Struggling species finding its footing
CABEZA PRIETA NATIONAL WILDLIFE REFUGE
Through the summer of 2002, John Hervert cut open the stomachs of Sonoran pronghorn corpses in the desert. They were full of cholla fruit, one of the few foods left for the pronghorns after months of drought, the worst on record in southern Arizona.
The plants they usually ate had dropped their leaves, said Hervert, an Arizona Game and Fish Department biologist. The cholla fruit had some semblance of moisture but lacked nutrients.
He watched the animals “slowly starve to death.” They would lose weight, their muscles eroding away and bones protruding everywhere underneath their skin, until they’d lie down and die.
“We were within three to four weeks of losing the last few animals that were barely hanging on before the rains actually saved them.” John Hervert Arizona Game and Fish Department biologist, on Sonoran pronghorns
“We were within three to four weeks of losing the last few animals that were barely hanging on before the rains actually saved them,” Hervert said.
The pronghorns survived bad droughts in the past, but in 2002, the species was on the brink of extinction in southern Arizona. Westward expansion over more than a century had splintered, erased and dried up the majority of pronghorn habitat. The population grew smaller each year.
For the last few Sonoran pronghorns in the United States, the only hope for survival seemed to be their greatest menace over the past century: humans.
Sixteen years later, government wildlife managers have slowed the pronghorns’ decline, increasing its numbers through captive breeding programs and, in some areas, willing water into the ground.
In the midst of that effort, the species’ survival has moved to the center of American politics. The U.S.-Mexico border runs through the pronghorns’ range, and climate change promises worse droughts. Although the nation is at odds over what the border will look like, how to respond to climate change and what to do about a dwindling water supply in the West, wildlife managers say these animals are on course to survive.
How the pronghorns lost habitat
The spindly shadows of the Sonoran Desert hide “prairie ghosts,” a name the pronghorns earned by eluding observers. And a lucky glimpse of their bushy tails skimming over cactuses and creosote bushes is fleeting.
Before the pronghorns’ numbers dwindled, surveyors charting the U.S.Mexico border in the late 1800s saw the agile ungulates in every valley from Nogales, Sonora, to Yuma.
The United States’ southern border in Arizona sat on the northern edge of Sonoran pronghorn habitat when it ran along the Gila River, but as the state expanded southward, the new line divided pronghorn territory between the two countries.
Over the years, urbanization, railroads, highways, mines, livestock grazing, fences, agriculture, canals, transmission corridors and human disturbances have steadily blocked off habitat or erased it, halting pronghorn migration, isolating herds and severing access to rivers.
The resulting habitat fragmentation is now one of the most significant threats to Sonoran pronghorns, according to a federal plan to recover them.
A border wall like the one President Donald Trump has promised to build would add another obstacle, but Hervert said the damage is done. The habitat is already fragmented.
The threats date back decades. Sonoran pronghorns were among the original endangered species, protected in 1967 under the Endangered Species Preservation Act.
Even before the wall, Highway 2 in Mexico, which runs along the border, mostly severed migration, said Jim Atkinson, a U.S. Fish and Wildlife Service biologist and the Sonoran pronghorn recovery coordinator. Interstate 8 in the United States has similarly blocked the pronghorns’ movement.
Human activities disturb animals
Trump’s border wall would still harm the Sonoran pronghorns, said Dan Millis of the Sierra Club. Further bulldozing and fragmenting of their habitat would only undermine the species’ recovery.
Another border wall already cuts through a portion of pronghorn habitat, he said. At 15 feet tall, the “pedestrian barrier fence,” as the federal government calls it, stretches along 5 miles of border at Organ Pipe Cactus National Monument.
Sonoran pronghorns have bypassed it and crossed Highway 2 into Mexico. Hervert has seen a couple of radio-collared pronghorns do it over the years, although it is extremely rare.
While a wall stretching the entire border could cut off this movement into Mexico entirely, barriers to pronghorns don’t have to be walls or busy paved roads. People also block off habitat by disturbing the area. And the worst human disturbance pronghorns face is the cat-and-mouse game between the Border Patrol and undocumented bordercrossers, according to the recovery plan.
U.S. border-policing strategy pushed crossers away from urban areas to these remote parts, according to a National Park Service report, which assessed an increase in vehicle tracks across federal lands in Sonoran pronghorn habitat.
Hervert said the Trump administration’s border wall could decrease disturbances behind the wall, if it reduces Border Patrol chases. The agents are, after all, responding to undocumented border-crossers, he said.
But, Millis said, “to say that walling off an area will help the environment is wishful thinking.” The best way to reduce human disturbance along the border is to cut the Border Patrol’s budget, he said. With the number of crossers down and the number of agents up, the Border Patrol causes much more human disturbance, he said.
Hervert has studied Sonoran pronghorns since the 1980s, when he saw fewer agents and crossers. The “big picture,” he said, is that humans have harmed Sonoran pronghorns for a long time in all kinds of ways.
“That’s why I feel very strongly that we have to do what we can to mitigate all those harms,” Hervert said.
‘It all comes down to water’
After a half-century on the endangered-species list, Sonoran pronghorns occupy only about 12 percent of their historical range. “It all comes down to water,” said Atkinson, the Fish and Wildlife Service biologist.
Sonoran pronghorns can’t access it like they used to, or it’s mostly dried up.
The Gila, Colorado, Sonoyta and Sonora rivers, and the surrounding ribbons of vegetation on both sides of the border, likely sustained Sonoran pronghorns historically during times of scarce Continued on next page