Leaping from the Ledge – Bighorn Sheep in the California Desert

Unregulated hunting previously devastated populations, followed by livestock-grazing impacts, loss of water sources from human diversion, mining operations, habitat fragmentation, vehicle collisions, military activity and the spread of diseases via domestic livestock, which often results in fatal pneumonia. 

Virtually all mountain ranges within the deserts of California once supported bighorn populations, forming what is referred to as a connected “metapopulation” or separate populations connected by individual animals moving among groups. Today disease outbreaks are believed to have caused the extirpation or substantial reduction of bighorn in numerous mountain ranges, and major interstate highways have severed habitat linkages once connecting populations. New renewable energy and housing developments in some areas additionally pose steep challenges for long-term bighorn conservation as they also sever or compromise the ability of these wide-ranging animals to follow their historical migration paths.  

One particularly vulnerable pocket of desert bighorn sheep is found tucked into the southern portion of its range. Once recognized as a separate subspecies, the Peninsular bighorn found on the “Colorado desert” slopes of California’s Peninsular Ranges are part of a geographically isolated population that is found from the San Jacinto Mountains of California south to the U.S.-Mexico border. Here, bighorn sheep are limited to a sliver of appropriate elevation habitat, sometimes quite narrow in width, that extends along low desert slopes. 

Although California has protected bighorn sheep in the Peninsular Ranges under the California Endangered Species Act since 1971, the population declined from approximately 1,100 animals in the 1970s to about 400 by 2000 from habitat loss and modification, human disturbance, off-road vehicles, habitat fragmentation from roads and highways, livestock grazing, disease, poaching and fire suppression. Low sheep numbers were further impacted by mountain lion predation – typically not a concern in a healthy bighorn population. In 1998, the U.S. Fish and Wildlife Service listed this population as endangered, which was followed by critical habitat designation and recovery plan adoption. 

With this increase in protection, the range-wide estimate of Peninsular bighorn increased from 400 individuals in 2000 to 955 in 2010. However, this population remains extremely vulnerable. As such, a 2012 federal review of this population’s status indicated that Peninsular bighorn should remain listed as endangered. 

With multiple threats making bighorn recovery difficult, disease looms as one of the most significant problems. It can rapidly sweep through a herd causing high lamb mortality and secondary infections in adults, and temporary, widespread pneumonia outbreaks, or epizootics, have been reported in many western states. There is significant evidence that a species of Mycoplasma bacteria (Mycoplasma ovipneumoniae), a common respiratory pathogen in domestic sheep and goats, can cause pneumonia in bighorn. 

Authorized livestock grazing on public lands has significantly degraded bighorn habitat in several mountain ranges and very likely influenced conditions for disease spread. Livestock use much of the same forage as bighorn, extensively disturb surface soils, strip vegetation around waters, introduce non-native plant seed and congregate at waters to the exclusion of wildlife. Livestock operations also offer the double-edged sword of “improved” natural springs, which offer more reliable water but also promote joint livestock-bighorn use and require regular maintenance to ensure appropriate water quality and quantity.

Mycoplasma bacterial diseases typically enter populations with little resistance, rapidly causing mortalities. Surviving sheep become carriers and bighorn lambs often catch the disease within a few months and die. These pneumonia-related diseases typically affect a population for more than a decade, with outbreaks believed to have reduced bighorn herds in western states up to 90 percent.

Due to the outbreak of respiratory disease first documented in bighorn at Old Dad Peak, the California Department of Fish and Wildlife (CDFW) in 2013 renewed emphasis on data collection in its bighorn monitoring efforts, collaborating with Oregon State University, the National Park Service, the Bureau of Land Management (BLM), the Society for the Conservation of Bighorn Sheep and the California Chapter of the Wild Sheep Foundation. During 2013-2016, CDFW conducted eight ground surveys, flew the first helicopter surveys in five years, captured and collared over 170 bighorn sheep across 13 mountain ranges, recovered 25 bighorn mortalities and helped install, repair and fill bighorn drinker systems. Habitat improvement at critical wildlife waters and installation and maintenance of artificially created water drinkers has increased population numbers and expanded habitat use.

This data collection has so far indicated that the bacteria Mycoplasma ovipneumoniae has affected bighorn across the California desert as outlined below:

• In May 2013, a National Park Service employee inspecting wildlife water drinkers found four dead bighorn sheep on Old Dad Peak, southeast of Baker, California, within the Mojave National Preserve. Of the 72 animals collared and tested for disease at Old Dad Peak in November 2013, six died.
• Old Dad Peak was the only locale where bighorn carcasses were reported. However, few animals were encountered in 2013 displaying disease symptoms, and laboratory analysis of bighorn tissue samples taken during the November 2013 disease research effort demonstrated that the pneumonia outbreak was present across the desert in California.
• In early May 2015, a coughing bighorn was reported in Joshua Tree National Park many miles south of Mojave National Preserve and a dead lamb was found in August. The public was urged to refrain from releasing domestic sheep and goats into the wild because these domestic animals can carry the disease without exhibiting symptoms and pass it to bighorn.
• In December 2018, at least 20 bighorn sheep from the San Gorgonio population perished from pneumonia, including five near a golf course in La Quinta, California where there have been ongoing disease issues. The cause of this pneumonia outbreak is not confirmed at this time, but it should be noted that there is a high potential for transmission from domestic goats, sheep and pathogens associated with feral cattle. 
• In total, four of the reported dead in San Gorgonio were reported to Whitewater Preserve rangers in the Sand to Snow National Monument. Later in December, the remains of seven young rams or ewes were found on the east side of the Mission Creek, where groups reportedly just sat down and died together. 

To manage for genetic diversity, one of CDFW’s prime management goals is to prevent further fragmentation of bighorn populations—particularly between mountain ranges separated by interstate highways 15 and 40—and to reconnect four focal metapopulation fragments into a single functional metapopulation per the agency’s draft Desert Bighorn Sheep Management Plan. Working with the National Park Service, Oregon State University and the BLM, the agencies need to collect data that can provide the information they need to help the population. It requires fitting bighorn with global positioning system (GPS) and telemetry tracking collars to monitor animal survival, cause of mortality, habitat use and range connectivity.

And that is what will happen to the animal that just arrived at base camp by helicopter. With oversight provided by CDFW desert Bighorn biologist Paige Prentice, an onsite CDFW veterinarian, and a host of contributing volunteers all geared to specific tasks, the animal is carefully hobbled to limit movement, blindfolded to minimize stress, weighed, measured, GPS-collared and ear-tagged. Blood is collected for trace mineral analysis and tested for a variety of disease pathogens. With the examination complete, the bighorn is reloaded for transport back to his initial capture location for release. 

State and federal agencies, as well as Defenders, hope to use the information gleaned from him and others to inform conservation priorities, such as retrofitting major highways to allow bighorn to travel among mountain ranges. The protected areas previously designated in the BLM’s 2016 Desert Renewable Energy Conservation Plan and requisite conservation management actions associated with public land project permitting will also need to be kept intact and potentially expanded with a bighorn connectivity focus in mind. Several livestock allotments on public lands may also need to be retired or greatly reduced.

The goal is to ensure that the iconic symbol of our western mountains can continue to leap from the ledge to a safe landing spot in the deserts of California.