Bighorn Sheep Studies

Bio Brief

The ecology, distribution and population status of the desert bighorn sheep (Ovis canadensis) in Grand Canyon National Park is relatively unknown. Few studies have been conducted; none of which were comprehensive. Native Americans hunted desert bighorn sheep. Bandalier (1892) documented hunting by the Piutes in the North Kaibab region and recorded hunting by the Havasupai of southwestern Grand Canyon as “common”. In 1957, Bendt (NPS, 1982), a National Park Service (NPS) biologist, reported that bighorn sheep were common throughout the park and monument but offered no specific numbers regarding sheep density and distribution. In the late 1920’s there was an estimated 600 sheep in the canyon. By 1969 Hoffmeister revised the estimated population downward to only 150 animals. Wilson (1976) and Allen (1961) listed the bighorn in the Grand Canyon region as common throughout the length of the canyon into the San Francisco mountains (summers) and Little Colorado River Gorge (winters).

Blaisdell (1961) noted in his comprehensive United States survey, that he believed there was substantial evidence of competition from feral burros. More recent studies evaluated this putative impact of burros on the bighorn sheep population. Those estimates suggest that about 114,000 acres of the inner canyon were being utilized by 300 burros. In addition, surveys were conducted in the area between Red Canyon and Fossil Canyon on the south side of the canyon and estimated a population of 50 sheep. This was in the area of burro/bighorn competition and comprised about 15-20% of the entire bighorn habitat in Grand Canyon. Although four surveys were conducted, there were no solid conclusions on the total effect of burro competition because the combined surveys did not cover the entire park. In the 1980s the park responded to this threat to bighorn populations by removing the feral burros. Surprisingly, there has never been a follow-up survey to determine if the sheep population recovered and stabilized following the removal of the burro population.

The status of desert bighorn sheep population elsewhere is fairly well documented. Southwestern state game and fish agencies regularly survey herds due to their listing as a “big game animal”. Most of these herds are manipulated somewhat through hunting or transplanting, or have experienced human impacts from recreation, grazing, artificial water development, or disease from domestic livestock. A comprehensive three-year study of bighorn sheep populations was started in 1991 in the Rocky Mountain Region National Parks, excluding Grand Canyon National Park, but including adjacent national park lands, as well as tribal, federal and state lands. While bighorn sheep in the Grand Canyon are protected, populations to the north of Grand Canyon National Park in Glen Canyon National Recreational Area and to the southwest in Lake Mead National Recreational Area are hunted, compete for forage with livestock and are impacted by the dams.

In addition, there are impacts from hikers and tourists who float the river. Some evidence suggests that bighorn, just as elk and deer, will ingest food wrappers, tent plastics, and other human-related items. The ingestion of these materials can cause serious impacts to the digestive system of these animals. All of these pressures have reduced bighorn populations. Some populations of bighorn in the bordering states of California and New Mexico are believed by some wildlife officials to be on the edge of extinction (Zakin 1999 and CDFG, personal conversation).

Grand Canyon National Park may be the one remaining area in the southwest where desert bighorn have not been severely impacted by man. These animals have evolved “in situ” since the Pleistocene and their populations are almost certainly genetically intact, given that no transplanting has occurred in Grand Canyon National Park. Although hunting, grazing, and transplanting do not occur in the park itself, these activities do occur on the park boundaries and can affect park populations. In fact, a new cattle-grazing allotment adjacent to the park boundary added an additional 600 animal units grazing on a year-round basis from 1999-2002. Domestic sheep grazing also continues on federal and tribal lands to the north, east and west of the park boundaries.



Purpose of Research

Management concerns for bighorn sheep include impacts from domestic sheep and the transmission of disease, aircraft overflights, and recreational disturbance from hiking and river rafting. Unfortunately, the few studies that have been conducted in these areas provide inconclusive or conflicting results. There is a desperate need to obtain information regarding these impacts upon the sheep so we may fulfill our mandate for effective stewardship. We currently have no information regarding the numbers or health status of the Grand Canyon sheep population, therefore we do not know if the population is either in good standing or on the decrease. By studying the distribution, status and ecology of the bighorn population over time, park management can consider how current trends may be affected by increasingly high human impacts occurring in the park’s ecosystem. Wildlife managers in Grand Canyon National Park are currently compiling data on several prey species (deer, elk and antelope) of large carnivores, but are missing data on desert bighorn sheep. Due to the logistics required to monitor sheep in their native habitat, determining the status of the species is far more difficult and costly than that of the other ungulates, and beyond the resources that the park can put toward this determination.



Research Methods

Bighorn sheep, like all wildlife in the Grand Canyon, are difficult to survey due to the rugged terrain of the canyon. There are three main methods available for conducting a population survey of bighorn sheep – each with its positive and negative aspects. Counts can be done relatively easily from the rims, but visibility of sheep is limited in the steep side canyons. Counting can be done from the river, but again the visibility of sheep up the steep side canyons is limited. The third and most common method is to survey by helicopter. The helicopter survey allows for aerial views into remote areas of the canyon, but the danger of flying a helicopter in a canyon and the sheer expense make this method very limited. Helicopter use has been restricted to preserve the proposed wilderness areas of Grand Canyon National Park.

The 2002 Colorado River expedition utilized observers on rafts to find and count all sheep that were visible from this vantage point. All bighorn sheep spotted while floating down the river were recorded by number, river mile location, sex, and approximate age. Bighorn sheep can be aged by assessing the growth patterns on their horns, counting the rings as you would to determine the age of a tree.



Resource Management

Several research trips have produced consistent data for bighorn sheep observations. From the river, approximately 100-120 sheep were counted. At best, this information is a very rough estimate of the bighorn population. However, when combined with other data, it will help researchers create a predictive model to determine where bighorn sheep are most likely to be found and what numbers might be expected. With a better idea of population dynamics, resource managers will be better prepared to protect the desert bighorn.


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