Yellowstone provides summer range for an estimated 10,000–20,000 elk (Cervus canadensis) from 6–7 herds, most of which winter at lower elevations outside the park. These herds provide visitor enjoyment as well as revenue to local economies through hunting outside the park. As Yellowstone’s most abundant ungulate, elk comprise approximately 90% of winter wolf kills and are an important food for bears, mountain lions, and at least 12 scavenger species, including bald eagles and coyotes. Competition with elk can influence the diet, habitat selection, and demography of bighorn sheep, bison, moose, mule deer, and pronghorn. Elk browsing and nitrogen deposition can affect vegetative production, soil fertility, and plant diversity. Thus, changes in elk abundance over space and time can alter plant and animal communities in Yellowstone.
Number in Yellowstone
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Size and Behavior
Elk are the most abundant large mammal found in Yellowstone. European American settlers used the word “elk” to describe the animal, which is the word used in Europe for moose (causing great confusion for European visitors). The Shawnee word “wapiti,” which means “white deer” or “white-rumped deer,” is another name for elk. The North American elk is considered the same species as the red deer of Europe.
Bull elk are one of the most photographed animals in Yellowstone, due to their huge antlers. Bull elk begin growing their first set of antlers when they are about one year old. Antler growth is triggered in spring by a combination of two factors: a depression of testosterone levels and lengthening daylight. The first result of this change is the casting or shedding of the previous year’s “rack.” Most bulls drop their antlers in March and April. New growth begins soon after.
Growing antlers are covered with a thick, fuzzy coating of skin commonly referred to as “velvet.” Blood flowing in the skin deposits calcium that makes the antler. Usually around early August, further hormonal changes signal the end of antler growth, and the bull begins scraping the velvet off, polishing and sharpening the antlers in the process.
The antler growing period is shortest for yearling bulls (about 90 days) and longest for healthy, mature bulls (about 140 days). Roughly 70% of the antler growth takes place in the last half of the period, when the antlers of a mature bull will grow two-thirds of an inch each day. The antlers of a typical, healthy bull are 55–60 inches long, just under six feet wide, and weigh about 30 pounds per pair.
Bulls retain their antlers through the winter. When antlered, bulls usually settle disputes by wrestling with their antlers. When antlerless, they use their front hooves (as cows do), which is more likely to result in injury to one of the combatants. Because bulls spend the winter with other bulls or with gender-mixed herds, retaining antlers means fewer injuries sustained overall. Also, bulls with large antlers that are retained longer are at the top of elk social structure, allowing them preferential access to feeding sites and mates.
The mating season (rut) generally occurs from early September to mid-October. Elk gather in mixed herds—many females and calves, with a few bulls nearby. Bulls bugle to announce their availability and fitness to females and to warn and challenge other bulls. When answered, bulls move toward one another and sometimes engage in battle for access to the cows. They crash their antlers together, push each other intensely, and wrestle for dominance. While loud and extremely strenuous, fights rarely cause serious injury. The weaker bull ultimately gives up and wanders off.
Calves are born in May and June. They are brown with white spots and have little scent, providing them with good camouflage from predators. They can walk within an hour of birth, but they spend much of their first week to ten days bedded down between nursing. Soon after they begin grazing with their mothers, and join a herd of other cows and calves. Up to ⅔ of each year’s calves may be killed by predators. Elk calves are food for black and grizzly bears, wolves, coyotes, cougars, and golden eagles. Female elk can live 17–18 years. Rare individuals may live 22 years.
The high elevation grasslands of the park provide summer habitat for 10,000–20,000 elk. However, less than 5,000 elk spend winter in the park. Climate is an important factor affecting the size and distribution of elk herds. Many ungulates migrate to increase their access to high-quality food. They prefer to feed on young plants, which are the most nutritious. In winter, colder temperatures and snowfall decrease the amount of forage that grows, and decrease the amount of forage that is accessible to wildlife. This forces elk to migrate to areas where forage is more available. The timing and routes of Northern Yellowstone elk migration closely follow the areas of seasonal vegetation growth and changes in snow depth. After winters with high snowpack, elk delay migration. In years with lower snowpack and earlier vegetation green-up, elk migrate earlier.
Ungulates that migrate typically give birth around periods of peak vegetation green-up to overlap with high-nutrition plant phases. Nutritious food allows mothers and calves to build up fat reserves. Changes in climate will undoubtedly impact newborn elk, but it is difficult to predict whether that impact will be positive or negative. Earlier spring could lead to a longer snow-free season where migration and access to food are not encumbered. However, a longer growing season, without increased access to high-quality forage, might have a negative impact. Warmer temperatures could increase the rate of green-up, causing the plants to complete their growth cycle faster, and shorten the period of time that food is available and accessible. Also, earlier spring could result in a mismatch in the timing of calving and the date of peak plant nutrition, resulting in high mortality of newborn calves.
Elk on the Northern Range
Yellowstone’s largest elk herd winters along and north of the park’s winter boundary. With more moderate temperatures and less snowfall than the park interior, this area can support large numbers of wintering elk. The herd winters in the area of the Lamar and Yellowstone river valleys from Soda Butte to Gardiner, Montana. Now, the majority of the northern herd migrates outside of the park into the Custer Gallatin National Forest and onto private land.
