Endangered Sheep Need an Open Border

California's border with Baja California is a complex region with unique environmental issues. Our Borderlands series takes a deeper look at this region unified by shared landscapes and friendship, and divided by international politics.

California's Anza-Borrego desert is an arid and rugged place, part of the larger Colorado Desert region. To a casual observer, it looks barren: rocks, peaks, and valleys, studded by the occasional mesquite and cactus.

But life thrives out here in the seemingly inhospitable desert in many forms, from tiny lichens on the underside of rocks to the huge array of birds that visit the Salton Sea. None, however, are seemingly more improbable than the keen-eyed peninsular bighorn sheep, which live in the deserts of California and Baja California, stalking their cliffs and lofty crags and splitting open barrel cacti with their horns for water.

The federally endangered peninsular bighorn sheep, a close relative of the more common desert bighorn, has always been an elusive animal, rarely seen despite signs along Interstate 8 that warn of its presence. Its numbers have dwindled dramatically in recent years, however, due to new roads, buildings, and other development around the border region. The sheep are wary and migratory, passing seasonally through the international border between the United States and Mexico.

The animals' numbers have been low for decades, but dropped alarmingly after the 1960s when new development brought in devastating changes to the desert environment. Roads cut through the sheep's pathways; farms brought in diseases to which the animals had no immunity; proposed wind and solar farms and expansion of the US-Mexico border wall continue to threaten their habitat. All of these things have critically endangered the peninsular bighorn.

But there is hope. Modern developments might have played a major role in bighorns' dwindling numbers, but now new technology may also be pulling the sheep back from the brink.

The habits of the peninsular bighorn have been poorly understood until relatively recently, when a collaborative, cross-border genetic study by researchers at San Diego Zoo Global and the Universidad Autónoma de Baja California in Ensenada, Mexico found that gene flow between what had been thought were two separate populations was, in fact, the norm.

Studies of the sheep are tricky: researchers capture them without tranquilizers and equip them with collars that transmit their locations almost in real-time. Because it is nearly impossible to track the fleet-footed animals in rugged terrain, they are tracked down by helicopter, and then netted - with a gun that shoots nets! - from the air. Once their blood is drawn, samples taken, and their GPS-equipped collars secured, the bighorns are released back into the wild.

"It's very efficient," said Mathias Tobler, a scientist at San Diego Zoo Global who specializes in wildlife ecology. "We can capture five to six individuals in one day. We track their movements, and that's how we know what habitat they use, where the barriers are, so we see the animals walk right up to the road, turn around, just move back and not cross the road at all." Major genetic differences have been seen on either side of California's 78 freeway and Mexico's Highway 2, although the sheep regularly cross under the far busier 8 freeway, suggesting that structure of the roadways need to be taken into consideration in the survival of the species.

Asako Navarro is a senior genetic researcher who co-authored a study on the sheep which found that they often travel through the remote peaks around La Rumorosa, which lies across the border from Jacumba Hot Springs. "There's a break in the border fence because there's very rough terrain in that area, and there's no fence through the Sierra Juarez range," she said. "We are seeing not only movement back and forth from the border, but use of the habitat by the peninsular bighorn sheep."

Thanks to conservation efforts, the population of the peninsular bighorn sheep is beginning to return, although the numbers remain in the hundreds -- a far cry from the thousands, if not millions, that once roamed the peaks and desert valleys from the San Jacinto Mountains to the southern tip of Baja California Sur.

"[The sheep used] the whole Peninsular Ranges in southern California all the way to the tip of Baja California Sur," said Lisa Nordstrom, a conservation ecologist at the San Diego Zoo's Institute for Conservation Research, which is devoted to using technology to preserve biodiversity. "Because bighorn sheep rely on this small population structure, connectivity is a huge issue for maintaining a viable population," Nordstrom said. Construction and development in the region -- whether it is another extension to the border wall or a solar farm -- would be devastating to the recovering population if it interrupts that connectivity.

If that connection is lost, it means the already-fragile population of the endangered peninsular bighorn sheep will likely not rebound again. Connectivity means more than simply allowing the sheep to pursue their paths across an international border, however. It also means connectivity and collaboration between countries.

"There's been very low coordination between Mexico and the US," said Tobler. "On the policy side, the research side, and everything -- so that's really what we're trying to do here. In the end it's a cross-border issue, and we can't manage populations on one side and ignore the other side."

Keeping lines of research and communication open will ensure that both Mexico and the United States can benefit from the a commitment to biodiversity -- a goal that is not just a means to an end in and of itself, but a way to protect life forms across the world that may be interconnected in ways that have not yet been discovered, even in the seemingly empty desert regions in which creatures like the peninsular bighorn once thrived, and may again.

"It's definitely a unique habitat," said Lisa Nordstrom. "It's definitely worth conserving."