columbia, they often only eat the skin, brain and
roe of the fish. In doing so, they provide a food source for
gulls,
ravens, and
foxes, all of which eat salmon as well; this benefits both the bear and the smaller predators. ==Interaction with humans==