After decades of debate over whether this range was overgrazed by too many elk, public concern has shifted to the herd’s small size. The winter count, which was approximately 17,000 when wolf reintroduction began in 1995, fell below 10,000 in 2003. It fluctuated between 6,000 and 7,000 as the wolf population on the park’s northern range declined from 94 in 2007 to 50 by the end of 2015. The elk count dropped to 3,915 in early 2013, the lowest since culling ended in the park in the 1960s. However, a March 2016 helicopter survey counted and classified 6,913 elk on the Northern Range, suggesting changes in population trends. Decreased numbers have been attributed to large carnivore recovery (wolves, cougars, bears), hunter harvest, and drought-related effects on pregnancy and survival. The State of Montana has reduced the permits issued for this herd so that hunting of females now has little impact on population size.
There are some indications that elk–wolf interactions are contributing to a release of willows and other woody vegetation from the effects of herbivory on the northern range. Wolves have altered elk behavior that influences group sizes, habitat selection, movements, distribution, and vigilance; while the proportion of browsed aspen, cottonwood, and willow leaders has decreased in some areas during recent years, and cottonwood and willow heights have increased significantly. Others argue that lower elk densities over the last two decades—resulting from the combined effects of predators (wolves, cougars, bears), human hunters, and weather—has necessarily altered the impact of elk browsing. Research is underway to determine the relative effects of climate, hydrology, carnivore predation/avoidance, and herbivory on these woody species.
Elk in the Interior
Only one herd lives both winter and summer inside the park. The Madison–Firehole elk herd (less than 100 animals) has been the focus of a research study since November 1991. Researchers are examining how environmental variability effects ungulate reproduction and survival. Prior to wolf restoration, the population was naturally regulated by severe winter conditions to a degree not found in other, human-hunted elk herds. The elk are also affected by high fluoride and silica levels in the water and plants they eat, which affect enamel formation and wear out teeth quickly—thus shortening their lives. The typical life span is 13 years; elk on the northern range regularly live to about 18 years. Information gained in this study will be useful in comparing non-hunted and hunted elk populations.
Elk in the Greater Yellowstone Ecosystem
The Greater Yellowstone Ecosystem is home to approximately 30,000–40,000 elk. For most of the last two decades, the Jackson herd, which currently numbers about 12,000, has been larger than the northern Yellowstone herd. Some ranges and migratory routes overlap, and some interchange occurs among the herds. Summer range in the southern part of Yellowstone National Park is used by part of the Jackson herd as well as by elk from the North Fork Shoshone and northern Yellowstone herds. Because the wildlife responsibilities of the National Park Service, the US Fish and Wildlife Service, the US Forest Service, and state wildlife agencies also coincide, elk management in Greater Yellowstone requires substantial coordination among government agencies with different priorities.
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The Northern Range is the hub of wildlife in Yellowstone National Park. Occupying just 10 percent of the park, it is winter range for the biggest elk herd in Yellowstone and is arguably the most carnivore-rich area in North America. Early management of predators caused dynamic changes to the ecosystem. The reappearance of carnivores on the landscape has had significant and sometimes unexpected impacts on the resident grazers and their habitat.
Disease in Greater Yellowstone
Many elk and bison in the Greater Yellowstone Ecosystem have been exposed to the bacterium that causes brucellosis. Brucellosis is a contagious bacterial disease that originated in livestock and often causes infected cows to abort their first calves. It is transmitted primarily when susceptible animals directly contact infected birth material. No cure exists for brucellosis in wild animals.
The prevalence of brucellosis in Yellowstone elk is low; the rate of exposure to brucellosis in 100 adult female elk captured on the park’s northern range during the winters of 2000 to 2005 was 2%; it was 3% in 130 neonatal elk on the park’s northern range during the summers of 2003–2005; and it was 3% in 73 adult female elk captured in the park’s Madison–Firehole drainages during winters of 1996–1998. Elk are commonly observed within 100 yards of bison during late winter and spring when brucellosis-induced abortion or calving occurs in Yellowstone.
Because of their high densities, elk that are fed in winter have sustained high levels of brucellosis; winter feeding on the northern range stopped more than 50 years ago. Elk are fed during the winter at the National Elk Refuge in Jackson, Wyoming, in addition to 22 Wyoming-run feed grounds. The feed grounds were created in the 1900s to maintain Wyoming’s elk herds and limit depredation as migratory routes from summer range to lower elevation winter ranges became blocked by settlement in the Jackson area. Transmission of brucellosis from feed ground elk, where an average of 30% have tested positive for exposure to the bacteria, was the apparent source of infection in Wyoming cattle in 2004.
Chronic Wasting Disease
Elk, deer, and moose in and near Yellowstone National Park are at risk for infection by chronic wasting disease (CWD). This fatal infection, transmitted by animal contact or through the environment, has spread to within 10 miles of the park. National Park Service staff and partners will continue surveillance and, if necessary, take action to minimize both transmission of the disease and the effects of intervention on the elk population and other park resources.
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