Desert Bighorn Sheep and the Barriers Between Mountains in the Mojave Desert

Desert mountains give conditions and resources that differ from the surrounding valleys. Photo: NPS.

From a car on a desert interstate, the Mojave looks lifeless. But the mountains rising from it like islands, and the playas between, are alive, and the sheep that live on them need to travel. The Mojave Desert spans California, Arizona and parts of Nevada and Utah, and its mountain groups act as sky islands where many plants and animals survive. One resident is the desert bighorn sheep (Ovis canadensis nelsoni).

Islands that need bridges

Bighorn sheep move skillfully over rugged terrain and can cross great distances of valley, as long as nothing blocks them. Photo: NPS.

Desert bighorns like steep, rocky ground. Their mountain ranges are cooler and trap more rain than the desert floor, so they support plants the sheep eat, and offer water, food and safety from predators. But the ranges are small compared with the Rockies or Sierra Nevada and ringed by low valleys, so each holds only a few sheep. Small herds are vulnerable unless they can reach other herds and resources in neighboring mountains. Movement between ranges links the populations into a network called a metapopulation, which helps maintain genetic diversity, strengthen faltering populations and recolonize places where a herd has disappeared.

What blocks them

Highways, canals and renewable energy developments can partly or completely stop bighorn movement. That reduces gene flow and genetic diversity and can stop recolonization, limiting the animals' ability to adapt to climate change, disease and other stresses.

A section of Interstate 40 divides the Bristol Mountains and keeps sheep from available habitat. Will a new water source near this underpass encourage them to cross safely? Photo: C. Aiello.

National park lands protect healthy habitat and genetic diversity, but the land between parks has to allow movement for neighboring sheep to benefit. The National Park Service is working with Oregon State University, the California Department of Fish and Wildlife, the Bureau of Land Management, the Department of Defense, CalTrans and the Society for Conservation of Bighorn Sheep on a project around Death Valley National Park, Mojave National Preserve and Joshua Tree National Park. It looks at how bighorns meet human-made barriers, how to make crossings easier, and where there is evidence of recolonization.

Underpasses and water

Researchers are testing whether existing highway underpasses work for bighorns. The sheep are wary of enclosed spaces that limit their view of predators, and underpasses are built for water flow, not necessarily in the best place for sheep. Preliminary data, even with more monitoring over two years, suggest that they do not use underpasses to cross between ranges very often. The team added water sources, called guzzlers, to lure the sheep through, though it may take time for them to find a brand-new resource. To see what works, they fit GPS radio collars with mini solar panels, and use remote cameras, track and sign surveys and DNA from scat to identify crossings.

Looking for new arrivals

Some ranges once held bighorns but have none now. Evidence that sheep have moved back is a sign that the landscape is working as connected habitat.

Two researchers head into a range to look for signs that bighorns have recolonized it. Photo: C. Aiello.

Sheep are hard to see because they cross rocky ground quickly, so researchers read tracks, scat and bedding sites. They confirm with genetic analysis that scat really came from bighorn sheep, and, because the genetics of these populations have been monitored for decades, DNA can even show where an animal came from.

Just because you do not see them does not mean they are not there. Desert bighorn sheep need mountain and lowland habitat to thrive in the Mojave. Photo: NPS.

The studies take years, and animal behavior sets the pace. Researchers were still collecting data, but expect the results to show how to protect bighorn populations across the Mojave into the future.

Sources

• National Park Service, Desert Bighorn Sheep: Connecting a Desert Landscape; rewritten in hubnx's own words. Photographs of named researchers are not reproduced.