Monitoring Desert Bighorn Sheep in Joshua Tree National Park

A bighorn sheep at Ryan Mountain, Joshua Tree National Park. NPS photo by Preston Jordan Jr.

Desert bighorn sheep are one of Joshua Tree National Park's signature animals, and spotting one among the rocks can be the highlight of a visit. The park's well-protected habitat makes it important for bighorn conservation across the species' range, and genetic studies show it is also an open corridor linking neighboring habitats and a source of genetic diversity for herds outside the park.

Threats

• Water. In the hot summer months bighorns depend on surface water at springs and oases. Since 2006, as springs have dried up, the park's herd has lost 50 percent of its usual summer habitat: of 45 documented springs, 6 were dry in 2006 and 33 in 2016. Sheep have been seen in neighboring towns such as Joshua Tree and Twentynine Palms, drinking from birdbaths and backyard pools, which raises the risk they will catch deadly diseases from domestic animals. People have manipulated water in the area since ranchers built dams in the early 1920s, and between 1968 and 1982 the park built "guzzlers" for wildlife, most of which are now in poor repair.
• People. Park visitation more than doubled from 2013 to 2019 and keeps climbing. Some of the most popular spots, including Barker Dam, 49 Palms Canyon and Lost Palms Oasis, are also prime bighorn habitat, and people's presence can keep sheep from water.
• Disease. A sheep-specific pneumonia (Mycoplasma ovipneumoniae) has been found in the park. Its exact origin is unknown, but the evidence suggests a domestic goat or sheep infected bighorns near the park, which then infected the park's animals. The disease has killed up to 90 percent of some Mojave Desert herds. Since the park's first outbreak in 2015, it has found four more probable cases and two known deaths, and more have likely gone undetected for lack of comprehensive monitoring.
• Lost connections. Development has narrowed the corridors linking the park's bighorns to other herds in the Southwest. With climate change and more development expected in the Mojave, open corridors will matter more and more.

Desert bighorn sheep near Barker Dam, one of the park's few reliable year-round sources of surface water. NPS photo by Michael Vamstad.

Tracking the herd

In 2020 and 2021, 16 bighorn sheep were fitted with GPS collars to track where they go and how they use the land. The goals are to:

• understand how sheep use their habitat and identify threats;
• document population trends, including numbers, distribution, health, reproduction, demographics and disease;
• learn how rising visitor use affects sheep in their favored habitat;
• study how sheep behave around water, and what role guzzlers and water in nearby towns play in supporting the herd.

Releasing a bighorn sheep fitted with a transmitter collar. National Park Service.

The monitoring guides management. In 2021, blood samples from six captured sheep were tested for disease, and five were positive for pneumonia antibodies. The park is seeking support to build disease transmission models and outbreak response plans. Biologists are also using 20 years of trail-use data to see how heavier use of certain trails affects sheep, and the park is reviewing its guzzlers, deciding whether to repair or remove each depending on whether sheep use it.

The park's goal is a natural, stable bighorn population with as little intervention as possible, taking into account changing climate, visitor numbers and water supply.

Sources

• National Park Service, Joshua Tree National Park, "Monitoring Desert Bighorn Sheep." https://www.nps.gov/articles/000/mdbs.htm