Bighorn and Big Rail Can Be Friends

The Brightline West high-speed railway could be bad news for desert bighorn sheep in Mojave National Preserve. We know how to fix that.

By Christina Aiello, Kass Bissmeyer, Lara Rozzell, Paige Prentice, Jeff Villepique, Debra Hughson, Mike J. Gauthier, Clinton W. Epps, and Nathan L. Galloway

About this article

This article was originally published under “Perspectives” in Park Science magazine, Volume 35, Number 1, Winter 2021 (December 8, 2021).

Bighorn populations in the Mojave tend to be small. They rely on interactions with nearby populations for long-term survival.

Image credit: NPS

Interstate 15 forms the northern boundary of the Mojave National Preserve in the Southern California desert. On a busy weekend, more than 70,000 vehicles may drive this section of highway. The traffic creates problems for travelers, air quality, and local wildlife. A future high-speed rail line—Brightline West—would run along the I-15 corridor to connect the suburbs of Los Angeles to Las Vegas. The rail could reduce traffic and pollution. But with the current design, it would also further fragment wildlife populations, including desert bighorn sheep. We would be more enthusiastic about a high-speed train in the Mojave Desert if the development plan included wildlife crossings.

Room to Roam

Before California’s highways existed, desert bighorn sheep moved as they pleased throughout this region. That room to roam helped them survive and adapt to the harsh conditions of the Mojave Desert. Bighorn sheep prefer rugged mountains. But in the dry southwest, each mountain might only support a small population. When small populations are isolated, they tend to be more vulnerable to threats and local extirpation. But by keeping connected, bighorn sheep manage to thrive as a “metapopulation.”

A metapopulation is a collection of interconnected populations that interact through animal movement. This interaction keeps the gene pool diverse, giving the animals a greater chance to adapt successfully to changing conditions. Migrants from nearby populations can also help strengthen a struggling population or recolonize an area where a population failed. This support system only works when the habitat is well-connected and animals can move freely. Within protected areas like Mojave National Preserve and Death Valley National Park, bighorn populations benefit from high connectivity, which helps them survive setbacks like drought and disease. What happens, though, when you insert a major physical barrier between populations?

Bighorn Sheep Can’t Get Around Like They Used To

The I-15 corridor, and other structures like I-40, I-10, and the California aqueduct divide this once-connected landscape. Many prior routes traveled by bighorn are now extremely risky or altogether impossible. The National Park Service has worked with Oregon State University and the California Department of Fish and Wildlife for decades to understand how desert bighorn sheep interact and move in this modern landscape. We first recognized that these barriers posed a problem for bighorn in the early 2000s, after analyzing DNA samples collected throughout the region. Gene flow appeared lower than expected between populations separated by barriers, which in the long term, could limit genetic diversity.

Tracking collars then gave us a closer look at how bighorn move across the landscape. Each collar placed on a bighorn sheep records its location at regular time points over multiple years. After recording over 100 movement paths over eight years around I-15, we now know that bighorn do sometimes travel across roadways to reach nearby habitat—even I-15. But with high traffic volumes, crossing the highway presents a major risk and is generally avoided. In the last documented I-15 crossing, in early 2020, a young ram was struck by a vehicle and killed.

Data from recent trail camera surveys also tell us that bighorn sheep avoid using existing structures like underpasses and culverts along I-15. During three years of monitoring some of the most likely underpasses, we saw no evidence that bighorn passed under the highway using these structures. Bighorn prefer big, open views and rough terrain to avoid predators. They have traveled under bridges elsewhere that were large, open, and located near rocky terrain or ridges. But along I-15, we found that the largest bridges were built in flat valleys, far from areas where bighorn spend most of their time.

We have over a decade of data from monitoring bighorn movements and genetics. These data suggest that building structures without wildlife in mind can limit use of important wildlife corridors. The new railway project gives us an opportunity to improve instead of harm bighorn connectivity. We can alleviate the barrier effect at key locations along the I-15 corridor through well-designed highway overpasses.

An Ounce of Prevention

For safety, the Brightline West design includes high walls along the track that will prevent people and animals from crossing at the level of the roadway. But these walls also harden the existing barrier for bighorn sheep, further reducing connectivity. This loss of connectivity is likely to reduce resiliency of the desert bighorn metapopulation and adversely affect these animals well beyond the highway footprint and for decades to come.

The good news is that we know how to restore and prevent further degradation of wildlife connectivity. Highway wildlife overpasses work well when we intentionally design and locate them. Desert bighorn already use three overpasses on U.S Highway 93 in Arizona as well as a recently constructed wildlife overcrossing spanning Interstate 11 in Nevada. Other animals in other states benefit from human infrastructure designed to support wildlife trying to cross major roads. California Department of Fish and Wildlife and its cooperators are planning an overpass in the Los Angeles metropolitan area to protect mountain lions.

The secret to a successful overpass is to use data to evaluate wildlife movement and combine the project with already planned construction. We already have the data, and we used it to propose three ideal locations on the northern boundary of Mojave National Preserve where bighorn would benefit the most. It will be much easier and cost-effective to build overpasses while new railroad construction is underway, including construction of overpasses for emergency vehicles, than to add them later. This is a chance to be proactive and build a railway that will reduce both fossil fuel emissions and our impact on iconic native wildlife. Now that’s a plan we can get behind.

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About the authors

<sup>Christina Aiello is a National Park Foundation science fellow at Mojave National Preserve, Joshua Tree National Park, and Death Valley National Park. She conducts research on desert bighorn sheep movement and habitat use as a postdoctoral fellow at Oregon State University. See Christina’s “Dare to Imagine” profile here.

Kass Bissmeyer is a program specialist with the National Park Service Biological Resources Division. She leads the NPS Connected Conservation community of practice, which focuses on landscape-scale conservation.

Lara Rozzell is the external energy and minerals coordinator with the National Park Service Pacific West Regional Office (serving Regions 8, 9, 10, and 12). She is an ecologist and works on landscape-scale conservation in the context of energy development.

Paige Prentice was the desert bighorn program biologist for California Department of Fish and Wildlife for six years. She continues to study desert bighorn through her Master of Science degree at Oregon State University.

Jeff Villepique supervises wildlife programs across Southern California’s mountains, valleys, and deserts for the California Department of Fish and Wildlife, He has studied bighorn sheep since 2001.

Debra Hughson is chief of the Science and Resource Stewardship Division at Mojave National Preserve. She has coordinated research and habitat conservation efforts to conserve bighorn sheep since 2005.

Mike J. Gauthier is the superintendent of Mojave National Preserve.

Clinton W. Epps is a professor in the Department of Fisheries, Wildlife, and Conservation Sciences at Oregon State University. He has studied genetics, movement, and population dynamics of desert bighorn sheep in the Mojave Desert since 1999.

Nathan L. Galloway is a wildlife biologist with the National Park Service Biological Resources Division, which conducts applied research to inform management of park resources.</sup>